If the pressure at a pump’s inlet drops below the liquid’s vapour pressure, bubbles form and collapse violently – cavitation – eroding the impeller. NPSH available must exceed the pump’s required NPSH (from the datasheet) with a margin.
How to use the NPSH Calculator
- Liquid level
- Pressure on the liquid surface
- Height difference Z
- Suction line friction loss
- Vapour pressure of the liquid
- Liquid density ρ
- NPSH required by the pump (optional)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose whether the liquid is above or below the pump and enter the values. Water’s vapour pressure is about 2.34 kPa at 20 °C.
Inputs explained
| Field | What to enter |
|---|---|
| Liquid level | Choose one: Above the pump (flooded suction), Below the pump (suction lift) |
| Pressure on the liquid surface | Units: kPa, Pa, MPa, bar, atm, psi |
| Height difference Z | Units: m, cm, mm, km, in, ft … |
| Suction line friction loss | Units: m, cm, mm, km, in, ft … |
| Vapour pressure of the liquid | Units: kPa, Pa, MPa, bar, atm, psi |
| Liquid density ρ | Units: kg/m³, g/cm³, g/mL, lb/ft³, lb/in³ |
| NPSH required by the pump (optional) | Units: m, cm, mm, km, in, ft … |
What each result means
- NPSH available = ([Pressure on the liquid surface] – [Vapour pressure of the liquid])÷([Liquid density ρ]×9.80665) + [Liquid level]×[Height difference Z] – [Suction line friction loss]
- Margin over NPSH required = [NPSH available] – [NPSH required by the pump (optional)]
Example values
How the main result changes when Pressure on the liquid surface changes and everything else stays at the example values above:
| Pressure on the liquid surface (kPa) | NPSH available (m) |
|---|---|
| 50.66 | 6.4374 |
| 75.99 | 9.0256 |
| 101.33 | 11.6139 |
| 126.66 | 14.2021 |
| 151.99 | 16.7904 |
| 202.65 | 21.9669 |
Formula
NPSHa = (P_atm − P_v)/(ρ·g) ± Z − h_f (+Z when the liquid is above the pump, −Z for a suction lift)
Worked example
Inputs — Liquid level: Above the pump (flooded suction); Pressure on the liquid surface: 101.325 kPa; Height difference Z: 2 m; Suction line friction loss: 0.5 m; Vapour pressure of the liquid: 2.34 kPa; Liquid density ρ: 998 kg/m³; NPSH required by the pump (optional): 3 m.
Results — NPSH available = 11.6139 m; Margin over NPSH required = 8.6139 m; Cavitation risk = OK.
Key terms
- Cavitation
- Formation and collapse of vapour bubbles in a liquid.
Frequently asked questions
What is the difference between NPSHa and NPSHr?
NPSHa is what your system provides; NPSHr is what the pump needs.
What margin is recommended?
At least 0.5–1 m, or 10–35% depending on the application.
How can I increase NPSHa?
Raise the liquid level, cool the liquid, shorten or enlarge the suction line.
Why does hot water cause cavitation?
Its vapour pressure is much higher.