Well Pressure Tank Sizing Calculator — Drawdown, the Acceptance Factor, and Minimum Pump Run Time

Pressure Tank Drawdown Chart

Drawdown is the water a tank delivers between the pump switching off at cut-out and switching on again at cut-in. It is a fraction of the shell, and this is what that fraction looks like.

Computed on the field convention, with the pre-charge 2 psi below cut-in, a tank delivers about 35 percent of its shell at 20/40, about 30 percent at 30/50, about 26 percent at 40/60 and about 23 percent at 50/70.

Applied to common tank sizes at 40/60, that gives about 5.2 gallons from a 20 gallon tank, 8.3 from a 32 gallon tank, 11.3 from a 44 gallon tank, 22.2 from an 86 gallon tank and 30.7 from a 119 gallon tank. In litres those are 19.7, 31.4, 42.8, 84.0 and 116.2.

At 30/50 the same tanks deliver about 5.9, 9.4, 13.0, 25.4 and 35.1 gallons. At 20/40, about 6.9, 11.0, 15.2, 29.7 and 41.0.

Manufacturer figures for real products sit in the same region and are not identical to these. The Well-X-Trol WX-202, a 20 gallon tank, publishes 8.0, 6.2 and 5.4 gallons at 20/40, 30/50 and 40/60. The WX-203 at 32 gallons publishes 9.9 at 30/50 and 8.6 at 40/60. The WX-250, sold as 44 gallons, publishes 17.7, 15.0 and 12.9. The Flexcon FlexLite FL 5, a 15 gallon tank, publishes 6.0, 5.1 and 4.4.

Two things follow from comparing those with the computed figures, and both are covered in their own section further down.

The nominal name on a tank is not reliably its shell volume. The WX-250 publishes drawdowns about 10 percent above what 44 gallons produces at every one of its settings, and the FL 5 runs 14 to 16 percent above what 15 gallons produces at every one of its settings. A consistent offset across all settings points to a shell larger than the name rather than to an error in the arithmetic.

And manufacturers do not share one pre-charge convention. Flexcon states on every sheet that its drawdown figures assume a pre-charge 2 psi below cut-in. Amtrol states no convention at all.

So use a computed chart to understand what changes when the switch setting changes, and use the manufacturer's chart for the exact model to select a tank.

Tank Volume Versus Usable Water

The number on a pressure tank is the volume of the shell. What comes out of it between pump starts is a quarter to a third of that, and the difference is the single most common misunderstanding on the subject.

A 20 gallon tank delivers about 5.2 gallons at 40/60. A 32 gallon tank delivers about 8.3. A 44 gallon tank delivers about 11.3. An 86 gallon tank delivers about 22.2.

The rest of the shell is compressed air, and it is not dead space. It is the entire mechanism. The pump fills the tank against that air, compressing it until the pressure reaches cut-out. When a tap opens, the air expands and pushes the water out. Without it the tank would hold water at whatever pressure the pump left it at and deliver almost none of it before the pressure collapsed.

That is why a tank with more water in it and less air would deliver less rather than more. A waterlogged tank, one that has lost its air charge entirely, is a shell full of water that delivers almost nothing, and the pump starts on every draw.

The practical error this produces is specific. Someone with a 10 gallon per minute pump reasons that a 20 gallon tank gives two minutes of run time. It gives about thirty seconds, which is half the one minute floor and a quarter of the two minute figure the manufacturer recommends above three quarters of a horsepower.

The ratio is not fixed either. It falls as the pressure switch setting rises, which is the next section.

Pressure Switch Settings and Drawdown

The same tank delivers different amounts of water depending on where the pressure switch is set, and the relationship is not obvious until it is written down.

On the field convention a tank delivers about 35 percent of its shell at 20/40, 30 percent at 30/50, 26 percent at 40/60 and 23 percent at 50/70. Higher settings deliver less.

The reason is the pressure ratio. Drawdown depends on how much the trapped air expands between cut-out and cut-in, and at higher absolute pressures the same 20 psi differential is a smaller proportional change. Twenty psi is a large fraction of a 40 psi absolute range and a small fraction of a 70 psi one.

This produces the field pattern the page exists to catch. A household wants better pressure. Someone moves the switch from 30/50 to 40/60. A 44 gallon tank that delivered about 13.0 gallons now delivers about 11.3, a loss of about 13 percent, and a pump that ran for 78 seconds now runs for 68. On a system that was already near the floor, that is the difference between adequate and short cycling, with nothing changed but a screw on the switch.

The honest qualification is that the run time does not always fall by the full amount. Run time is drawdown divided by pump flow, and a pump delivers less at higher pressure, so the required drawdown falls at the same time as the available drawdown. At a 5 percent flow drop the run time penalty is about 8 percent rather than 13. At a 13 percent flow drop it disappears. So the drawdown loss is a fact about the tank and the run time loss depends on the pump curve, which makes it an upper bound rather than a prediction.

The arithmetic runs the other way too, and this is worth knowing when a larger tank is not immediately available. Lowering the switch increases the drawdown: the same 44 gallon tank delivers about 15.2 gallons at 20/40. Dropping a setting is a real intervention on a short cycling system rather than a compromise, provided the lower pressure is acceptable at the fixtures.

Pre-Charge Setting and Short Cycling

The air pre-charge is the only adjustment on a pressure tank, and it decides the drawdown as directly as the switch setting does. It is also the first thing to check when a pump starts short cycling, because a wrong charge produces exactly the symptom of an undersized tank.

The rule is to set the charge 2 psi below the pressure switch cut-in. That is 18 psi on a 20/40 switch, 28 psi on a 30/50 and 38 psi on a 40/60.

The check has a condition that is easy to miss and invalidates the reading if it is skipped. It must be done with the tank empty of water and the system depressurised. A tank holding water reads system pressure at the air valve, which tells you nothing about the charge behind the diaphragm.

What a wrong charge costs is larger than most people expect. On a 44 gallon tank at 40/60 with a 10 gallon per minute pump, the correct 38 psi charge delivers about 11.3 gallons and a 68 second run. At 30 psi it delivers 9.6 gallons and 58 seconds. At 20 psi it delivers 7.5 gallons and 45 seconds, a third of the drawdown gone. At 5 psi it delivers 4.2 gallons and 25 seconds.

None of those tanks is undersized. The one running at 45 seconds is a correctly sized tank behaving like a small one, and the repair is a tyre gauge and a few minutes.

A charge that is too high causes a different failure. If the pre-charge sits above the cut-in pressure, the tank will not begin accepting water until the system reaches the charge, so part of the intended drawdown is unavailable from the other direction. This is a common condition on new installations, because tanks commonly ship with a factory charge of 38 psi. That suits a 40/60 switch and is 10 psi too high for a 30/50 system, so a brand new correctly sized tank can short cycle from the day it is commissioned unless the charge is bled down to 28.

A tank that has lost its charge completely is described as waterlogged. The shell fills with water, the drawdown approaches nothing, and the pump starts on every draw. On a bladder tank that usually means the bladder has failed and the tank is replaced. On a diaphragm tank the diaphragm cannot be replaced either.

Manufacturer Chart Versus Boyle's Law

This calculator computes the acceptance factor rather than reading it from a table, and that choice is worth explaining because it has both an advantage and a limit.

The advantage is that a computed factor works everywhere a table does not. It handles a non standard switch pair such as 35/55, which no published chart covers. It makes the pre-charge assumption visible instead of hidden inside a figure. And it allows the sensitivity comparison across settings to exist at all, because a chart gives one number rather than a relationship.

The limit is that a computation from a nominal tank volume is not the same thing as a manufacturer's measured figure for a specific product, and the gap is larger than rounding.

Two manufacturers make the point between them.

Flexcon prints its convention on every specification sheet: total drawdown assumes the tank pre-charge is set at 2 psi below cut-in pressure. That is the field rule, stated openly.

Amtrol prints no convention. Its Well-X-Trol WX-202 at 20 gallons and WX-203 at 32 gallons publish figures that match a computation at the cut-in pressure closely, which would suggest the other convention.

But the figures cannot settle it, because a second unknown moves the same way. Flexcon's FlexLite FL 5 is a 15 gallon tank publishing 6.0, 5.1 and 4.4 gallons at the three standard settings. Computed on 15 gallons at Flexcon's own stated convention those come to 5.17, 4.43 and 3.87, so the published figures run 14 to 16 percent above the computation at the convention the manufacturer says it used. The Well-X-Trol WX-250, sold as 44 gallons, runs about 10 percent above a computation on 44 gallons at every setting.

A consistent offset across every switch setting points to a shell that holds more than its nominal name, not to a wrong convention. So the difference between a published figure and a computed one mixes two things, the pre-charge convention and the real shell volume, and neither can be extracted from the numbers alone.

The conclusion is the practical one. Compute to understand what is happening: why a higher switch setting costs drawdown, what a wrong pre-charge is doing, how much a longer run time costs in tank size. Read the manufacturer's chart for the exact model to make the selection, because it reflects the real shell and the manufacturer's own stated conditions.

The pressure ratings differ between brands too, which is a smaller version of the same lesson. Well-X-Trol tanks carry a maximum working pressure of 150 psig except the WX-252 at 100. Flexcon tanks are rated 125 psig. A switch setting that is safe on one is not automatically safe on the other.

Minimum Pump Run Time

Everything on this page follows from one number that has nothing to do with plumbing: how long the pump motor should run before it is allowed to stop.

Every start draws heavy current and puts heat into the motor. A motor that starts constantly never gets the running period that carries that heat away, and it wears out. Published guidance puts the cost of short cycling at a pump life falling from around fifteen years to five or less, and an undersized tank is described as the most common cause of premature pump failure in well systems.

The manufacturer position is the most specific available. Amtrol defines Effective Sizing Protection I as a tank selection based on approximately one minute of running time, recommended for pumps up to three quarters of a horsepower. Effective Sizing Protection II is based on approximately two minutes. On that basis a 1 horsepower pump is already in the two minute category.

Trade guidance is generally looser, commonly quoting one minute below 2 horsepower and two minutes at 2 horsepower and above. Several sources recommend two minutes regardless of size. The one minute figure is a floor rather than a target in every version of the rule.

What that costs is straightforward and worth seeing before the choice is made. A 10 gallon per minute pump at 40/60 needs about 39 gallons of tank for a one minute run and about 78 gallons for two minutes. Doubling the run time doubles the shell, which is why the manufacturer chart has two columns and why the decision is worth making deliberately rather than by default.

The run time is also the best way to express a shortfall. A tank that is three gallons short of its requirement is an abstraction. The same tank described as giving the pump forty-five seconds instead of sixty is not.

Captive Air Versus Air Over Water Tanks

Three tank types are in service and they are not interchangeable, particularly for the pre-charge instruction this page produces.

A bladder tank holds the water inside a replaceable rubber bladder within the shell. The air charge sits outside the bladder, sealed. If the bladder fails it can be changed on many models without replacing the vessel.

A diaphragm tank uses a fixed diaphragm bonded into the shell to separate air from water. It works the same way in service and cannot be repaired: a failed diaphragm means a new tank.

Both are captive air tanks. The charge is sealed on the air side, it can be measured at a valve with the tank drained, and the rule of setting it 2 psi below cut-in applies to both.

An air over water tank, often an older galvanized vessel, has no barrier at all. The air sits directly on the water, dissolves into it over time, and leaves with the water. These tanks lose their air steadily rather than through a failure, and they are maintained with an air volume control that admits air automatically, or by draining and recharging them periodically.

Boyle's law still describes the air in an air over water tank at any instant, so the drawdown arithmetic is not meaningless. What does not apply is the pre-charge procedure. There is no sealed charge to set to a target and no valve reading that means what it means on a captive air tank, so this calculator reports the arithmetic for such a tank and withholds a pre-charge figure rather than printing an instruction that does not exist on that vessel.

If you are diagnosing a tank and do not know which type it is, the air valve is the clue. A captive air tank has a valve like a tyre valve, usually on the top, and the tank rings hollow above the water line. An air over water tank generally has an air volume control fitted to the pump or the tank rather than a simple charging valve.

Constant Pressure Systems Are Different

Not every well system is sized this way, and using the drawdown method on the wrong kind of system produces a tank several times larger than it needs to be.

A conventional system runs the pump at one speed between two pressures. The pressure at the tap therefore varies constantly between cut-in and cut-out, and the tank exists to give the motor a decent run between starts.

A constant pressure system, usually built around a variable frequency drive, changes the pump speed instead. It holds a target pressure by matching the pump output to the demand, so the pressure at the tap stays roughly constant and the pump does not stop and start with every draw.

That changes what the tank is for. On a variable speed system the tank is a small buffer that absorbs minor fluctuations and lets the drive settle, rather than a store that has to carry a full pump cycle. Such systems typically use a much smaller tank than a conventional one of the same duty, and some use none at all.

So the drawdown times run time method does not apply. There is no fixed cut-in and cut-out pair to compute an acceptance factor from, and the motor is not being protected from frequent starts in the same way.

This calculator asks the system type as its first question for that reason, and routes a constant pressure system away rather than returning a number that would be wrong by a wide margin. Sizing a variable speed system means following the drive manufacturer's own guidance, which is specific to the equipment.

What Is a Well Pressure Tank

A well pressure tank is a vessel that stores water under pressure between pump cycles. It is what lets a tap run without the pump starting, and it is the reason a well system does not switch on every time someone washes their hands.

Inside a modern tank the water is separated from a charge of compressed air by a rubber bladder or a diaphragm. As the pump fills the tank, the air is compressed and the pressure rises. When it reaches the cut-out setting the pressure switch stops the pump. As water is drawn off, the compressed air expands and pushes it out, the pressure falls, and at the cut-in setting the switch starts the pump again.

The water that comes out between those two events is the drawdown, and it is a fraction of the shell rather than all of it. How large a fraction depends on the pressure ratio between cut-in and cut-out, which is why the same tank delivers less at a higher switch setting.

What the tank is really protecting is the pump motor. Every start draws heavy current and generates heat, and a motor that starts constantly wears out fast. The tank exists so that each start is followed by a run long enough to matter, which is why sizing it begins with a run time rather than with a household.