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Construction Calculators

Gallons Per Minute (GPM) Calculator

Estimate the flow rate in gallons per minute (GPM) through a pipe from its inside diameter and the water's velocity.

in
ft/s
Typical design velocity for residential plumbing is about 4-8 ft/s.
Flow rate —
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About the Gallons Per Minute (GPM) Calculator

This calculator gives a simplified estimate of the flow rate through a pipe from its cross-sectional area and the water's velocity, using the standard continuity relationship between flow, area and speed.

Formula: Cross-Sectional Area (sq ft) = π × (Diameter ÷ 2)² (converted to feet); Flow (cu ft/sec) = Area × Velocity; GPM = Flow (cu ft/sec) × 448.831

This is a simplified velocity-based estimate — actual flow rate also depends on pressure, pipe friction and fittings. For sizing a well pump, irrigation system or fixture supply, consult manufacturer flow charts or a licensed plumber.

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