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Discharge Pipe Size Calculator
Quick answer: Enter your pump's flow at your head in gallons per hour. The calculator shows water velocity in 1¼, 1½ and 2-inch Schedule 40 pipe. Many planners aim for roughly 2 to 8 feet per second: fast enough to carry grit, slow enough to limit noise and water hammer. Never reduce below the pump's outlet size.
The formula
Velocity (ft/s) = GPM ÷ 448.83 ÷ pipe area in square feet, using typical Schedule 40 inside diameters (1.38, 1.61 and 2.067 inches). Higher velocity means more friction head and louder check-valve closure.
Worked example
A pump delivering 2,000 GPH (about 33 GPM) runs at about 7.1 ft/s in 1¼-inch pipe, 5.2 ft/s in 1½-inch and 3.2 ft/s in 2-inch. All are within the planning band; the 2-inch line would cut friction and hammer the most.
Illustrative note (2026-10-29): This example was calculated on 2026-10-29 for illustration.
Limits
Local code may set minimum sizes and discharge locations. A licensed plumber should confirm pipe size, routing and freeze protection.