The GPM Flow Rate Calculator converts between water flow rate (in gallons per minute) and flow velocity (in feet per second) for a pipe of a given inside diameter. It's the same math plumbers, irrigation designers, and pump sizers use to check whether a pipe size will deliver adequate flow without running too fast or too slow.
How the GPM Flow Rate Calculator works
The calculator uses the continuity equation for incompressible flow: GPM = V × A × 448.83, where V is velocity in feet per second and A is the pipe's cross-sectional area in square feet. The area is computed from the inside diameter you enter (A = π/4 × d², with d converted from inches to feet). The 448.83 constant converts cubic feet per second into gallons per minute (7.48052 gallons per cubic foot × 60 seconds per minute). Solving for velocity instead of GPM simply rearranges the same equation: V = GPM / (A × 448.83).
Inputs and what they mean
Enter the pipe's actual inside diameter, not its nominal trade size — Schedule 40 PVC, copper type L, and steel pipe all have inside diameters that differ from their nominal labels by varying amounts. For the velocity-to-GPM direction, enter the water speed in feet per second (a value you might measure, estimate from a pump curve, or take from a design target). For the GPM-to-velocity direction, enter a known or required flow rate, such as a fixture's rated demand or a pump's output.
Limits and edge cases
This calculator assumes full, steady, single-phase water flow through a circular pipe — it does not model partially-full flow (see a drain-slope calculator for that), compressible fluids, or two-phase flow. A pipe diameter of zero or a negative value has no physical meaning, so the calculator asks for a valid diameter before showing a result. The 4–8 ft/s recommended range is a general plumbing-industry guideline, not a universal code requirement — always check your local plumbing code and manufacturer specifications for the pipe material and application you're using.