Water pipe size calculator

How the design flow and the tube size are calculated

This page turns the fixture units on a water pipe into a design flow and picks the copper tube that carries it. Flow comes from the Hunter curve as printed in Table E103.3(3) of IPC Appendix E, in the column for flush tank systems or for flushometer valve systems, with any continuous load added in GPM. Tube size is the smallest Type L or Type M copper, from 1/2 inch up, whose velocity stays at or under 8 ft/s for cold water or 5 ft/s for hot water, the limits of the Copper Tube Handbook. Friction is the handbook's Hazen-Williams figure with C = 150. Fixture branches, the pressure budget of Appendix E and loads above 1,000 WSFU are outside this page.

Hunter demand

demand GPM = Table E103.3(3) value for the load in WSFUIPC prints the demand in GPM against the load in WSFU, with one column for systems predominantly of flush tanks and one for flushometer valves. 10 WSFU is 14.6 GPM on flush tanks and 27.0 GPM on flushometer valves, and 20 WSFU is 19.6 and 35.0. The curve rises more slowly than the load, so doubling the fixtures from 10 to 20 WSFU adds 34.2 percent to the flow and not 100.

Between printed loads

GPM = lower + (load - lower load) / (upper load - lower load) x (upper - lower)The table prints the loads in steps. A load of 8.6 WSFU lies between 8 WSFU at 12.8 GPM and 9 WSFU at 13.7 GPM, and the page reads a straight line, which gives 13.3 GPM. That reading is the page's own and not a rule of the code.

Continuous load

design flow = Hunter demand + continuous GPMAppendix E adds continuous loads such as lawn sprinklers and air conditioners to the table demand in GPM. 50 WSFU on flush tanks is 29.1 GPM, and 5 GPM of sprinkler flow on top of it makes 34.1 GPM.

Velocity limit

limit = 8 ft/s cold, 5 ft/s hot up to 140°FThe Copper Tube Handbook says not to exceed 8 feet per second for cold water and 5 feet per second in hot water up to about 140°F, to avoid noise and erosion-corrosion. It asks for 2 to 3 feet per second where the water routinely runs hotter, and for lower velocities in 1/2 inch and smaller tube. Page 27 gives 5 to 8 feet per second for plumbing systems, the lower limit applying to smaller sizes.

Velocity

v ft/s = 0.408 x GPM / d^2The inside diameter d is in inches and the flow in GPM. Caleffi idronics 16 prints the constant 1.26 for 1/2 inch Type M and 0.62 for 3/4 inch Type M, which this formula reproduces. At 14.6 GPM, 3/4 inch Type L runs at 9.67 ft/s, over the 8 ft/s limit, and 1 inch Type L runs at 5.67 ft/s.

Friction

psi per ft = 4.52 x GPM^1.85 / (150^1.85 x d^4.87)This is the Hazen-Williams formula of Table 14.6 in the Copper Tube Handbook, with C = 150 for copper. Caleffi idronics 31 works 3/4 inch Type L at 6 GPM to 0.0381 psi per foot and 5.72 psi over 150 feet, and this page gives the same figures. 1 inch Type L at 14.6 GPM loses 5.39 psi per 100 feet.

Smallest tube

tube = first size from 1/2 inch whose velocity is at or under the limitNothing is rounded before the comparison, so a flow exactly at the limit stays in that size. The page shows the neighbors on each side with their velocity, friction and the flow each carries at the limit. Type M has a slightly larger bore than Type L and never needs a larger size.

Worked examples and the published figures

The default case is 10 WSFU on flush tanks. Table E103.3(3) prints 14.6 GPM for it. In Type L copper, 3/4 inch would run at 9.67 ft/s, over the 8 ft/s limit for cold water, and 1 inch runs at 5.67 ft/s and loses 5.39 psi per 100 ft. On hot water, with the 5 ft/s limit, the same flow takes 1-1/4 inch.

A private house with two bathroom groups on flush tanks (3.6 WSFU each in total) and a kitchen sink (1.4 WSFU) is 8.6 WSFU. That load falls between two printed rows, 8 WSFU at 12.8 GPM and 9 WSFU at 13.7 GPM, so the page reads 13.3 GPM and picks 1 inch.

The table below sets each figure that the sources print beside the value this page computes. The Hunter points are the code's own, and the friction and velocity figures are Caleffi's worked values for the same Hazen-Williams formula.

Eight cases below run through the same functions the form uses. The 20 WSFU pair shows the two columns of the table: 19.6 GPM on flush tanks and 35.0 GPM on flushometer valves, which moves the pipe from 1 inch to 1-1/2 inch. The 1 WSFU case is the smallest load in the table and the only one that lands in 1/2 inch tube.

Fixture loads from IPC Table E103.3(2), in water supply fixture units. Add the total column for a pipe that feeds cold and hot fixtures, the cold column for the cold pipe after the water heater and the hot column for the hot pipe. The first eight rows are printed by two reproductions of the code and the last four by one study guide.

Standards and data sources

• International Code Council, International Plumbing Code, Appendix E, Tables E103.3(2) and E103.3(3), as reproduced by up.codes (2015 IPC, Connecticut State Building Code) - Table E103.3(3) turns the fixture load in WSFU into GPM for systems predominantly of flush tanks or flushometer valves, and Table E103.3(2) gives the load of each fixture. ICC's own site refused automated access while this page was written, so the figures were read from reproductions of the code and checked against each other. Reproduced here are 10 WSFU at 14.6 and 27.0 GPM, 100 WSFU at 43.5 and 67.5, and 1,000 WSFU at 208.0 on both.
• Open Exam Prep, ICC plumbing plans examiner study guide, Water Supply Fixture Units (WSFU) and Demand Estimation - A listing of Table E103.3(3) from 10 to 1,000 WSFU in both columns and of the fixture loads of Table E103.3(2), including the private bathroom group on a flush tank at 2.7 cold, 1.5 hot and 3.6 total WSFU. It is a study guide and not the code, so the page checks it against the other reproductions.
• pdfcoffee.com, IPC E103.3 excerpt, Table for estimating demand - The full row list used here, 1 to 1,000 WSFU, including the flush tank entries of 3.0, 5.0, 6.5 and 8.0 GPM for 1 to 4 WSFU, where no flushometer entry is printed. Its text extraction is garbled in places, so every row it shares with another reproduction was matched against it, and rows above 1,000 WSFU are not used.
• Engineering ToolBox, Water Supply Fixture Units, WSFU vs GPM - Tables without and with flush valves. Its values for 1, 2, 4, 8, 15, 30 and 50 WSFU without flush valves and 5, 7, 10, 15, 20, 30 and 50 WSFU with them agree with the IPC figures at every load they share, such as 30 WSFU at 23.3 and 42.0 GPM.
• Copper Development Association, Copper Tube Handbook (A4015-26/24) - Water velocity limits of 8 ft/s for cold water and 5 ft/s for hot water up to about 140°F, the dimensions of Type L and Type M tube from Tables 14.2b and 14.2c, and the Hazen-Williams friction formula of Table 14.6 with C = 150.
• Caleffi, idronics issues 16 and 31 - The velocity formula v = 0.408 x GPM / d^2 with its constants of 1.26 and 0.62 for Type M, and the worked friction example of 3/4 inch Type L at 6 GPM, 0.0381 psi per foot. Both are reproduced in the check table on this page.

Limitations of this calculator

• Systems are read as predominantly flush tanks or predominantly flushometer valves, the two columns of Table E103.3(3). The page does not blend the columns for a building with both. Flushometer valves have no printed demand below 5 WSFU, and the page refuses that case and does not borrow the flush tank column.
• Continuous loads such as sprinklers and process water are added as plain GPM after the conversion and get no diversity. They are entered as a flow because the table gives them no fixture units. Fixtures the table does not list, such as a hose bibb, are given a value by comparison with a similar fixture, which this page does not do. A load that runs only part of the day still counts in full.
• Loads run from 1 to 1,000 WSFU, and the table continues above that. This page does not read rows beyond 1,000 WSFU. Between printed loads the demand is a straight-line reading that the page chose.
• Only copper Type L and Type M are offered. PEX, CPVC and steel pipe are not covered, because the open sources read for this page do not agree on inside diameters for PEX and none gave a clean size table for the others. Take those sizes from the pipe maker's tables.
• Friction is the Hazen-Williams figure for cold water and straight tube. Fittings and valves, the water meter, backflow preventers, elevation head and the pressure available at the main are not included, so the loss shown is only one part of a pressure budget.
• The velocity limits are the Copper Tube Handbook figures of 8 ft/s for cold water and 5 ft/s for hot water up to about 140°F. Hotter water and circulating hot water need 2 to 3 ft/s, which the page does not size for. Recirculation loops are not covered.
• The minimum size the code gives for a fixture branch or a service, and local amendments to it, are not applied. UPC Appendix A has its own fixture unit table, and this page reads only the IPC figures. Check which code your jurisdiction has adopted.
• This is a planning estimate, not a design drawing. A licensed plumber or engineer, the pipe manufacturer's data and the authority having jurisdiction decide the pipe sizes and the installation.

Common water pipe sizing mistakes

1. Adding up the fixture flows. Hunter's curve gives a probable peak demand for the group, not the sum of the fixtures. Doubling the load from 10 to 20 WSFU adds 34.2 percent to the demand, from 14.6 to 19.6 GPM.
2. Using the wrong column. The same load needs very different flow on the two kinds of system. 20 WSFU is 19.6 GPM on flush tanks and 35.0 GPM on flushometer valves, which moves the pipe from 1 inch to 1-1/2 inch Type L. Pick the column for the fixtures that dominate the building.
3. Using the total WSFU on the hot pipe. Table E103.3(2) gives a cold, a hot and a total load for each fixture. A private lavatory is 0.5 WSFU on the cold pipe, 0.5 on the hot pipe and 0.7 in total. The hot pipe takes the hot column, and the cold pipe after the water heater takes the cold column.
4. Leaving out the continuous load. Irrigation runs for long periods, and Appendix E adds continuous loads of that kind to the table demand in GPM. 50 WSFU on flush tanks is 29.1 GPM, and with 5 GPM of sprinkler flow the design flow is 34.1 GPM and the pipe is 1-1/2 inch Type L.
5. Sizing by velocity alone. A tube within the velocity limit can still lose too much pressure over a long run. 5.39 psi per 100 feet in 1 inch Type L at 14.6 GPM is 10.77 psi over 200 feet. Check the loss against the pressure available at the main, less the elevation and the meter.
6. Running small tube near the top of the range. Smaller tube takes the lower end of the handbook's 5 to 8 ft/s range, and the handbook asks for lower velocities in 1/2 inch and smaller tube. 1/2 inch Type L reaches 8 ft/s at 5.8 GPM, and 1 WSFU on its own is already 3.0 GPM.

Common questions about water pipe size

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