Water Pipe Size Calculator
Minimum inside diameter for a water flow at a max velocity, the nearest copper L/M, PEX, PVC or steel size, its real velocity and Hazen-Williams psi loss.
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About this toolWhat is the Water Pipe Size Calculator?
Water pipes are sized so the flow stays below a velocity limit and the pressure lost to friction is affordable. Velocity sets the minimum bore: 10 gpm at a cold-water limit of 8 ft/s needs at least 0.715 in inside diameter. The smallest copper type L tube with that bore is 3/4 in (0.785 in ID), where the water actually runs at 6.6 ft/s and loses about 11 psi per 100 ft.
Enter the design flow in gpm or L/s and a maximum velocity, or tap one of the typical limits for cold or hot water. The calculator returns the minimum inside diameter, the nearest standard nominal size in your chosen material with its published inside diameter, the actual velocity, and the friction loss per 100 ft (or 100 m) from Hazen-Williams with a C value you can adjust. A table runs the same flow through copper type L and M, PEX, PVC Schedule 40 and steel Schedule 40 so you can compare them directly.
- Minimum inside diameter from flow and maximum velocity
- Published inside diameters for copper L and M, PEX SDR 9, PVC and steel Schedule 40
- Actual velocity and Hazen-Williams friction loss with editable C
- Total loss over a length in psi, kPa and head
- Side-by-side comparison of every material for the same flow
How to use the Water Pipe Size Calculator
- Choose US units (gpm, ft/s, psi) or metric (L/s, m/s, kPa).
- Enter the design flow, the peak demand the pipe will see, for example from fixture units in the plumbing code.
- Enter a maximum velocity or tap a typical limit for cold water, hot water or quiet branches.
- Pick the pipe or tube material, check the Hazen-Williams C value, and enter a length if you want the total loss.
- Read the minimum ID, the nominal size, the actual velocity and the friction loss, and compare materials in the table.
Worked examples
Cold water main in copper (page default)
Input
10 gpm, 8 ft/s max, copper type L, C 140, 100 ft
Output
Minimum ID 0.715 in, 3/4 in type L (0.785 in), 6.63 ft/s, 11.1 psi per 100 ft
The 3/4 in tube passes the velocity check, but 11 psi lost in 100 ft is a lot for a house on 50 psi mains. Going to 1 in would cut the loss to about 3 psi.
Same flow as hot water
Input
10 gpm, 5 ft/s max, copper type L
Output
Minimum ID 0.904 in, 1 in type L (1.025 in), 3.89 ft/s, 3.0 psi per 100 ft
The lower hot-water velocity limit pushes the size up one step, which also takes care of the pressure loss.
PEX for the same cold flow
Input
10 gpm, 8 ft/s, PEX SDR 9, C 150
Output
1 in PEX (0.875 in ID), 5.34 ft/s, 5.8 psi per 100 ft
3/4 in PEX has a 0.681 in bore, below the 0.715 in minimum, so PEX goes up a size where copper did not.
Background and insight
Nominal sizes are not inside diameters. Copper tube to ASTM B88 has an outside diameter 1/8 in larger than its nominal size, and type K, L and M differ only in wall thickness, so type M has the largest bore. PEX to ASTM F876 uses the same copper tube sizes (CTS) outside but a much thicker SDR 9 wall, so 3/4 in PEX has an inside diameter of only about 0.68 in. Schedule 40 PVC and steel pipe follow the iron pipe size system, where 3/4 in nominal has a 1.050 in OD and a 0.824 in ID.
Velocity limits protect against noise, water hammer and erosion of the pipe wall. The Copper Development Association and the Uniform Plumbing Code limit copper systems to 8 ft/s (2.4 m/s) for cold water and 5 ft/s (1.5 m/s) for hot water, and the CDA recommends 2 to 3 ft/s above 140 °F, where erosion corrosion is faster. Many designers keep residential branches nearer 4 ft/s for quiet.
Friction loss here uses the Hazen-Williams formula, an empirical fit for water at ordinary temperatures in turbulent flow. Its roughness constant C is about 150 for new plastic pipe, 140 for copper, 120 for new steel and 100 or less for old corroded steel. In US units, head loss per 100 ft = 0.2083 (100/C)^1.852 x gpm^1.852 / d^4.8655. Because loss scales with almost the fifth power of diameter, going up one pipe size typically cuts friction by more than half.
Common use cases
Residential water service and mains
Check that a service line or main carries the peak demand without exceeding the velocity limit or losing too much pressure.
Copper to PEX substitutions
See which PEX size matches the bore of an existing copper line before a repipe.
Hydronic and process loops
Pick a pipe size for a circulator flow at a target velocity, then read the head loss for pump selection.
Irrigation and outbuildings
Size a PVC run to a garden, barn or workshop and estimate the pressure left at the far end.
Tips and limitations
- Design flow is peak demand, not the sum of every fixture. Use the fixture-unit method in your plumbing code (IPC or UPC) to get it.
- Add equivalent length for elbows, tees and valves. A rough allowance is 25 to 50% of the straight length for typical domestic runs.
- Elevation costs 0.433 psi per foot of rise, on top of friction. Check the pressure left at the highest fixture.
- PEX insert fittings reduce the bore at every joint, so PEX systems are often upsized or laid out as home runs from a manifold.
- Hazen-Williams is less accurate for hot water and small or very smooth pipes. For critical work use Darcy-Weisbach with the real viscosity.
Frequently asked questions
How do I calculate water pipe size from flow?
Minimum inside diameter d = √(4Q / πV). In US units, d (in) = √(0.4085 x gpm / V ft/s). Then choose the smallest standard pipe whose inside diameter is at least d.
What size pipe do I need for 10 GPM?
At an 8 ft/s cold-water limit, 3/4 in copper type L works (6.6 ft/s). At 5 ft/s for hot water, or if pressure loss matters, use 1 in. PEX needs 1 in either way because of its thicker wall.
What is the maximum water velocity in copper pipe?
The Copper Development Association and UPC give 8 ft/s (2.4 m/s) for cold water and 5 ft/s (1.5 m/s) for hot water up to 140 °F, with 2 to 3 ft/s recommended above 140 °F.
What is the Hazen-Williams C value for PEX and copper?
Typical values are 150 for PEX, PVC and other smooth plastics and 140 for copper tube. New steel is about 120 and older corroded steel 100 or lower.
What is the inside diameter of 3/4 inch PEX?
About 0.681 in for SDR 9 PEX to ASTM F876: 0.875 in outside diameter minus twice the 0.097 in minimum wall. 3/4 in type L copper is 0.785 in.
How much pressure is lost in 100 feet of 3/4 inch copper?
At 10 gpm, about 11 psi in type L with C 140. At 5 gpm it is about 3 psi, since loss rises with roughly the 1.85 power of flow.
Is there an in-depth guide to the Water Pipe Size Calculator?
Yes. “Water Pipe Sizing by Velocity and Hazen-Williams Friction Loss” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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