NPSH Available

Net positive suction head at the pump inlet — the cavitation margin.

InputNPSHa = (P_surface − P_vapor)/(ρ·g) + z_static − h_friction

Pressure on liquid surface (absolute) (kPa)or enter psi — converts to kPaFluid vapor pressure (absolute) (kPa)or enter psi — converts to kPaLiquid level above pump centerline (negative = lift) (m)or enter ft — converts to mSuction-line friction loss (m)or enter ft — converts to mFluid density (kg/m³)

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The engineering

NPSHa is how far above boiling the liquid arrives at the pump eye, expressed as head. The pump's datasheet demands NPSHr; if available doesn't beat required — by a margin, 0.5–1 m minimum, more for hot or volatile service — the impeller eye flashes vapor and the bubbles collapse against the vanes like tiny jackhammers.

The knobs are all in the formula: hot water is the classic trap (vapor pressure at 90 °C is 70 kPa, stealing 7 m of your head), high altitude quietly shaves the atmospheric term, and every elbow on the suction side eats the h_loss row. Suction lines are short, fat, and straight for a reason.

Where this math comes from

Cavitation announced itself in 1897 when Charles Parsons' revolutionary turbine ship Turbinia couldn't reach speed — her propellers were spinning in their own vapor. Parsons built the first cavitation tunnel to see it, and marine engineering met the enemy that pump engineering would formalize.

Dietrich Thoma's cavitation number (circa 1924) gave the phenomenon a similarity parameter, and the pump industry, marshaled by the Hydraulic Institute, recast it as the NPSH bookkeeping used on this card — a clean split between what the installation provides (yours) and what the pump requires (the vendor's), so cavitation arguments have a contract line to point at.

1897Charles Parsons / TurbiniaPropeller cavitation discovered at sea; first cavitation tunnel.
1924Dietrich ThomaCavitation similarity number σ (circa).
1917Hydraulic InstituteStandards body that codified NPSH practice.

See the full timeline of the math behind every calculator →

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