A flow test (hydrant or similar) usually gives three numbers: static pressure with little or no flow, residual pressure while water is flowing, and the test flow in gpm. The drop from static to residual is what the supply spent to deliver that flow. This page explains that sketch and gives a free browser calculator: pressure drop, and an educational available-fire-flow style estimate at a target residual using the common N1.85 form. It is an approximation for learning and quick checks—not a substitute for project water-supply tables, utility data, or review by a licensed design professional.
What residual pressure is telling you
- Record static pressure Ps (psi) with outlets shut (or at the baseline your procedure uses).
- Open the flowing outlet(s), measure residual pressure Pr (psi) while flowing, and measure or calculate test flow Qt (gpm)—often from pitot reading and outlet coefficient in a full hydrant test.
- Pressure drop ΔP = Ps − Pr at that Qt. Larger drop at the same flow means a weaker or more restricted supply.
- Educational available flow at a target residual Pt: Q ≈ Qt × ((Ps − Pt) / (Ps − Pr))1/1.85. A common reporting target is 20 psi residual; confirm what your authority having jurisdiction or water utility uses.
- Do not treat this sketch as a sealed hydraulic calculation. Seasonal variation, open hydrants elsewhere, elevation, and meter/backflow losses all matter on real projects.
Free calculator
Educational approximation only. Nothing you enter leaves your browser. For a fuller walkthrough of hydrant test reading, see the hydrant flow test calculator.
Worked example
Static 70 psi, residual 55 psi at 1,000 gpm. Drop = 15 psi. Available-flow sketch at 20 psi residual ≈ 1,000 × ((70 − 20) / (70 − 55))1/1.85 ≈ 1,940 gpm. Same arithmetic as the hydrant guide; this page centers the residual/drop reading so you can compare tests or talk through why residual fell.
Next steps: the free hydrant flow test calculator for the same curve framed as a full test read, the fire pump curve check when you are matching supply to pump demand, and the Fire Pump & Tank Sizing workbook when you need a project workbook with supply curve and tank duration.