NPSH calculator
A pump cavitates when the liquid at its inlet falls close to its vapour pressure. NPSHa is the suction head the installation actually provides; the pump’s NPSHr is what it needs. This engine builds NPSHa from pressure, static head and friction, then reports the margin against the requirement.
| Term | Head (m) |
|---|---|
| Atmospheric pressure | 10.35 |
| Less vapour pressure | −0.24 |
| Static head | 2.00 |
| Less friction loss | −1.20 |
| NPSHa | 10.91 |
| Less NPSHr | −3.00 |
| Margin | 7.91 |
Negative static head means the liquid level is below the pump inlet (a lift).
(p_atm − p_v)/(ρg) + z − h_f · Absolute pressures; z positive when the level is above the inlet.
NPSHa − NPSHr · Keep at least 0.5 m, more for hot or throttled duty.
(p_atm − p_v)/(ρg) · Falls sharply as the liquid approaches its boiling point.
A clear path from inputs to results.
A pump cavitates when the liquid at its inlet approaches its vapour pressure. NPSHa is what the installation provides; NPSHr is what the pump needs. This calculator builds the available head from pressure, static head and friction loss, then reports the margin against the requirement.
How to use
- Enter atmospheric pressure and the liquid’s vapour pressure at the operating temperature.
- Set the density.
- Enter the static head above the pump inlet — negative for a suction lift.
- Subtract the suction-line friction loss and compare with the pump NPSHr.
The formula
Reference: Hydraulic Institute standards for rotodynamic pumps; standard cavitation margin practice.
Worked example
Water at 20 °C, 2 m static head, 1.2 m friction, NPSHr 3 m
- Atmospheric 101.3 kPa, vapour pressure 2.34 kPa.
- Density 998 kg/m³.
- Static head +2 m.
- Friction 1.2 m.
- NPSHr 3 m.
- 1
Pressure head: (101.3 − 2.34) × 1000 / (998 × 9.80665) = 10.11 m.
- 2
Add static head: 10.11 + 2 = 12.11 m.
- 3
Subtract friction: 12.11 − 1.2 = 10.91 m of NPSHa.
- 4
Margin over NPSHr: 10.91 − 3 = 7.91 m.
- 5
The required 0.5 m margin is comfortably met.
NPSHa 10.91 m · margin 7.91 m · safe.
Interpreting the result
The pressure term is the one people forget. At 80 °C water’s vapour pressure is 47.4 kPa, which cuts the available head by about 4.6 m — enough to turn a safe installation into a cavitating one without changing a single pipe. Suction lift is the other lever: every metre you raise the pump costs a metre of NPSHa, which is why pumps are mounted low next to the sump.
Assumptions & limitations
- Steady-state, single-phase flow with the stated density and vapour pressure.
- Friction loss is entered directly; it is not computed from pipe length and fittings.
- The pump’s NPSHr is taken from its curve at the duty flow — it rises with flow, so check the value at the operating point.
- Transient effects, entrained gas, vortexing and dissolved-air release are not modelled.
Frequently asked questions
What margin should I keep above NPSHr?
0.5 m is a common minimum for clean, cold duty; 1 m or more for hot liquids, throttled flow or critical service.
Does a bigger suction pipe help?
Yes — it reduces friction loss, which adds directly to NPSHa. Lowering the pump or raising the tank level does the same thing.
Why does hot water cavitate more easily?
Its vapour pressure is much higher, so less of the atmospheric pressure is available to push liquid into the impeller.
Can I fix cavitation by throttling the discharge?
It reduces flow and therefore NPSHr, so it can help, but it wastes energy. Fixing the suction side is usually the better answer.
Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.