← Back to All Calculators

NPSH Available Calculator

Check pump suction head, vapor pressure, NPSH margin, and cavitation risk

Quick answer: NPSH available is pressure head above vapor pressure, plus static suction head, minus suction-line loss. Compare it with the pump curve's NPSH required at the highest actual flow.

Inputs

Use barometric pressure for an open tank or absolute system pressure for a closed vessel.
Positive when the liquid surface is above the pump; negative for suction lift.

Formula reference

NPSHa = (Pabs − Pv) / (ρg) + Hstatic − Hsuction loss

Required check = NPSHr + stated safety margin

What this calculator answers

This tool estimates NPSH available for a water pump from the absolute pressure at the liquid surface or closed system, the water vapor pressure at the selected temperature, static suction head, and suction-line friction loss. It then compares that value with the pump manufacturer's NPSH required and a margin that you state.

Use the actual operating point

  • Read NPSHr from the manufacturer's pump curve at the highest actual flow.
  • Use measured or calculated suction pressure and friction loss at that flow.
  • Use fluid density and vapor pressure appropriate to the liquid and temperature.

Boundaries

The water vapor-pressure interpolation and head equation are preliminary. This page does not select a pump, certify cavitation safety, model transients, or replace a manufacturer Q-H-NPSHr curve.

For refrigerants, glycol mixtures, flashing liquids, altitude, or unusual suction arrangements, use verified fluid properties and a qualified review.

FAQ

Can NPSHa be negative?

Yes. A negative result means the assumed suction condition does not provide positive pressure head above vapor pressure after the stated losses.

Why does temperature matter?

Vapor pressure rises rapidly with temperature, reducing the pressure head available to keep the liquid from vaporizing.

Sources and related tasks