Free Online NPSH & Suction Lift Calculator

A pump cavitates when the suction pressure falls below the water's vapour pressure, collapsing vapour bubbles that erode the impeller. This calculator finds the Net Positive Suction Head available (NPSHa) from atmospheric pressure, suction lift, friction loss and water temperature, so you can confirm it exceeds the pump's required NPSH.

The golden rule is NPSHa > NPSHr, with a safety margin of at least 0.5-1.0 m (or 1.3× for critical duties). NPSHa falls with higher suction lift, more suction friction, hotter water and higher altitude. When NPSHa is tight, lower the pump, shorten and fatten the suction pipe, or cool the liquid.

Example: A pump lifts 30 °C water 3 m with 0.6 m of suction friction at sea level. Atmospheric head ≈ 10.33 m, vapour-pressure head at 30 °C ≈ 0.43 m. NPSHa = 10.33 − 0.43 − 3 − 0.6 = 6.3 m. If the pump needs 4 m NPSHr, the 2.3 m margin is healthy; a hot-water or high-altitude duty would need re-checking.

NPSHa must always exceed the pump's NPSHr by a safety margin. Water vapour-pressure head rises steeply with temperature, eating into available NPSH:

Improving a Tight NPSH

If NPSHa is close to NPSHr: lower the pump (reduce suction lift or flood it), increase the suction pipe diameter and remove fittings to cut friction, cool the liquid, or select a pump with a lower NPSHr. Never throttle the suction side. A margin of 0.5-1.0 m is usual; critical or hot duties use 1.3× NPSHr.

Atmospheric head also falls ~1.2 m per 1000 m of altitude - account for it at hill sites.