NPSH available / required calculator

Net positive suction head (NPSH) sketches show up when a fire pump or centrifugal pump lesson checks whether the suction side can supply enough absolute pressure head at the impeller to avoid cavitation. A common teaching form for NPSH available is NPSHa ≈ H_atm + H_s − H_vp − H_f, with all terms in feet of fluid: atmospheric pressure head at the site, static suction head (positive) or lift (negative), vapor-pressure head of the liquid at the pumping temperature, and friction (plus minor) losses in the suction piping. NPSH required (NPSHr) comes from the pump manufacturer curve at the operating flow—not from this page. This calculator is for learning and quick checks—not a substitute for manufacturer data, the criteria adopted by your authority having jurisdiction, a sealed calculation, or review by a licensed design professional.

How NPSH is used

1. Enter atmospheric pressure head H_atm (ft). Freshwater at sea level is often sketched near 33.9 ft (14.7 psi × 2.31); reduce for altitude.
2. Enter static suction head H_s (ft): positive when the water surface is above the impeller datum, negative for suction lift.
3. Enter vapor-pressure head H_vp (ft) for the liquid at pumping temperature (cold water teaching values are often a few tenths of a foot; hot water is much larger).
4. Enter suction friction / minor-loss head H_f (ft) for the suction path at the flow of interest.
5. Optionally enter manufacturer NPSHr (ft) at that flow and compare margin = NPSHa − NPSHr. Confirm units, datum, and the adopted edition before you rely on a result.

Free calculator

Educational approximation only. Formula sketch: NPSHa ≈ H_atm + H_s − H_vp − H_f (ft). Nothing you enter leaves your browser. For PSI ↔ feet of head see the PSI to feet of head calculator. Elevation pressure notes: elevation pressure calculator. Pump curve and tank sizing live in the Fire Pump & Tank Sizing workbook.

Worked example

H_atm = 33.9 ft, H_s = +5 ft (flooded suction), H_vp = 0.8 ft, H_f = 3 ft: NPSHa ≈ 33.9 + 5 − 0.8 − 3 = 35.1 ft. If the manufacturer NPSHr at the operating flow is 10 ft, the teaching margin is about 25 ft. Always confirm altitude, temperature, losses, datum, and the pump curve—not this page alone.

Next steps: convert pressure and head with the free PSI to feet of head calculator, check elevation effects with the elevation pressure calculator, review a pump curve sketch with the fire pump curve check, and size pump/tank suction and storage in the Fire Pump & Tank Sizing workbook ($49).