Last updated: October 14, 2026
Net positive suction head available (NPSHa) is the margin of absolute pressure, expressed as head, that the liquid has at the pump suction above its vapour pressure. If NPSHa falls below what the pump needs (NPSHr, from the pump curve), the liquid starts to boil in the impeller eye. The vapour bubbles collapse as pressure rises, which is cavitation: noise, vibration, lost head and damaged impellers.
The equation
For a pump drawing from an open tank, sump or wet well:
NPSHa = hatm − suction lift − hf,suction − hm,suction − hvap- hatm: atmospheric pressure as head of the liquid. It falls with elevation.
- Suction lift: height of the pump centreline above the liquid surface. For a flooded suction, it is negative and adds to NPSHa.
- hf and hm: friction and fitting losses in the suction line only.
- hvap: vapour pressure of the liquid as head. It rises steeply with temperature.
Pressure is converted to head with h = P / (ρg). For water near 25 °C, 1 m of head is about 9.78 kPa; at SG 1.0, 1 psi is 2.31 ft.
Worked example
This uses the suction side of the TDH example: 40 L/s (634 USGPM) through 8 m of 8" HDPE DR17 with an entrance and one elbow (ΣK = 1.4), and a 3.0 m (9.84 ft) suction lift. The site is at 300 m (984 ft) elevation and the water is at 25 °C (77 °F).
Step 1: Atmospheric head
Standard atmosphere at 300 m: P = 101.325 × (1 − 2.25577×10⁻⁵ × 300)5.2559 = 97.77 kPa (14.18 psia). Water at 25 °C has SG 0.9971, so hatm = 97.77 / (9.807 × 0.9971 × 0.99997) = 10.00 m (32.81 ft).
Step 2: Vapour pressure head
Vapour pressure of water at 25 °C is about 3.16 kPa (0.458 psia), which is hvap = 0.323 m (1.06 ft).
Step 3: Suction losses
From the TDH example: friction 0.064 m (0.21 ft) and fitting losses 0.137 m (0.45 ft).
Step 4: NPSHa
| Term | Metric (m) | US (ft) |
|---|---|---|
| Atmospheric head | +10.000 | +32.807 |
| Suction lift | −3.000 | −9.843 |
| Suction friction | −0.064 | −0.214 |
| Suction minor losses | −0.137 | −0.449 |
| Vapour pressure head | −0.323 | −1.060 |
| NPSHa | 6.48 m | 21.24 ft |
The US column uses the US form of Hazen-Williams for suction friction, so it differs slightly from the converted metric result.
What changes NPSHa
| Case | NPSHa |
|---|---|
| Base case: 300 m elevation, 25 °C | 6.48 m |
| Same, at sea level | 6.84 m |
| Same, with water at 60 °C (140 °F): vapour head 2.06 m | 4.88 m |
- Suction lift is the biggest lever. Every metre of lift removes a metre of NPSHa.
- Elevation costs a little over 1 m of head per 1,000 m of altitude.
- Temperature matters most for hot liquids: vapour pressure head goes from about 0.3 m at 25 °C to about 2 m at 60 °C.
- Suction line losses are small here because the suction line is short and generous. A long, undersized or partly blocked suction line can take metres of NPSHa.
Margin
Compare NPSHa with the pump's NPSHr at the actual operating flow, at the lowest liquid level you expect. NPSHr is usually defined at a 3% head drop, so cavitation is already happening at that point. Leave a margin: commonly at least 10% or 0.6–1.5 m (2–5 ft), and more for pumps that run far from best efficiency or that pump wastewater with entrained air or solids.
Convert pressures and heads with the free unit converter, see how NPSH fits into the whole design in the course, or start with how to calculate TDH.
Worked examples are for learning. Check your own designs against manufacturer data and have them reviewed by a qualified professional. See the disclaimer.