A hydrant flow test gives you three numbers: static pressure, residual pressure at a measured flow, and that flow. From those three points you can estimate available flow at any other residual pressure (commonly 20 psi) using the exponential form of the water supply curve.
The method
- Record static pressure Ps (psi), residual pressure Pr (psi) at test flow Qt (gpm).
- Available flow at a target residual Pt: Q = Qt × ((Ps − Pt) / (Ps − Pr))1/1.85.
- 20 psi residual is a common reporting target for available fire flow. Confirm the residual your authority having jurisdiction or water utility uses.
- This screening estimate ignores elevation, hydrant coefficient, and main friction beyond the test. Use it to size a pump or compare supplies, then confirm with a full hydraulic analysis.
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Results
Worked example
Static 70 psi, residual 55 psi at 1,000 gpm. Available flow at 20 psi = 1,000 × ((70 − 20) / (70 − 55))1/1.85 ≈ 1,000 × (50/15)0.5405 ≈ 1,940 gpm.
Sizing a fire pump against that supply? The Fire Pump & Water Storage Tank Sizing Workbook combines the supply curve with pump net pressure, suction losses and tank volume.