PSI to GPM Calculator

A rough estimate of flow through a sharp-edged orifice. Not a substitute for a pump curve.

How the Math Works

The PSI to GPM Calculator uses the formula GPM ≈ 29.7 × D² × √PSI, which stems from fluid dynamics principles. This equation estimates volumetric flow rate (GPM) based on pressure (PSI) and orifice diameter (D). The constant 29.7 arises from unit conversions between imperial measurements and the square root relationship between pressure and flow velocity in ideal fluid conditions. The D² term reflects how flow area increases with the square of diameter, while the square root of PSI represents how velocity increases with pressure. This approximation assumes a sharp-edged orifice under steady flow conditions.

Practical Applications

To use this calculator, first measure or obtain the orifice diameter in inches. Input your system's current pressure reading in PSI. Multiply the diameter squared by the square root of your PSI value, then multiply by 29.7 to estimate gallons per minute. For example, a 2-inch orifice at 100 PSI yields approximately 118.8 GPM (29.7 × 4 × 10). This calculation helps size pumps, select appropriate piping, and verify system performance during installation or maintenance tasks.

Day-to-Day Use

This calculation helps homeowners and technicians troubleshoot irrigation systems, water wells, and hydraulic equipment. When replacing a pump, you can estimate if your new unit will deliver sufficient water flow for lawns, pools, or industrial processes. Plumbers use it to size proper pipe diameters and prevent costly undersized installations. Emergency responders might estimate water delivery rates from fire hydrants, while farmers calculate irrigation efficiency. Understanding this relationship helps prevent equipment damage, reduces utility costs, and ensures systems operate efficiently in everyday applications.

Worked example

1 in orifice at 40 psi → ~188 GPM.

FAQ