Water Flow Rate Calculator

Estimate the water flow rate through a pipe from its inside diameter, supply pressure, and run length.

flow rate
FLOW RATE
— GPM
(approximate)

Usage Tip

Use the actual inside diameter and your measured supply pressure for the closest estimate.

THE MATH
GPH = 149 × d2 × √(P ÷ L)
Where: d = diameter (in)  •  P = pressure (PSI)  •  L = length (ft)
• GPM = GPH ÷ 60
• Approximate — real flow varies with fittings
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.