Water flow Calc

Three modes from a shared Hazen-Williams core. Size a pipe for a known flow, find available flow through a known pipe, or check delivered flow at a fixture.

01Water flow

Water flow calculator.

Three modes from a shared Hazen-Williams core. Size a pipe for a known flow, find available flow through a known pipe, or check delivered flow at a fixture.

01Mode

02Parameters

Pipe material

[01]

Pipe length

[02]

Available pressure drop

[03]

Pipe diameter

[04]

Fixture type

[05]

Hot water line

[06]

Treat this run as a hot supply line — applies the 5 ft/s erosion-corrosion limit instead of the material's general ceiling.

Fittings (optional)no fittings counted

Count each fitting in the run. The calculator converts these to an equivalent straight-pipe length and adds them to the friction calculation. L/D values per Crane TP-410.

90° elbow (standard) L/D = 3090° elbow (long-radius) L/D = 1645° elbow L/D = 16Tee — through run L/D = 20Tee — branch flow L/D = 60Gate valve (fully open) L/D = 8Ball valve (fully open) L/D = 3Globe valve (fully open) L/D = 340Swing check valve L/D = 100

Verify against your local code

This calculator applies general code rules that vary by jurisdiction and code edition. Confirm the code edition adopted in your area and verify with the authority having jurisdiction (AHJ) before installation. Use at your own risk. Read full terms.

Frequently Asked Questions

What size water pipe do I need for a given GPM?

In Find pipe size mode, enter your target flow and the tool sizes the diameter for no more than 75% of the material velocity ceiling, then rounds up to the next standard nominal size. Friction loss at that size is computed with the Hazen-Williams equation using the material C-factor, including any equivalent length you add for fittings. It also warns if velocity still exceeds the ceiling.

How much water flow can a pipe carry?

Find available flow mode solves the Hazen-Williams equation in reverse, computing the maximum GPM a known diameter and material can carry within the available pressure drop you enter over the run length. It reports the resulting velocity and friction loss so you can confirm the pipe stays within its velocity ceiling. Fittings are included as equivalent straight-pipe length per Crane TP-410.

Why does the hot water line option lower the velocity limit?

Hot copper supply lines are limited to about 5 ft/s to prevent erosion-corrosion (pinholing) of the tube wall, per ASTM B88 and ASPE guidance. Checking Hot water line overrides the material general ceiling with this 5 ft/s cap, which sustained over-velocity in hot copper can otherwise breach in 5–15 years.