CPVC Pipe Sizing Calculator — CTS SDR 11 Bore and Temperature Derating

CPVC Pressure Rating and Temperature Derating

This is the single most important table on the page, and it is one table read two ways. The factor column tells you how much of the rating survives; the rating column tells you what is left.

For CTS SDR 11 CPVC the published points are:

At 73 F, factor 1.00, rating 400 psi, which is 2,758 kPa.

At 80 F, factor 1.00, rating 400 psi.

At 90 F, factor 0.91, rating 360 psi, which is 2,482 kPa.

At 100 F, factor 0.82, rating 325 psi, which is 2,241 kPa.

At 120 F, factor 0.65, rating 260 psi, which is 1,793 kPa.

At 140 F, factor 0.50, rating 200 psi, which is 1,379 kPa.

At 160 F, factor 0.40, rating 160 psi, which is 1,103 kPa.

At 180 F, factor 0.25, rating 100 psi, which is 690 kPa.

In Celsius those temperatures are 22.8, 26.7, 32.2, 37.8, 48.9, 60.0, 71.1 and 82.2.

Four things about this table decide how it should be used.

The rating holds flat from 73 F to 80 F. The first reduction appears at 90 F, so a cold water line in a warm plenum loses nothing.

The rating is not always the factor multiplied by 400. At 90 F, 400 times 0.91 is 364 and the table publishes 360. At 100 F, 400 times 0.82 is 328 and the table publishes 325. Two rows are rounded, so the published rating is the figure to use rather than a recomputed product.

The curve between the points is not a straight line, which matters more than it sounds. Draw a line from 1.00 at 73 F to 0.25 at 180 F and it gives 0.530 at 140 F where the published factor is 0.50. That overstates the rating by about 6 percent, in the unsafe direction, in the middle of the domestic hot water range. At 90 F the same line gives 0.881 against a published 0.91, so it understates there. A linear fit is wrong in both directions and is not conservative in either.

And the derating applies to a rating that already carries its design factor. The published figure includes a design factor of 0.5, and the temperature derating multiplies that published figure. Nothing further is added by the user, and a second margin applied on top will oversize the wall for no benefit.

One consequence for anything not on the list. If the operating temperature is 135 F, it is not a published point. The nearest published points are 120 F at 0.65 and 140 F at 0.50. This calculator names those points and withholds the pressure verdict rather than interpolating between them, because the arithmetic above shows that interpolation on this curve is not safe. The manufacturer's table for the specific product is the place to get an intermediate figure.

CTS Versus IPS CPVC Pipe

A nominal size is a complete specification in copper and is not one in CPVC, because the same number describes more than one pipe.

CTS is copper tube size, to ASTM D2846, in SDR 11. It shares the outside diameter of copper tube of the same nominal size and runs from 1/2 inch to 2 inch. This is the tube used for most residential and light commercial hot and cold water distribution, and it is what most people mean when they say CPVC pipe in a plumbing context.

IPS is iron pipe size, to ASTM F441, in Schedule 40 or Schedule 80. It uses a different outside diameter and a different wall, so a nominally identical pipe has a different bore and a different pressure rating. IPS covers the larger sizes and industrial work, and ASTM F442 covers SDR pipe in the same family.

The practical consequence is that a drawing calling for 1 inch CPVC has not specified a pipe. The outside diameter differs, the wall differs, the bore differs, the fittings differ, and the pressure rating differs. Two contractors reading the same note can order pipe that will not join.

It also means a rating cannot travel between families. The 400 psi at 73 F figure quoted throughout this page belongs to CTS SDR 11. Applying it to an IPS Schedule 40 pipe because the nominal size matches is the same error as reading a copper rating off a plastic datasheet.

This version of the calculator computes CTS SDR 11 only. IPS Schedule 40 and Schedule 80 remain in the family list, and selecting one returns an explanation rather than a number. That is deliberate: removing the option would hide the distinction, and the distinction is the point. A user who did not know the families differ learns it from the state rather than from an absence.

CPVC Bore Versus Nominal Size

The nominal size is a trade designation. The bore is a dimension, and only the second one appears in a calculation.

The bores used here are read from manufacturer dimensional tables published to ASTM D2846, and they were cross-checked between two of them before being used. Working through the CTS sizes: a 1/2 inch tube has an outside diameter of 0.625 inch and a published average bore of 0.469 inch, or 11.9 mm. A 3/4 inch tube is 0.875 inch outside and 0.695 inch bore, or 17.7 mm. A 1 inch tube is 1.125 inch outside and 0.901 inch bore. At 1-1/4 inch it is 1.375 and 1.105. At 1-1/2 inch, 1.625 and 1.309. At 2 inch, 2.125 and 1.716.

Notice that at every size the bore is smaller than the nominal figure, and at some sizes it is smaller than the next size down's nominal. A 1-1/4 inch CTS tube has a bore of 1.105 inch, narrower than the outside diameter of a 1 inch tube.

That gap does real work in two places. Velocity varies with the inverse square of the bore, so using the nominal figure understates it. At 8 gallons per minute through a 3/4 inch tube the published bore gives about 6.77 feet per second and the nominal figure gives about 5.81, an understatement of 14 percent. Friction varies with the bore to the power 4.8655, where the same error is larger again.

Why the published table and not the SDR ratio. SDR 11 defines the wall as one eleventh of the outside diameter, so it is tempting to take the bore as nine elevenths of it and stop there. That works from 3/4 inch up, where the published minimum wall lands within half a percent of the ratio. It fails at 1/2 inch, where the published minimum wall is 0.068 inch against an OD/11 of 0.0568 inch, twenty percent thicker, because the standard sets a wall floor at the small end rather than following the ratio down. The ratio returns 0.511 inch at 1/2 inch where the published average is 0.469 inch, which understates velocity by 19 percent and friction by 52 percent on the most common size in the range, and it understates in the unsafe direction.

There are two published bores rather than one, and the distinction matters. The minimum wall is a manufacturing floor, so the bore it implies is the widest the pipe is allowed to be: 0.489 inch at 1/2 inch, 0.715 at 3/4. The published average bore sits a constant 0.020 inch below that across the range, which is the average wall running thicker than the minimum. This calculator uses the average, because it is what a length of pipe actually runs at and it is the column the manufacturer's own friction and velocity tables are built on. The result reports both, so the width of the band is visible rather than hidden inside a single figure.

CPVC Velocity and Friction

Once the bore is known the hydraulics are ordinary arithmetic, with one deliberate omission.

Velocity is 0.4085 times the flow in gallons per minute divided by the bore in inches squared. At 8 gallons per minute through a 3/4 inch CTS tube with a published bore of 0.695 inch, that is about 6.77 feet per second, or 2.06 metres per second.

Friction uses Hazen-Williams with a roughness coefficient of 150 for CPVC, against about 130 to 140 for copper. CPVC is smoother, so it loses less head over the same length at the same bore.

The omission is a velocity limit. This page does not apply one, because no universal velocity limit for CPVC was verified. Manufacturer data may set a limit for a specific product and a project specification may set its own, and either is a legitimate criterion. What would not be legitimate is a limit invented here and presented as a property of the material, so the calculator reports the velocity and issues a verdict only against a limit you supply, naming it as your criterion.

That brings up the substitution question, which is where the two material properties collide.

Swapping copper for CPVC at the same nominal size changes the hydraulics in two opposing directions. The CPVC bore is narrower, because the CTS outside diameter is shared with copper while the CPVC wall is thicker, so the same flow runs faster. And the CPVC surface is smoother, so the friction coefficient is better. One effect pushes velocity up and the other pushes head loss down.

Which one dominates is an arithmetic question rather than a matter of judgement, and the answer needs both bores as published figures rather than as estimates. It has them now.

Both bore sets are now published figures, so the calculator computes the comparison rather than describing it. Select copper Type L and it returns the two bores, the difference in flow area, the difference in velocity at your flow, and the difference in friction, at the nominal size you chose.

The result overturns the assumption the smoothness argument usually leads to. At 3/4 inch the CPVC bore of 0.695 inch against copper Type L at 0.785 inch is 22 percent less flow area, so the same flow runs 28 percent faster. On friction, the better roughness coefficient of 150 against about 130 to 140 pulls head loss down by 12 to 23 percent on its own, but the narrower bore multiplies it by 1.8, because the bore carries an exponent of 4.8655 against the roughness term's 1.852. The two do not cancel and they are not close: CPVC loses roughly 39 to 59 percent more head than copper Type L at the same nominal size and the same flow. That pattern holds at every size in the CTS range, widening to 59 to 83 percent at 1/2 inch.

So the substitution moves both hydraulic figures the wrong way at once. A branch sitting comfortably inside a velocity limit in copper can cross it in CPVC at the same nominal size, and the pressure drop over the run rises at the same time rather than offsetting it. None of that makes CPVC the wrong material, which turns on rating, corrosion, cost and temperature rather than on head loss. It makes the diameter something that has to be recomputed rather than carried across.

CPVC Thermal Expansion

This is the largest single difference between CPVC and metal pipe, and it is measured in inches rather than in tolerances.

The coefficient of linear expansion for CPVC is about 3.4 times ten to the minus five inches per inch per degree Fahrenheit. Copper is about 9.2 times ten to the minus six. CPVC therefore moves roughly four times as far for the same run and the same temperature change. Manufacturer literature also publishes 3.2 times ten to the minus five for some compounds, a difference of about six percent on the result, so the figure used should be named and the manufacturer value used for final design.

Manufacturer guidance states it in a form worth remembering: a 25 F change produces about 1 inch of movement in a 100 foot straight run.

The movement is 12 times the coefficient, times the run in feet, times the temperature change in degrees Fahrenheit. A 60 foot run installed at 70 F and operating at 140 F moves about 1.71 inches, or 43.5 mm. The same run in copper moves about 0.46 inches, or 11.8 mm.

Two things about that calculation are easy to get wrong.

The length is the straight run between anchors or changes of direction, not the hydraulic developed length used for friction. A pipe with three offsets in it has a developed length that includes all of them and a set of shorter straight runs that each move independently. Using the developed length overstates the movement of any one segment.

And the temperature change is the operating temperature minus the installed temperature, which carries a sign. A cold water line installed at 70 F in a warm building and running at 50 F has a change of minus 20 F and contracts by about 0.49 inches over 60 feet. Contraction pulls on joints where expansion pushes on them, and a design that only considered growth has provided for the wrong direction.

The movement has to be absorbed by an expansion loop, an offset or a change of direction. Supports should restrict lateral movement and direct the axial movement into the loop, and hangers and guides belong in the loop or at the change of direction rather than clamped across the middle of a run that has to move. A hanger there either restrains the pipe, which loads the joints, or lets it buckle.

This page reports the movement and states that it must be absorbed. It does not dimension the loop, because loop and guide geometry depend on the modulus at temperature and belong to the manufacturer's engineering data. The site does carry a pipe expansion loop sizing calculator, which solves the guided cantilever leg length for a given movement and allowable stress range. Confirm before relying on it that it covers CPVC at your operating temperature rather than metal pipe alone: its material list is carbon steel, stainless steel and copper, and a leg length derived from a metal modulus does not transfer to a thermoplastic that loses stiffness as it warms.

CPVC Support Spacing at Temperature

Support spacing for CPVC is not a single number, and the reason is the same property that drives the pressure derating.

Thermoplastics soften as they warm. A CPVC line at 180 F is less stiff than the same line at 73 F, so it sags further between the same supports. Maximum support spacing therefore tightens as the operating temperature rises, and it also varies with pipe size, because a larger pipe carries more weight of water per foot while gaining stiffness at a different rate.

The practical consequence is one of the more common field errors on hot water work. A hanger schedule taken from a cold water table, or carried across from copper practice, will leave a hot CPVC line under-supported. The pipe does not fail; it sags between hangers, which puts a bend into every joint and pools water at the low points.

Spacing also interacts with the expansion question. Supports on a run that has to move are not there only to carry weight: they restrict lateral movement and direct the axial movement into the loop or offset. A support added simply to close a sag can defeat the expansion arrangement if it clamps rather than guides.

This calculator does not print a spacing figure. The published table by size and temperature has not been obtained, and printing a number from a generic hanger schedule or from memory would be worse than printing nothing, because a spacing figure looks authoritative and a wrong one produces exactly the sag it was meant to prevent. The page states the requirement and the direction of the relationship and points at the manufacturer table, and the figures will be added when that table is in hand. The site does carry a pipe support spacing calculator, which solves the maximum span from bending stress and deflection. Confirm before relying on it that it covers CPVC at your operating temperature rather than metal pipe alone: its material list is carbon steel, stainless steel and copper, and a span computed from a metal modulus will be too long for CPVC, and longer still as the line runs hot.

Where CPVC Cannot Be Used

Some of the answers on this page end the calculation rather than adjusting it, and they are worth knowing before a material is specified.

Compressed air and gas. CPVC is not permitted for the distribution of compressed air or gas, and manufacturer literature allows oil free air handling only to a low pressure limit. The reason is the failure mode rather than the rating. A water line that fails leaks, because water is nearly incompressible and there is no stored energy to release. A thermoplastic line carrying a compressed gas stores energy along its entire length, and when it fails it fails by shattering. This is a manufacturer restriction with a physical reason behind it rather than a code citation, and it is the one answer on this page that stops the calculation on safety grounds.

Fire sprinkler service. CPVC sprinkler pipe exists and is a separately listed system with its own approvals, its own sizing rules and its own fittings and cements. It is not the same product as potable water distribution CPVC and a rating or bore from this page does not transfer to it.

Drain, waste and vent. CPVC DWV is a non pressure product under different standards. Nothing on this page applies to it.

Industrial and other non potable liquids. Chemical compatibility governs before any pressure, temperature, bore or friction figure is worth computing. CPVC is incompatible with petroleum based products, aromatic hydrocarbons and chlorinated solvents, and the fluid has to be checked against the manufacturer's chemical resistance data before the pipe is considered at all.

That incompatibility list reaches beyond the fluid inside the pipe, which is the part most often missed. Thread sealants and pipe dope, fire stopping materials, some insulation, and cutting oils left on a surface can all attack CPVC from the outside. A system correctly specified for its contents can still fail because of what was put on it during installation.

CPVC Calculator Versus Water Pipe Sizing

These are two different calculations and running them in the wrong order wastes the first one.

Water pipe sizing answers what diameter carries the flow. It starts from fixture units, converts them to a design flow, and works down to a diameter under velocity and friction criteria. It is material aware only to the extent of a roughness coefficient, and it applies to copper, CPVC, PEX or anything else.

This page answers whether CPVC can be that diameter, and what changes when it is. It takes the size as given and returns the family, the wall, the published bore, the pressure rating at the operating temperature, the velocity the real bore produces, and the movement the run will make.

Neither substitutes for the other. Running this page without a diameter leaves you nothing to qualify. Running the sizing page and stopping there leaves a nominal size that does not yet describe a CPVC pipe, a rating that has not been derated for the water temperature, and a run whose movement has not been considered.

The order is therefore: size it there, qualify it here. In practice the second step sometimes sends you back to the first, because a diameter that worked in copper can produce a velocity in CPVC that the project will not accept, and that is a sizing decision rather than a material one.

This page will not choose a diameter under any circumstances. It does not take fixture units, it has no demand builder, and it does not convert a flow into a size. That boundary is deliberate, because two calculators answering the same question with slightly different assumptions is worse than one answering it well.

What is CPVC Pipe Sizing

CPVC is chlorinated polyvinyl chloride, a thermoplastic used for hot and cold potable water distribution where copper is expensive or where the water chemistry attacks it. It resists chlorine, chloramines and pH variation, and it does not suffer the copper ion catalysed oxidation that affects some other plastics.

Sizing it is not the same exercise as sizing copper, because a CPVC pipe carries more variables than a diameter.

It comes in two families, CTS in SDR 11 and IPS in Schedule 40 or 80, and the same nominal figure on a drawing can mean either.

Its strength depends on temperature in a way metal's does not. Thermoplastics lose strength as they warm, so a CPVC pressure rating is meaningful only alongside a temperature, and the published figure is at 73 F.

And it moves, at around four times the rate of copper, so a straight run in hot water service has to be designed with somewhere for the movement to go.

Sizing CPVC therefore means qualifying a diameter rather than choosing one: confirming the family and wall, working out the real bore, derating the pressure rating for the operating temperature, checking the velocity the real bore produces, and providing for the movement.