NPSH Available Calculator (NPSHa for Pumps)
By Santaji GadepatilinUpdated November 9, 2026
A centrifugal pump cavitates when the pressure at its eye falls to the liquid's vapour pressure, so the suction system must supply more head than the pump needs. This NPSH available calculator works out NPSHa from the pressure on the liquid surface, the vapour pressure and density of the liquid, the static height and the suction-line friction, then compares it with the pump's NPSHr — the check explained in NPSH and cavitation.
How to use the NPSH available calculator
Enter the absolute pressure on the liquid surface (atmospheric pressure for an open tank), the liquid’s vapour pressure at the pumping temperature, its density, the liquid level relative to the pump centreline (negative for a suction lift), the suction-line losses at the design flow and the pump’s NPSHr from its curve. The calculator gives each head term, the NPSH available, the margin in metres and the NPSHa ÷ NPSHr ratio.
NPSH available formula
NPSHa = (Psurface − Pv) ÷ (ρ g) + hz − hf
All pressures are absolute. Calculate NPSHa at the maximum flow (highest friction and NPSHr), the lowest liquid level and the highest liquid temperature you expect.
Worked example
Water at 40 °C (Pv = 7.384 kPa, ρ = 992.22 kg/m³) lifted from an open sump at sea level, liquid level 3 m below the pump, 1.2 m suction loss, pump NPSHr = 4 m:
- Pressure head = 101,325 ÷ (992.22 × 9.807) = 10.41 m
- Vapour pressure head = 7,384 ÷ (992.22 × 9.807) = 0.759 m
- NPSHa = 10.41 − 0.759 − 3 − 1.2 = 5.45 m
- Margin = 5.45 − 4 = 1.45 m, ratio 1.36 — comfortable
Vapour pressure of water and NPSH from an open tank (sea level)
The last column is the NPSHa with zero static height and zero suction loss — the most a sea-level open tank can provide at that temperature.
| Temperature | Vapour pressure | Density | Vapour head | Max NPSHa, open tank |
|---|---|---|---|---|
| 10 °C | 1.228 kPa | 999.70 kg/m³ | 0.13 m | 10.21 m |
| 20 °C | 2.339 kPa | 998.21 kg/m³ | 0.24 m | 10.11 m |
| 30 °C | 4.247 kPa | 995.65 kg/m³ | 0.43 m | 9.94 m |
| 40 °C | 7.384 kPa | 992.22 kg/m³ | 0.76 m | 9.65 m |
| 50 °C | 12.351 kPa | 988.03 kg/m³ | 1.27 m | 9.18 m |
| 60 °C | 19.946 kPa | 983.20 kg/m³ | 2.07 m | 8.44 m |
| 70 °C | 31.201 kPa | 977.76 kg/m³ | 3.25 m | 7.31 m |
| 80 °C | 47.415 kPa | 971.79 kg/m³ | 4.98 m | 5.66 m |
| 90 °C | 70.182 kPa | 965.31 kg/m³ | 7.41 m | 3.29 m |
| 100 °C | 101.418 kPa | 958.35 kg/m³ | 10.79 m | −0.01 m |
Vapour pressure from the IAPWS-IF97 saturation equation.
Effect of altitude (open tank, no static height or losses)
| Site altitude | Atmospheric pressure | NPSHa, water 20 °C | NPSHa, water 60 °C |
|---|---|---|---|
| 0 m | 101.3 kPa | 10.11 m | 8.44 m |
| 500 m | 95.5 kPa | 9.51 m | 7.83 m |
| 1,000 m | 89.9 kPa | 8.94 m | 7.25 m |
| 1,500 m | 84.6 kPa | 8.40 m | 6.70 m |
| 2,000 m | 79.5 kPa | 7.88 m | 6.18 m |
| 3,000 m | 70.1 kPa | 6.92 m | 5.20 m |
Practical notes
- A suction lift of more than about 4–5 m with cold water usually leaves too little NPSHa — move the pump lower or use a flooded suction.
- Reduce suction losses: use a suction pipe one size larger than the pump inlet, keep it short, and avoid strainers or valves that can clog close to the pump.
- For liquids at their boiling point (condensate, deaerator outlet, refrigerants) Psurface = Pv, so NPSHa is just the static height minus losses.
- If you have a suction gauge reading, NPSHa = (Pgauge + Patm − Pv) ÷ ρg + v²/2g + gauge height above the pump centreline.
Frequently asked questions
How do you calculate NPSH available?
NPSHa = (P_surface − P_vapour) ÷ (ρg) + h_static − h_friction, using absolute pressures. h_static is positive when the liquid level is above the pump centreline and negative for a suction lift.
What is the difference between NPSHa and NPSHr?
NPSHa is a property of the suction system — the head above vapour pressure available at the pump inlet. NPSHr is a property of the pump, from the manufacturer's curve, usually defined as the NPSH at which head drops by 3% (NPSH3). NPSHa must exceed NPSHr.
How much NPSH margin is needed?
Hydraulic Institute guideline ANSI/HI 9.6.1 suggests NPSHa ÷ NPSHr ratios from about 1.1 up to 2.5 depending on the pump's suction energy and service; many specifications ask for at least 1 m (3 ft) of margin or a 1.3 ratio.
Why does hot water cause cavitation more easily?
Vapour pressure rises steeply with temperature — from 2.3 kPa at 20 °C to 101 kPa at 100 °C — so the vapour head term eats almost all of the atmospheric head. Boiling or near-boiling liquids need a flooded suction with enough static height.