Flow Rate Calculator

1. Select pipe diameter. Choose the nominal pipe size you are evaluating. The calculator uses actual inside diameter for the chosen size.

2. Choose pipe material. Each material has a different Hazen-Williams C factor that affects friction. Smoother pipes (higher C) deliver more flow at the same pressure.

3. Enter available pressure. Use the PSI measured at the start of the pipe run. For city water, this is typically 40-80 PSI. For well systems, use the pump's operating pressure.

4. Enter pipe length. Measure the developed (actual) length of the pipe run, including all horizontal and vertical sections.

5. Enter elevation change. If the outlet is higher than the inlet, enter a positive number. Each foot of rise costs 0.433 PSI. Downhill runs (negative) gain pressure.

This calculator uses the Hazen-Williams equation, which is the standard method for pressurized water systems in plumbing design.

The equation balances friction losses against available pressure to determine the maximum steady-state flow. Higher pressure, larger diameter, and smoother pipes all increase the flow rate.

Verifying fixture performance. Will a 3/4" line deliver enough GPM to run a rain showerhead at the second floor? Enter the pipe details and check against the fixture's rated flow.

Irrigation system design. Calculate how many GPM your supply line can deliver to determine how many sprinkler zones you can run simultaneously.

Troubleshooting low flow. If a customer reports poor flow at a specific fixture, calculate the theoretical maximum for that pipe run. If the calculated flow is adequate, the problem is elsewhere (valve, fixture, blockage).