Pipe Size Calculator

1. Enter total WSFU. Add up the Water Supply Fixture Units on the pipe section you are sizing. Use our Fixture Unit Calculator if you need help with the count.

2. Select pipe material. Each material has a different Hazen-Williams C factor that affects friction loss. PEX and PVC (C=150) have less friction than copper (C=130).

3. Enter available pressure. This is the supply pressure at the start of the pipe run, typically the street pressure or pressure after a PRV.

4. Enter pipe length and fittings. Measure the actual developed length of the pipe run. Each fitting adds equivalent length — the calculator adds approximately 2.5 ft per fitting.

5. Choose line type. Hot water lines have a lower maximum velocity (5 fps vs 8 fps) to prevent erosion and water hammer in heated pipes.

This calculator converts fixture units to GPM using Hunter's Curve, then tests each pipe diameter from 1/2" up until it finds one that satisfies both velocity and friction loss constraints.

The Hazen-Williams equation relates flow, pipe roughness, and diameter to friction loss per 100 feet. Smaller pipes create more friction and higher velocity, so the calculator steps up sizes until both limits are satisfied.

New construction rough-in. Size every supply branch and main during the design phase. Getting pipe size right at rough-in avoids costly re-work after walls are closed.

Addition or remodel. Adding a bathroom or kitchen? Calculate whether the existing supply pipe can handle the additional fixture units or if it needs to be upsized.

Low-pressure troubleshooting. If a customer complains about low pressure at distant fixtures, this calculator can verify if the pipe size is adequate for the fixture count and run length.