Hose Bibb and Irrigation Demand Calculator — Why a Running Hose Is Added After the Fixture Unit Curve, Not Into It

Calculate

This selector governs every field, label, result and export here and takes priority over the site header switch. Switching converts what you entered rather than reinterpreting it, so 5 gpm becomes 18.9 L/min and returns to exactly 5. Fixture units are dimensionless and read the same in both systems.

The Code You Are Sizing To

Name the code your authority has adopted. The two model codes do not agree on every fixture, so an unknown basis stops the page rather than issuing values that may not be yours.

The edition appears beside the code on every result, because a fixture unit value is only checkable against the edition it was read from.

The published tables carry separate private and public columns. In the cited table both columns give a hose bibb the same value, so this selects the column the rest of the building total is read from.

The Outdoor Outlets, Counted by Use

Outlets used to fill a bucket or wash a car, never left running. These are the ones that contribute fixture units. Enter 0 if there are none.

Outlets that feed a sprinkler, a soaker or anything left on for an extended period. These contribute gallons per minute instead. Enter 0 if there are none.

Counted separately and excluded from both totals, with both scenarios reported. Enter 0 if the use of every outlet is known.

Optional. The reduced loading for additional bibbs applies to a shared segment as well as to the building total, and never to the branch serving one outlet.

What the Running Outlets Draw

Required where the continuous count is above zero. This page defaults no figure, because what a hose delivers depends on the outlet, the hose, the nozzle and the pressure.

The result reports a confidence beside the figure: measured and design flows are strong, a specification is medium because it applies at its own stated pressure, and an estimate is weak.

The Irrigation

The arrangement decides where the flow goes rather than how large it is. A manual system on a bibb is already in the hose figure, and a separate service line does not load the building supply at all.

Required where the arrangement is an automatic system. A zone is a group of heads fed by one valve.

Required where the arrangement is an automatic system. This figure comes from the irrigation design rather than from any plumbing table, and no sprinkler head count is converted into it.

Required where the arrangement is an automatic system. An unknown setting returns no continuous total rather than an assumption, because assuming sequential operation is the answer that undersizes.

Required only where a stated number was chosen above. It cannot exceed the number of zones.

The Rest of the Building

Optional. Entering it returns the combined fixture unit figure to convert. Converting that total to gallons per minute is done on the water pipe sizing page, not here.

Overview

A hose bibb has a fixture unit value and a running hose does not, and the reason is what the fixture unit curve is built on.

The curve is probabilistic. It assumes fixtures are used briefly and rarely at the same moment, so a house at thirty fixture units converts to about twenty gallons per minute rather than the sum of everything it could draw at once. A hose left running for an hour has no odds. It is running.

Published code says so directly: fixture units cannot be applied to constant-use fixtures such as hose bibbs, lawn sprinklers and air conditioners, and those must be assigned a gallons per minute value instead. The same tables also give a hose bibb 2.5 fixture units, and both are correct, because they answer different questions.

An outlet in continuous use goes into the gallons per minute figure instead of the fixture unit total, not as well as it. Put a five gallon per minute hose through the curve as 2.5 fixture units and the total comes out at 21.2 gallons per minute where the code method gives 25, a shortfall in the direction of undersized pipe.

What to Look at First

Read the classification block first, before either number. Deciding which of a property's outdoor outlets belongs in the fixture unit total and which belongs beside it as a continuous figure is what this page is for, and the numbers follow from it.

Then read the pair. A fixture unit total for the curve and a continuous flow to add after it, never a single combined figure, because combining them needs the conversion table that belongs to the water pipe sizing calculation.

How to Use This Calculator

  1. Name the code basis and its edition. The two model codes do not agree on every fixture, and this page does not present one set of values as universal.

  2. Count the hose bibbs three ways: those used occasionally only, those that run continuously, and those whose use you do not know. A total of four with at least one running does not say whether one, two or four run, and the difference is the whole continuous figure.

  3. Say what the irrigation is. An automatic system on its own branch, a manual system fed from a hose bibb, a system on a separate service line, or none. A manual system on a bibb is already in the hose figure and is not added again.

  4. Enter the flow for anything in continuous use, and say where the figure came from. This page does not default a hose flow, because it depends on the outlet, the hose, the nozzle and the pressure.

  5. For an automatic system, enter the zone flow and say how many zones the controller can run at once. Four zones at eight gallons per minute is eight or thirty two depending on nothing but a setting, and without both figures there is no irrigation demand to report.

  6. Read the answer as a pair. A fixture unit total for the curve and a continuous figure to add after it. The pipe sizing page converts the fixture unit side through the published table first, then adds the continuous side to the result.

Inputs & Outputs

Inputs

•Unit System : Options: US / Imperial (gpm), SI / Metric (L/min)
•Code Basis : Options: Not selected, Uniform Plumbing Code, International Plumbing Code appendix method, Local jurisdiction table, Unknown
•Adopted Edition : Options: Not selected, 2024 edition, 2021 edition, 2018 edition, 2015 edition, An earlier or superseded edition, Edition not stated
•Occupancy : Options: Not selected, Private, a dwelling, Public
•Hose Bibbs in Occasional Use Only (outlets)
•Hose Bibbs in Continuous Use (outlets)
•Hose Bibbs Whose Use Is Not Known (outlets)
•More Than One Bibb on a Single Distribution Segment : Options: Not stated, Yes, one segment serves several bibbs, No, each bibb has its own run
•Continuous Flow Per Running Hose (gpm / L/min)
•Where the Flow Figure Came From : Options: Not selected, Measured at the outlet, From a nozzle or sprinkler specification, From an irrigation zone design, An estimate
•Irrigation Arrangement : Options: Not selected, No irrigation system, Automatic system on a dedicated branch, Manual system fed from a hose bibb, Irrigation on a separate service line, Unknown
•Number of Irrigation Zones (zones)
•Continuous Flow Per Zone (gpm / L/min)
•Zones the Controller Can Run at Once : Options: Not selected, One at a time, A stated number at once, All zones may run, Unknown
•How Many Zones Run Together (zones)
•Existing Fixture Unit Total for the Rest of the Building (WSFU)

Outputs

•Each outlet group with the category it fell into and why
•The branch load for every outlet, reported separately because it does not move
•The fixture unit contribution of the occasional outlets, with the reduced loading and its scope
•The continuous demand in gallons per minute, with its basis, confidence and zone arrangement
•The irrigation demand for a separate service line, reported on its own
•The pair to carry to the water pipe sizing page, never combined here
•The fixed worked example showing what the wrong method would have given
•The backflow requirement named for the outlets being counted
•The code and edition the fixture unit values came from

Hose Bibb and Irrigation Demand Formula

Two quantities that never combine on this page, and one rule that applies to every outlet whatever it does.

Fixture units, one or more occasional bibbs

wsfu_occasional = 2.5 + 1.0 * (occasional_count - 1)

Where the occasional count is zero, the contribution is zero. This equation applies from a count of one upward, because written as a single line it returns 1.5 at a count of zero.

Building fixture unit total

wsfu_total = wsfu_building + wsfu_occasional

Branch load, every bibb

wsfu_branch = 2.5

Every hose bibb, occasional or continuous, has a branch sized on 2.5 fixture units.

Continuous demand from hoses

gpm_hoses = continuous_count * gpm_per_hose

Continuous demand from irrigation

gpm_irrigation = zones_at_once * gpm_per_zone

Total continuous demand on the building supply

gpm_continuous_building = gpm_hoses + gpm_irrigation

Unit conversion

L/min = gpm * 3.785411784

Fixture units are dimensionless and identical in both systems, and nominal pipe sizes stay in inches with the millimetre figure alongside.

Where the irrigation is a manual system fed from a hose bibb, its flow is already in the hose figure and the irrigation term is zero. Where it is on a separate service line, the irrigation flow is reported on its own as a separate service demand and does not enter the building figure, so the two are never blended.

No equation here converts fixture units to gallons per minute, and none produces a combined flow. This page leaves that conversion to the pipe sizing calculator so that the same published table and the same sizing workflow are used consistently.

Why Running Hoses Are Added After the Curve

There are two kinds of water demand in a supply calculation and a hose bibb can produce either one.

A fixture unit is a probabilistic, intermittent load. A gallons per minute figure is a constant one. The same tap belongs to whichever category matches how it is being used, and the fixture cannot tell you which.

The fixture unit method works by converting a total through a published curve, and the curve is where the assumption lives. It was built on the observation that plumbing fixtures are used briefly and unpredictably, so the more fixtures a building has, the smaller the share of them likely to run at any moment. That is why a house at thirty fixture units converts to about twenty gallons per minute, which is 75.7 litres per minute, rather than to the sum of everything it could draw at once.

A lavatory runs for twenty seconds. A water closet fills for a minute. A dishwasher draws in short bursts. Each of those has odds attached to it, and the curve is a way of writing down the odds.

A hose left running for an hour has none. It is not likely to be running, it is running. There is no probability to apply and nothing for the curve to reduce, so pushing a continuous load through it does not produce a smaller number for a good reason. It produces a smaller number for no reason.

Published code puts constant-use fixtures outside the method rather than inside it with a different value. Fixture units cannot be applied to constant-use fixtures, such as hose bibbs, lawn sprinklers and air conditioners, and these must be assigned the gallons per minute value. The table note says the same from the other side: for fixtures or supply connections likely to impose continuous flow demands, the required flow in gallons per minute shall be determined and added separately to the demand for the distribution system.

The word separately is doing the work. The continuous figure joins the answer after the curve has converted the fixture units, not before it.

What that costs when it is done the other way is worth seeing in numbers. On a house at thirty fixture units, a five gallon per minute hose treated as 2.5 fixture units takes the total to 32.5, which converts to about 21.2 gallons per minute. Added correctly it gives 25. The shortfall is 3.8 gallons per minute, which is fifteen percent of the correct total. At a ten gallon per minute hose the curve method still gives 21.2 against a correct 30, a shortfall of 8.8 or twenty nine percent.

The direction matters as much as the size. Engineers are used to approximations that err toward safety and this one does not: it produces a number smaller than the truth, toward undersized pipe, an undersized meter and an undersized service. And nothing downstream flags it, because the fixture unit total is plausible, the conversion is a legitimate lookup, and the resulting pipe size is a real one.

Hose Bibb Fixture Unit Branch Rule

One rule on this page does not move whatever the outlet does, and it is easy to lose because a second rule sitting beside it does move.

Published table notes state that the fixture branch to each hose bibb shall be sized on the basis of 2.5 fixture units. Every hose bibb. The one used to fill a bucket twice a summer and the one feeding a soaker hose all afternoon have the same branch rule, because a branch has to carry whatever the outlet draws when it is open, and both open the same way.

The rule that does move is the reduced loading. The same notes state that the reduced fixture unit loading for additional hose bibbs is used where sizing the total building demand and for pipe sizing where more than one hose bibb is supplied by a segment of water distribution pipe. So the first outlet counts 2.5 and each additional one counts 1.0, and that reduction applies to the building total and to a shared segment.

It does not apply to a branch. A branch serving one outlet is 2.5 fixture units even if it is the fourth hose bibb on the property.

Three bibbs on a property therefore produce three separate answers rather than one. Each branch is sized on 2.5. A segment carrying all three is sized on 2.5 plus 1.0 plus 1.0. And the building total takes the same 4.5, counting only the outlets being treated as fixture units at all.

That last clause is where this rule meets the rest of the page. An outlet in continuous use still has a branch sized on 2.5 fixture units, because the branch has to carry the flow. What changes is that its demand joins the building total as gallons per minute rather than as fixture units.

So a property with one occasional bibb and one continuous bibb has two branches at 2.5 each, 2.5 fixture units in the building total, and a continuous figure in gallons per minute. The branch rule and the building total are answering different questions and they do not have to agree.

Do Not Count a Running Hose Twice

This is the accounting rule, and it is the most common way to get this calculation wrong.

An outlet in continuous use contributes gallons per minute instead of fixture units for that sizing condition. Not as well as them.

The temptation runs the other way, because counting it in both places feels careful. It is not careful, it is double counting: the same outlet appearing twice in one demand total under two different names. The code sentence about constant-use fixtures exists to prevent exactly that, and the giveaway is that it says fixture units cannot be applied rather than that they should be supplemented.

Worked on two hose bibbs, one occasional and one feeding a soaker at five gallons per minute: the building total takes 2.5 fixture units from the occasional bibb and five gallons per minute from the running one, and both branches are sized on 2.5 fixture units. The answer is not 3.5 fixture units and five gallons per minute.

There is a second trap of the same shape and it catches people more often, because nothing about it looks like double counting.

A manual sprinkler fed from a hose bibb is one flow through one outlet. If the bibb is recorded as running continuously and the sprinkler is recorded as an irrigation system, the same water is counted as a hose and again as irrigation. There is no second connection and no second draw: the sprinkler is what the hose is doing.

That is why this calculator asks for the irrigation arrangement rather than just asking whether irrigation exists. A manual system fed from a hose bibb contributes its flow once, through the hose figure, and the irrigation term is zero.

Both traps produce a number larger than the truth, which is the opposite direction from the error in the previous section, and both are still errors. A demand calculation is not made safer by counting things twice; it is made wrong in a way that is harder to argue with, because oversizing rarely gets challenged.

Irrigation Zone Demand

Where a property has an automatic irrigation system, the continuous demand depends on something that is not a plumbing property at all.

A zone is a group of heads fed by one valve. The system's designer sets the flow each zone draws, and that figure comes from the irrigation design rather than from any plumbing table. This calculator takes it as an input and does not estimate it.

What decides the demand on the supply is how many zones the controller can run at the same time.

Four zones at eight gallons per minute each impose eight gallons per minute if the controller runs them one at a time, which is 30.3 litres per minute. They impose thirty two if it runs all four together, which is 121.1. Two at once is sixteen, or 60.6.

Same pipes, same heads, same zones, four times the demand, decided by a setting.

That setting can be changed by anyone with access to the controller, without a single alteration to the piping, and it can be changed after the system is signed off. A system sized for sequential operation and later reprogrammed to run zones together is drawing four times what its supply was sized for, and nothing about the installation records that it happened.

So the arrangement is asked and never assumed, and where the answer is unknown this calculator returns no continuous irrigation total rather than a default. Assuming sequential operation is the comfortable answer and it is the one that undersizes.

Sprinkler heads are worth a note of their own. Published fixture unit tables carry a value for a lawn sprinkler head, and this calculator does not use it. A head on an automatic zone is a constant-use fixture by definition, running for as long as the zone runs, so the same rule that keeps a running hose out of the fixture unit curve keeps a running zone out of it. Irrigation demand on this page is a zone flow in gallons per minute.

Separate Irrigation Service

Some properties feed the irrigation from its own service line rather than from the domestic supply, and some jurisdictions require it.

Where that is the arrangement, the continuous irrigation demand does not load the building supply at all. The domestic pipe sizing sees the hose bibbs and nothing else, and the irrigation demand belongs to a separate calculation for a separate pipe.

This calculator reports the two separately rather than dropping the irrigation figure, because the separate line still has to be sized and the demand is the same demand. It is simply loading a different pipe.

Three things follow that are worth knowing before assuming the separation solves anything.

The separate line has its own sizing, and the zone arrangement question applies to it identically. Four zones at eight gallons per minute is still eight or thirty two on a dedicated line.

The backflow requirement does not go away. A separate service to an irrigation system is a connection to the potable supply and carries its own protection requirement, and on a system where chemicals can be introduced the hazard level is what sets the device.

And local requirements govern whether the separation is optional in the first place. Some jurisdictions require irrigation to be on a separate service or separately metered, and where that applies the question is not whether to separate but how the separate line is sized and protected.

Where a jurisdiction does not require separation, published state guidance describes the alternative directly: estimate the continuous demand separately from the intermittent demand and add it to the fixture demand in gallons per minute, which is what this page does for the combined case.

Backflow Protection for Hose Bibbs

Every hose threaded connection on a building is a cross connection, because a hose end can be lying in a bucket, a pool, a bucket of detergent, a chemical sprayer or a drain.

Published code at Section 608.15.4.2 of the International Plumbing Code and Section P2902.4.3 of the International Residential Code requires sill cocks, hose bibbs, wall hydrants and other openings with a hose connection to be protected by an atmospheric type or pressure type vacuum breaker, or by a permanently attached hose connection vacuum breaker where the highest point of usage is less than 10 feet, which is 3.0 metres, above it.

The exceptions are narrow: water heater and boiler drain valves provided with hose connection threads and intended only for draining, and water supply valves intended for connection of clothes washing machines where backflow prevention is otherwise provided or is integral with the machine.

The device standards are worth keeping straight, because two of them belong to a different question. A hose connection vacuum breaker is ASSE 1011, a pipe applied atmospheric vacuum breaker is ASSE 1001, and a pressure vacuum breaker assembly is ASSE 1020. Those three are the devices named for hose threaded openings.

A double check valve assembly at ASSE 1015 and a reduced pressure zone assembly at ASSE 1013 are backflow prevention assemblies for higher hazard connections and for irrigation service, not choices for a hose bibb. For an irrigation system, published guidance names the atmospheric vacuum breaker, the pressure vacuum breaker, the spill resistant vacuum breaker and the reduced pressure principle assembly, and the hazard at the connection is what selects between them. A fertiliser injector is the case that moves it.

Installation position is part of the requirement rather than a detail. The critical level of an atmospheric vacuum breaker sits not less than 6 inches, which is 152 mm, above all downstream piping and above the flood level rim. A pressure vacuum breaker sits not less than 12 inches, which is 305 mm.

Two things that are not backflow protection are worth naming. A single check valve is not one of the devices the code names for this duty. And a plumbing permit does not settle the question on its own: the building department enforces the code and the water utility enforces cross connection control separately, both approvals are required, and where the two conflict the more stringent applies.

The required device also costs pressure, which connects this back to the demand figures above. A manufacturer product guide plots the pressure loss through a three quarter inch atmospheric type vacuum breaker on a scale that reaches 12 psi, which is 83 kPa or about 28 feet of head, across the device's flow range. So the device that makes the connection legal takes pressure off the flow the outlet was assumed to deliver, and that loss belongs in the pressure budget rather than being discovered afterwards. This page names the loss and does not verify what any particular outlet delivers.

Continuous Pressure and Vacuum Breakers

This is where the two halves of this page meet, and it is a code statement rather than an observation.

Published code states that atmospheric vacuum breakers, including hose bibb vacuum breakers, shall not be subjected to continuous water pressure. It also states that shutoff or control valves shall not be installed downstream from an atmospheric vacuum breaker.

Now read that against what puts an outlet into this page's continuous demand category. A hose left connected with the tap open, a soaker running all afternoon, a sprinkler on a timer fed from a bibb.

Those outlets are under continuous pressure. The condition that makes them a continuous demand on the main is the same condition the commonest backflow device fitted to them is not rated for, and the manufacturer of one such device says the same thing in its own product guide: not to be used under continuous pressure or where a back pressure condition may exist.

The downstream valve rule closes the other route. A hose end shutoff, a spray gun that stops flow, or a valve on a manifold left connected to the bibb all put a control valve downstream of the vacuum breaker, and all of them hold pressure against it when they are closed.

So a permanently connected system is not a hose bibb with a hose on it. It is a fixed connection to the potable supply, and it needs a device selected for continuous pressure rather than the one that threads onto the spout.

The practical shape of it is simple enough. An outlet used the way a hose bibb is used, opened and closed, drained and disconnected, is served by a hose connection vacuum breaker. An outlet left connected and pressurised is an irrigation connection in everything but name, and the device follows the duty rather than the fitting.

That is the same logic the rest of this page applies to the demand. The fixture does not decide the answer. The use does, on both sides of the question.

What Is Hose Bibb and Irrigation Demand

Water supply demand for a building is estimated by counting fixture units and converting the total through a published probability curve. That method works because ordinary plumbing fixtures are used briefly and unpredictably.

Hose bibbs and irrigation break the assumption the method rests on.

A hose filling a bucket behaves like a plumbing fixture. The same hose feeding a sprinkler for an hour behaves like nothing in the curve at all.

Deciding which side of that line each outdoor outlet falls on is what this calculation is. The fixture cannot tell you, because the same tap does both. Only the use can.

Key Facts

  • Published fixture unit tables give a hose bibb 2.5 water supply fixture units and each additional hose bibb 1.0.
  • Published appendix guidance states that fixture units cannot be applied to constant-use fixtures, such as hose bibbs, lawn sprinklers and air conditioners, and that these must be assigned the gallons per minute value.
  • Published table notes state that for fixtures or supply connections likely to impose continuous flow demands, the required flow in gallons per minute shall be determined and added separately to the demand for the distribution system.
  • An outlet in continuous use contributes gallons per minute instead of fixture units, not as well as them.
  • The fixture branch to each hose bibb is sized on 2.5 fixture units whatever the outlet's use. The reduced loading of 1.0 for additional bibbs applies to the building total and to a segment serving more than one bibb, never to a branch.
  • The fixture unit curve is probabilistic: a house at thirty fixture units converts to about twenty gallons per minute, which is 75.7 litres per minute, on a published flush tank conversion table.
  • A five gallon per minute hose pushed through the curve gives a total of 21.2 gallons per minute against 25 by the code method, a shortfall of 3.8 or fifteen percent of the correct total. At ten gallons per minute the shortfall is 8.8, or twenty nine percent.
  • The error is toward undersizing and nothing downstream flags it, because the fixture unit total is plausible and the resulting pipe size is a real one.
  • Four irrigation zones at eight gallons per minute impose eight, sixteen or thirty two gallons per minute depending only on how many the controller runs at once. In litres per minute those are 30.3, 60.6 and 121.1.
  • Published tables carry a lawn sprinkler head fixture unit value, and this calculator does not use it, because a head on an automatic zone is a constant-use fixture.
  • Section 608.15.4.2 of the International Plumbing Code and Section P2902.4.3 of the International Residential Code require sill cocks, hose bibbs, wall hydrants and other hose connection openings to be protected by an atmospheric type or pressure type vacuum breaker, or by a permanently attached hose connection vacuum breaker. The exceptions are water heater and boiler drain valves used for draining and clothes washing machine supply valves where protection is otherwise provided.
  • The devices named for hose threaded openings are the hose connection vacuum breaker at ASSE 1011, the atmospheric vacuum breaker at ASSE 1001 and the pressure vacuum breaker at ASSE 1020. The double check valve assembly at ASSE 1015 and the reduced pressure zone assembly at ASSE 1013 are assemblies for higher hazard connections and irrigation service rather than hose bibb options.
  • Published code states that atmospheric vacuum breakers, including hose bibb vacuum breakers, shall not be subjected to continuous water pressure, and that shutoff or control valves shall not be installed downstream from one.
  • The critical level of an atmospheric vacuum breaker sits not less than 6 inches or 152 mm above all downstream piping and above the flood level rim. A pressure vacuum breaker sits not less than 12 inches or 305 mm. A manufacturer product guide plots the loss through a three quarter inch atmospheric type on a scale reaching 12 psi, which is 83 kPa or about 28 feet of head.

Applications

  • A designer sizing a house with three outside taps decides which of them belong in the fixture unit total and which sit beside it, before the pipe sizing calculation starts.
  • A contractor adding an irrigation system to an existing house finds that the continuous demand is added after the curve rather than folded into it, and that the answer moves a pipe size.
  • A plumber checking why a shower drops when the sprinklers come on finds the demand was never counted as continuous.
  • A property owner with four zones learns that the controller setting decides the demand, and that running all four together quadruples it without a single change to the pipe.
  • A homeowner who leaves a soaker hose connected all summer learns that the outlet is a continuous demand on the main and that the vacuum breaker threaded onto it is not rated for continuous pressure.
  • An inspector checking outside taps confirms that every hose threaded connection has protection, and that the device fitted is rated for how the tap is actually used.
  • An estimator working from a drawing showing four hose bibbs finds the drawing does not say how any of them are used, which is the one thing the calculation needs.
  • A designer specifying an irrigation system with a fertiliser injector finds the hazard level changes the device, and that a hose thread vacuum breaker is not the answer.

Worked Examples

Example 1. The outlet counted once, not twice

Given: two hose bibbs, one used occasionally and one feeding a soaker hose at five gallons per minute, which is 18.9 litres per minute.

Result: 2.5 fixture units in the building total, five gallons per minute of continuous demand, and both branches sized on 2.5 fixture units. Not 3.5 fixture units and five gallons per minute.

The running bibb is in the gallons per minute figure instead of the fixture unit total rather than as well as it, which is what the constant-use rule requires. Counting it in both places looks conservative and is an error of accounting.

Example 2. What the wrong method costs

Given: a house at thirty fixture units, converting to about twenty gallons per minute on a published flush tank conversion table, with a hose running at five gallons per minute.

Treated as 2.5 fixture units the total becomes 32.5, which converts to about 21.2. Added as a continuous demand the total is 25.

Result: a shortfall of 3.8 gallons per minute, fifteen percent of the correct total. At a ten gallon per minute hose the curve method still gives 21.2 against a correct 30, a shortfall of 8.8 or twenty nine percent. The direction is toward undersized pipe, an undersized meter and an undersized service.

Example 3. Zero occasional bibbs is zero

Given: one hose bibb, used only to feed an irrigation manifold, and no other outside taps.

Result: zero fixture units from hose bibbs in the building total, and the continuous flow in gallons per minute. The 2.5 that applies to the first occasional outlet does not apply to an empty count, and the branch to the single bibb is still sized on 2.5.

Example 4. The controller decides the demand

Given: four irrigation zones at eight gallons per minute each.

Result: eight gallons per minute run one at a time, which is 30.3 litres per minute, and thirty two run together, which is 121.1. Same pipes, same heads, four times the demand, decided by a setting anyone can change without touching the plumbing.

Example 5. Automatic irrigation with missing zone data

Given: an automatic system selected, with no zone flow and no controller setting entered.

Result: no irrigation demand is reported. The page does not default a zone flow and does not assume the controller runs one zone at a time, because assuming sequential operation is the answer that undersizes.

Example 6. Code basis unknown

Given: two occasional hose bibbs, with the code basis left unknown.

Result: the method is explained and no fixture unit values are issued as final. The two model codes do not agree on every fixture, so the values and the branch rule are tied to a named code basis and edition before they are used.

Example 7. A specification without a verified pressure

Given: a sprinkler specification giving six gallons per minute at 50 psi, which is 22.7 litres per minute at 345 kPa, with the outlet pressure unknown.

Result: six gallons per minute is accepted with its basis recorded as a specification, and the result states that this page does not verify the outlet can deliver it. The specification applies at its stated pressure and nothing here confirms that pressure exists at the tap.

Example 8. The device that makes it legal takes pressure

Given: a hose running at eight gallons per minute through a required vacuum breaker.

Result: the demand is eight gallons per minute, which is 30.3 litres per minute, and the device costs pressure that comes off the same supply. A manufacturer product guide plots the loss through a three quarter inch atmospheric type on a scale reaching 12 psi, which is 83 kPa or about 28 feet of head, so the figure belongs in the pressure budget and is read from the chart for the device actually fitted. The demand figure and the pressure budget are different questions, and this page answers the first one.

Standards & References

  • Minnesota Administrative Rules 4714.0610, Size of Potable Water Piping The state adoption of the Uniform Plumbing Code table of water supply fixture units and minimum fixture branch pipe sizes, giving a hose bibb 2.5 fixture units and each additional hose bibb 1.0 in both the private and public columns. Note 5 carries the continuous flow rule, that for fixtures or supply connections likely to impose continuous flow demands the required flow in gallons per minute is determined and added separately to the demand for the distribution system. Note 8 carries the branch rule, that the reduced fixture unit loading for additional hose bibbs is used where sizing total building demand and for pipe sizing where more than one hose bibb is supplied by a segment of water distribution pipe, and that the fixture branch to each hose bibb shall be sized on the basis of 2.5 fixture units.
  • Seattle Department of Construction and Inspections, Seattle Residential Code Appendix P The appendix guidance on sizing the water supply system, containing the sentence this page turns on: estimate the continuous supply demand in gallons per minute for lawn sprinklers and air conditioners and add the sum to the total demand for fixtures, because fixture units cannot be applied to constant-use fixtures, such as hose bibbs, lawn sprinklers and air conditioners, and these types of fixtures must be assigned the gallons per minute value.
  • UpCodes, Supply Demand The same fixture unit table and its notes in the published code viewer, used here to confirm the continuous flow note and the hose bibb branch note independently of the state adoption above.
  • UpCodes, Demand in Gallons Per Minute Corresponding to Fixture Load in Water Supply Fixture Units The published conversion table behind the worked example, in its flush tank column: 18.5 gallons per minute at 28 fixture units, 20 at 30, 21 at 32, 21.5 at 33 and 22.5 at 35. A house at thirty fixture units therefore converts to about twenty gallons per minute, and 32.5 fixture units falls at about 21.2, which is where the comparison on this page comes from.
  • Illinois Administrative Code, Title 77 Part 890 Appendix A Table N The state fixture unit to gallons per minute conversion table for a flush tank system, and the state rule that where a separate service line is not required for irrigation or similar systems likely to impose continuous demands, the continuous demand is estimated separately from the intermittent demand and added to the fixture demand in gallons per minute.
  • International Code Council, CodeNotes: Backflow Preventers and Protection of Water Supply The requirement that hose bibbs, wall hydrants, sill cocks and other openings with a hose threaded connection be protected by an atmospheric vacuum breaker, pressure vacuum breaker or permanently attached hose connection vacuum breaker at Section 608.15.4.2 of the International Plumbing Code and Section P2902.4.3 of the International Residential Code; the exceptions for water heater and boiler drain valves and clothes washing machine connections; and the requirement that an irrigation system connection be protected by an approved device, named there as an atmospheric vacuum breaker, a pressure vacuum breaker, a spill resistant vacuum breaker or a reduced pressure principle assembly.
  • Louisiana Department of Health, 2015 International Plumbing Code Backflow and Cross Connection Control Requirements, amended The vacuum breaker rules quoted here in full: at Section 608.15.4 the critical level of atmospheric type vacuum breakers not less than 6 inches above all downstream piping and above the flood level rim, the prohibition on shutoff or control valves downstream from an atmospheric vacuum breaker, the statement that atmospheric vacuum breakers including hose bibb vacuum breakers shall not be subjected to continuous water pressure, and the critical level of pressure type vacuum breakers at not less than 12 inches. Section 608.15.4.2 carries the hose connection requirement, the permanently attached hose connection vacuum breaker route where the highest point of usage is less than 10 feet above it, and the two exceptions. Section 608.13 carries the device standards: ASSE 1001 for pipe applied atmospheric vacuum breakers, ASSE 1011 for hose connection vacuum breakers, ASSE 1020 for pressure vacuum breaker assemblies and ASSE 1015 for double check valve assemblies.
  • Watts Regulator Company, Vacuum Breakers Product Guide The manufacturer flow and pressure loss charts referred to on this page, plotting the loss through a three quarter inch atmospheric type vacuum breaker on a scale that reaches 12 psi across the device's flow range, and the manufacturer's own statement for its hose connection vacuum breaker that the device is not to be used under continuous pressure or where the possibility of a back pressure condition may exist.

Units

Flow is entered and reported in gallons per minute or litres per minute, at 3.785411784 litres per gallon exactly. Five gallons per minute is 18.9 litres per minute, eight is 30.3, sixteen is 60.6 and thirty two is 121.1.

Fixture units are dimensionless and identical in both unit systems. A hose bibb is 2.5 fixture units in Imperial and in Metric alike, and so is the branch serving it.

Nominal pipe sizes stay in inches in both systems, because branch sizes in these tables are stated that way, with the millimetre equivalent alongside. Vacuum breaker critical levels are in inches and millimetres at 25.4, so six inches is 152 mm and twelve inches is 305 mm. Pressure is in psi and kPa at 6.894757, and head in feet and metres at 0.3048, so twelve psi is 83 kPa, which is about 28 feet of head or 8.4 metres.

The internal unit selector governs the fields, labels, math, on-screen result and any exported result, and it takes priority over the site-wide unit switch. Switching converts the values you entered rather than reinterpreting them.

Limitations

  • This page returns a pair of figures and never a combined flow. It leaves the fixture unit to gallons per minute conversion to the pipe sizing calculator so that the same published table and the same sizing workflow are used consistently.
  • It does not size pipe, a meter or a service.
  • It does not estimate what a hose or a zone delivers. That figure depends on the outlet, the hose diameter and length, the nozzle or sprinkler and the pressure available, and no default is offered.
  • It does not verify that a flow taken from a nozzle or sprinkler specification is achievable at the pressure the outlet actually has.
  • It does not design an irrigation system. Zone layout, head selection, precipitation rate, coverage and scheduling are outside it, and the flow per zone arrives here as an input from that design.
  • It does not convert sprinkler heads into fixture units. Published tables carry a head value and using it would contradict the constant-use rule, because a head on an automatic zone is a constant-use fixture by definition.
  • It does not select a backflow device or assess a hazard. It names the requirement, the code sections and the device standards, and the choice depends on the connection and on the local authority.
  • It does not verify pressure at any outlet, including the loss through a required backflow device, which belongs in the pipe sizing and pressure budget calculation.
  • The fixture unit values differ between the two model codes, which is why the code basis and edition are required inputs and appear on the result.
  • The worked comparison uses a published conversion table and is an illustration rather than a calculation on the figures entered.
  • Local requirements govern. Some jurisdictions require irrigation to be on a separate service or separately metered, and where that applies the irrigation demand does not join the domestic building supply at all.

Common Mistakes to Avoid

  • Counting a running hose twice. An outlet in continuous use contributes gallons per minute instead of fixture units, not as well as them.
  • Pushing a continuous demand through the fixture unit curve. The curve is built on brief, occasional use and has no term for a tap left open, so the result is smaller than the truth rather than safely larger.
  • Assuming the curve method is the conservative one. It understates by fifteen percent on a five gallon per minute hose and twenty nine on a ten, and nothing downstream flags it.
  • Applying the reduced 1.0 loading to a branch. Every hose bibb branch is sized on 2.5 fixture units, and the reduced value applies to the building total and to a shared segment.
  • Applying the 2.5 to an empty count. Zero occasional bibbs contributes zero fixture units, not 1.5 and not 2.5.
  • Counting a manual sprinkler as a hose and again as an irrigation system. It is one flow through one outlet.
  • Assuming the irrigation controller runs zones one at a time. Four zones at eight gallons per minute is eight or thirty two, and the difference is a setting.
  • Recording a hose bibb count without recording how each one is used. Four taps with at least one running does not say whether one, two or four run, and that is the whole continuous figure.
  • Defaulting a hose flow. It depends on the outlet, the hose, the nozzle and the pressure, and a guessed figure carries the entire answer.
  • Taking a nozzle specification as the delivered flow. The specification applies at its stated pressure, which the outlet may not have.
  • Converting sprinkler heads into fixture units. A head on an automatic zone is a constant-use fixture and the code puts those outside the method.
  • Fitting a hose thread vacuum breaker to a permanently connected system. Atmospheric vacuum breakers, including hose bibb vacuum breakers, must not be subjected to continuous water pressure.
  • Putting a shutoff or control valve downstream of an atmospheric vacuum breaker. Published code prohibits it, and a hose end shutoff or spray gun does exactly that.
  • Reading a double check valve or a reduced pressure zone assembly as a hose bibb option. Those are assemblies for higher hazard connections and irrigation service.
  • Treating a single check valve as backflow protection. It is not one of the devices the code names for this duty.
  • Ignoring the pressure loss through the required device. The assumed hose flow may not be available once the protection is fitted.
  • Assuming the plumbing permit settles it. The water utility enforces cross connection control separately, both approvals are required, and where the two conflict the more stringent applies.

Frequently Asked Questions

How many fixture units is a hose bibb?
Published tables give 2.5 water supply fixture units for the first hose bibb and 1.0 for each additional one. That value applies when the outlet is used occasionally, and the branch serving any hose bibb is sized on 2.5 whatever the outlet's use.
Do I add irrigation demand as fixture units or gallons per minute?
Gallons per minute, added after the fixture unit conversion. Published appendix guidance states that fixture units cannot be applied to constant-use fixtures such as hose bibbs, lawn sprinklers and air conditioners, and that these must be assigned the gallons per minute value.
Why can a hose bibb be both?
Because the same tap does two different things. Filling a bucket is a short, unpredictable draw, which is what the fixture unit curve describes. Feeding a sprinkler for an hour is a constant load, which the curve has no way to express.
If a hose runs continuously, do I still count its fixture units?
No. It contributes gallons per minute instead of fixture units for that sizing condition, not as well as them. Counting it in both places double counts the same outlet.
What happens if I put a running hose through the fixture unit curve?
The answer comes out too small. On a house at thirty fixture units, a five gallon per minute hose treated as 2.5 fixture units gives 21.2 gallons per minute against 25 by the code method, and the error is toward undersized pipe.
How much water does a garden hose use?
That is the input this calculation needs and it is not a code figure. It depends on the tap, the hose diameter and length, the nozzle and the pressure available, so measure it or take it from the irrigation design rather than assuming a number.
Does every outside tap need a backflow preventer?
Published code requires sill cocks, hose bibbs, wall hydrants and other openings with a hose connection to be protected by an atmospheric type or pressure type vacuum breaker, or by a permanently attached hose connection vacuum breaker, at Section 608.15.4.2 of the International Plumbing Code. The exceptions are water heater and boiler drain valves used for draining and clothes washing machine supply valves where protection is otherwise provided.
Can I leave a hose connected to a tap with a vacuum breaker on it?
Not to an atmospheric type. Published code states that atmospheric vacuum breakers, including hose bibb vacuum breakers, shall not be subjected to continuous water pressure, and that shutoff or control valves shall not be installed downstream from one. A permanently connected system needs a device selected for that duty.
Does this page give me one final demand figure?
No. It returns a pair: a fixture unit total for the curve, and a continuous figure in gallons per minute to be added after that curve. The pipe sizing calculator performs the table conversion and the sizing.
What if I do not know how the hose bibbs are used?
The calculator does not choose for you. It reports the fixture unit case and the continuous case separately with the condition each depends on, and it does not issue a final pair until the use is known.
Can I use sprinkler heads as fixture units?
Not for an automatic zone. Published tables carry a head value, and the constant-use rule puts a running zone into gallons per minute instead, because a head on an automatic zone runs for as long as the zone runs.
Does a manual sprinkler count as both a hose and irrigation?
No. A manual sprinkler fed from a hose bibb is one flow through one outlet, counted once as continuous hose flow.
Why does the irrigation controller setting matter so much?
Because simultaneous zones add. Four zones at eight gallons per minute impose eight if one runs at a time and thirty two if all four can run together, and that is a setting rather than a property of the pipe.
Does a vacuum breaker reduce the flow from a hose?
It can. The required device costs pressure, and a manufacturer product guide plots the loss through a three quarter inch atmospheric type on a scale reaching 12 psi, which is about 28 feet of head. This page names the loss and does not verify what any particular outlet delivers.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Water Pipe Sizing Calculator

Water Service Line Sizing Calculator

Water Supply Fixture Unit Calculator

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