# NPSH Calculator (Net Positive Suction Head)

URL: https://speedycalculator.com/calculators/physics/npsh/
Last updated: 2026-10-30

> The NPSH calculator finds the net positive suction head available to a pump from the pressure on the liquid surface, the liquid level above or below the pump, suction friction losses and the liquid's vapour pressure.

## Inputs

- Liquid level
- Pressure on the liquid surface (kPa, Pa, MPa, bar, atm, psi)
- Height difference Z (m, cm, mm, km, in, ft, yd, mi)
- Suction line friction loss (m, cm, mm, km, in, ft, yd, mi)
- Vapour pressure of the liquid (kPa, Pa, MPa, bar, atm, psi)
- Liquid density ρ (kg/m³, g/cm³, g/mL, lb/ft³, lb/in³)
- NPSH required by the pump (optional) (m, cm, mm, km, in, ft, yd, mi)

## Results

- NPSH available
- Margin over NPSH required
- Cavitation risk

## Formula

```
NPSHa = (P_atm − P_v)/(ρ·g) ± Z − h_f  (+Z when the liquid is above the pump, −Z for a suction lift)
```

## Explanation

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

FieldWhat to enterLiquid levelChoose one: Above the pump (flooded suction), Below the pump (suction lift)Pressure on the liquid surfaceUnits: kPa, Pa, MPa, bar, atm, psiHeight difference ZUnits: m, cm, mm, km, in, ft …Suction line friction lossUnits: m, cm, mm, km, in, ft …Vapour pressure of the liquidUnits: kPa, Pa, MPa, bar, atm, psiLiquid density ρUnits: kg/m³, g/cm³, g/mL, lb/ft³, lb/in³NPSH 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)50.666.437475.999.0256101.3311.6139126.6614.2021151.9916.7904202.6521.9669

## 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.

## FAQ

**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.


## Sources

- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)
- [Hydraulic Institute](https://www.pumps.org/)