Pump NPSH Calculator

Calculates Net Positive Suction Head Available (NPSHa) — the actual suction-side energy margin above the fluid's vapor pressure at the pump inlet — and compares it against the required NPSHr from the pump's manufacturer curve. The formula is NPSHa = (surface pressure ÷ ρg) + static suction head − friction losses − (vapor pressure ÷ ρg), converting each pressure term into an equivalent head of the pumped fluid using its actual density. Static head is signed: positive when the liquid surface sits above the pump centerline (a flooded suction, which helps), negative when the pump must lift liquid up to itself (a suction lift, which hurts). The margin (NPSHa − NPSHr) and ratio (NPSHa/NPSHr) both indicate cavitation risk — the Hydraulic Institute's ANSI/HI 14.3 standard for rotodynamic pump design and application generally recommends keeping the ratio above about 1.2 for general service, since operating too close to NPSHr causes vapor bubbles to form and collapse inside the pump, eroding impellers and destroying performance over time.

Separately, the calculator estimates hydraulic power (ρgQH) and the larger shaft power once pump efficiency is factored in, giving a rough motor sizing figure. A key limitation: vapor pressure must match the actual pumping temperature, not a generic default — hot water or volatile solvents have dramatically higher vapor pressure than the 20°C water default, which directly eats into the available margin. Also remember NPSHr always comes from testing on the specific pump model and impeller trim; it cannot be estimated from first principles here.