Gpm Calculator
Calculate gallons per minute (GPM) flow rate for pipes, hoses, or irrigation systems. Input pressure, pipe diameter, and length to estimate flow.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to Use the GPM Calculator
This tool estimates the flow rate (in gallons per minute) through a pipe using the Hazen-Williams formula. All inputs are in imperial units (inches, PSI, feet).
- Enter Pipe Diameter (inches) – e.g., 1.5 for a 1.5-inch pipe.
- Enter Pressure (PSI) – the water pressure at the source.
- Enter Pipe Length (feet) – the total length of the pipe run.
- Select Roughness Coefficient (C factor) – choose the pipe material from the dropdown (e.g., PVC = 140, Steel = 120).
- Click "Calculate GPM" – the result appears as a flow rate in GPM, along with the formula used.
Note: All values must be positive numbers. Invalid inputs will show a red error border.
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Frequently Asked Questions
GPM stands for gallons per minute, a unit of flow rate measuring how many gallons of liquid pass a point in one minute.
We use the Hazen-Williams equation: GPM = 0.2083 × C × d^2.63 × (P/L)^0.54, where C is the roughness coefficient, d is pipe diameter in inches, P is pressure in PSI, and L is pipe length in feet.
The calculator includes common roughness coefficients for PVC (140), copper/brass (130), steel (120), galvanized (100), and cast iron (80).