NPSH Available Calculator
Net positive suction head available at a pump, the margin over what it requires, and the highest suction lift it allows.
Results
What this tool does
Cavitation is the pump boiling its own liquid at the impeller eye, and it destroys impellers quickly and noisily. A negative static head means the pump is above the liquid and sucking it up, which is the case that runs out of margin first. Water's vapour pressure is 2.34 kPa at 20 °C but 12.3 kPa at 50 °C and 47.4 kPa at 80 °C, so hot water is where suction problems live.
Formula
NPSHd = (p atm − p vapour) ÷ (ρ g) + h static − h losses
Variables
| Symbol | Meaning | Unit |
|---|---|---|
pa | Atmospheric pressure | kPa |
pv | Vapour pressure | kPa |
ro | Fluid density | kg/m³ |
hs | Static head (negative if lifting) | m |
hl | Head loss | m |
hr | NPSH required by the pump | m |
NA | NPSH available | m |
MG | Margin | m |
HB | Head loss | m |
HV | Head loss | m |
HX | Static head (negative if lifting) | m |
Worked example
- Atmospheric pressure101.325 kPa
- Vapour pressure2.34 kPa
- Fluid density998 kg/m³
- Static head (negative if lifting)-2 m
- Head loss1.2 m
- NPSH required by the pump3 m
- NPSH available6.9139 m
- Margin3.9139 m
- Head loss10.3530 m
- Head loss0.2391 m
- Static head (negative if lifting)5.4139 m
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
How much margin over the required NPSH is enough?
The published NPSH required is the point at which the pump has already lost three percent of its head to cavitation, not the point where cavitation begins — so matching it exactly means running a pump that is already damaging itself. Common practice is at least 0.5 to 1 metre of margin, and considerably more for large or high-energy pumps. Remember that hot liquid raises the vapour pressure sharply and that altitude lowers the atmospheric pressure, and both eat the margin.