NPSH Available Calculator
Compute NPSH available to protect a pump from cavitation using pressure, temperature and suction geometry.
Inputs
Water: 0.36 @ 70°F, 0.95 @ 100°F, 11.5 @ 200°F.
Positive if source is above pump centerline, negative for suction lift.
Results
Formula
NPSHa = 2.31·(P_atm − P_vap)/SG + h_static − h_friction. Require NPSHa ≥ NPSHr + safety margin (≥3 ft or 20%).
Worked example
Water @ 70°F, atmospheric tank, 5 ft flooded suction, 2 ft friction: NPSHa ≈ 36 ft; if pump NPSHr = 10 ft, margin ≈ 26 ft — safe.
Frequently asked questions
What safety margin is enough?
Hydraulic Institute recommends NPSHa ≥ 1.35 × NPSHr, or a minimum of 3 ft above NPSHr — whichever is larger.
Why does hot water cavitate so easily?
Vapor pressure rises non-linearly with temperature. At 200°F water is at 11.5 psia — atmospheric contributes almost nothing to NPSHa.
One short email a month. Free PDFs, worked examples, no spam.