Water Flow Rate Calculator (GPM)

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

  1. Pick a mode Choose Find pipe size, Find available flow, or Check fixture delivery. The mode determines which inputs appear below.
  2. Set material and length Select the Pipe material (each carries its Hazen-Williams C-factor and a max-velocity ceiling) and enter Pipe length in feet.
  3. Enter the mode-specific input For pipe sizing enter Target flow in GPM; for the other two modes enter Available pressure drop in PSI and select a Pipe diameter, plus a Fixture type for fixture-delivery mode.
  4. Refine and calculate Optionally check Hot water line to cap velocity at 5 ft/s and add fitting counts (converted to equivalent length per Crane TP-410), then press Calculate.

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.

What pressure drop should I assume per 100 feet for house water lines?

There is no single fixed pressure-drop value per 100 ft that works for every job, because loss changes with flow, diameter, material roughness, and fittings. Use Hazen-Williams with your actual run and equivalent fitting length, then confirm residual pressure at the fixture. As a field rule, keep losses moderate enough to preserve usable fixture pressure under simultaneous demand.

How do fittings change my water flow calculation?

Fittings add equivalent length, which raises friction loss and can reduce available flow for the same pressure budget. This calculator converts fitting counts to equivalent straight-pipe length and adds that to entered run length before solving Hazen-Williams. Ignoring fittings often overstates delivered GPM on real installations.

What velocity is too high for copper domestic water?

A common field ceiling is about 8 ft/s for cold copper and around 5 ft/s for hot copper lines, where erosion-corrosion risk rises. Sustained over-velocity also increases noise and water-hammer sensitivity. Use the hot-line option when appropriate and verify final velocity against your design and jurisdiction guidance.

Why does fixture flow show lower than pipe capacity?

In fixture-delivery mode, delivered flow is capped by the fixture peak demand, so it can be lower than the pipe's hydraulic capacity. That means the pipe could carry more, but the fixture itself will not draw beyond its demand profile. This prevents treating raw pipe capacity as guaranteed fixture discharge.

What is equivalent length in pipe sizing?

Equivalent length is the straight-pipe distance that causes the same friction loss as a fitting, valve, or transition. Adding these equivalent lengths to the measured run gives a more realistic total for pressure-drop and flow calculations. Without it, long fitting-heavy runs are routinely undersized.

Calculators