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NPSH Available Calculator

The NPSH Available Calculator finds the Net Positive Suction Head available (NPSHa) at a pump's inlet — the absolute head above the fluid's vapour pressure. Comparing NPSHa with the pump's NPSH required tells you whether the pump will run free of cavitation. Enter the atmospheric and vapour pressures, fluid density, static suction head and suction friction loss.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the atmospheric (or suction-vessel) pressure and the fluid's vapour pressure, both as absolute values in kPa. Defaults are 101.325 kPa and 2.34 kPa (water at 20°C).
  2. Enter the static suction head Hs in metres — positive when the source is above the pump (flooded suction) and negative for a suction lift.
  3. Add the suction-side friction loss Hf, then read the NPSH available and compare it against the pump's NPSH required plus a margin.

How it works

NPSHa = (Patm − Pvap)/(ρ·g) + Hs − Hf. The pressures are converted from kPa to Pa (×1000) and divided by ρ·g to give a pressure head in metres of liquid. The static suction head is added (or subtracted for a lift) and the suction friction loss is subtracted. The calculator uses g = 9.81 m/s². A larger vapour pressure (hot liquid), a suction lift, or high friction all cut NPSHa and push the pump toward cavitation.

Worked example

Worked example. With Patm = 101.325 kPa, Pvap = 2.34 kPa, water (1000 kg/m³), Hs = +2 m and Hf = 0.5 m: pressure head = (101325 − 2340) ÷ (1000 × 9.81) = 10.09 m. NPSHa = 10.09 + 2 − 0.5 = 11.59 m. If the pump needs 4 m NPSHr, the ~7.6 m of extra margin is ample.

Common mistakes

  • Using gauge pressure instead of absolute pressure for the atmospheric and vapour-pressure terms.
  • Getting the sign of Hs wrong — a suction lift must be entered as a negative value, which lowers NPSHa.
  • Ignoring the vapour pressure rise with temperature; hot water has a much higher Pvap and far less NPSH margin.

Frequently asked questions

What is the difference between NPSH available and NPSH required?

NPSH available (NPSHa) is a property of your system — how much suction head the installation provides. NPSH required (NPSHr) is a property of the pump, read from its curve. You must keep NPSHa above NPSHr plus a margin to avoid cavitation.

How much NPSH margin should I allow?

A common rule of thumb is at least 0.5–1 m above NPSHr, and more for large or high-energy pumps. Consult the pump manufacturer and relevant standards; hotter fluids and volatile liquids need extra margin.

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