NPSH calculator · NPSH available and cavitation check
Find the NPSH available at the pump inlet from water temperature, atmospheric pressure (altitude), suction tank pressure, suction level and suction loss. Add the maker's NPSHr to see the margin and cavitation risk.
NPSH and cavitation
When the pressure at the pump inlet falls below the vapour pressure at the water temperature, vapour bubbles form and collapse, eroding the impeller and causing noise, vibration and loss of head. This is cavitation.
NPSH available (NPSHa) is how far the inlet pressure stays above the vapour pressure, expressed as head (m). It is set by the system: pressure on the water surface, level, suction loss and temperature. NPSHa = (Ps − Pv)/(ρ g) + zs − hfs.
NPSH required (NPSHr) is what the pump needs to run without a set drop in performance; the maker finds it by test. NPSHa must stay well above NPSHr to avoid cavitation.
Vapour pressure and theoretical maximum suction lift by water temperature
Vapour pressure and saturated liquid density from IAPWS-IF97. The last column is the theoretical limit (Patm − Pv)/(ρ g) for a tank open to air (101.325 kPa), zero suction loss and zero NPSHr; a real suction lift must stay below this value minus the suction loss and NPSHr.
| Water temperature °C | Vapour pressure kPa | Density kg/m³ | Vapour pressure head m | Max. suction lift m |
|---|---|---|---|---|
| 10 | 1.228 | 999.7 | 0.13 | 10.21 |
| 20 | 2.339 | 998.2 | 0.24 | 10.11 |
| 30 | 4.247 | 995.6 | 0.43 | 9.94 |
| 40 | 7.384 | 992.2 | 0.76 | 9.65 |
| 50 | 12.351 | 988.0 | 1.27 | 9.18 |
| 60 | 19.946 | 983.2 | 2.07 | 8.44 |
| 70 | 31.201 | 977.7 | 3.25 | 7.31 |
| 80 | 47.415 | 971.8 | 4.98 | 5.66 |
| 90 | 70.182 | 965.3 | 7.41 | 3.29 |
| 100 | 101.418 | 958.4 | 10.79 | −0.01 |
Near 100 °C the limit drops to zero or below, so suction lift is impossible and the water level must be above the pump (flooded suction). At altitude the lower air pressure reduces the limit at the same temperature.
Frequently asked questions
What is the difference between NPSHa and NPSHr?
NPSHa (available) is what the system delivers at the pump inlet, calculated from surface pressure, level, suction loss and water temperature. NPSHr (required) is what the pump demands; the maker finds it by test and shows it on the curve, and it rises with flow. NPSHa must exceed NPSHr over the whole operating range to avoid cavitation.
Why can't a pump lift hot water?
As water gets hotter its vapour pressure rises quickly, and the vapour pressure head approaches the atmospheric head. From an open tank, 20 °C water can in theory be lifted about 10 m, but 80 °C water only about 5.7 m, and 100 °C water not at all. Suction loss and NPSHr come off these figures, so hot water is usually given a flooded suction with the level above the pump.
How much NPSH margin do I need?
Only one thing is certain: a margin of zero or less means cavitation risk. The margin you need depends on pump type, suction specific speed, operating range and liquid, so there is no single number. Check the maker's data and the NPSH margin ratio guidance in ANSI/HI 9.6.1. The common 0.5 to 1 m or more is only a rule of thumb.
Equations and sources
Equations used by the calculator and the table above. Pressures in kPa are multiplied by 1000 to get Pa.
| Item | Equation | Notes · basis |
|---|---|---|
| NPSH available | NPSHa = (Ps − Pv) / (ρ g) + zs − hfs | Ps absolute pressure on the surface, Pv vapour pressure, zs suction level (+ flooded, − lift), hfs suction loss |
| Surface pressure | Ps = Patm + Pg | Patm atmospheric pressure, Pg tank gauge pressure (0 if open, negative for vacuum) |
| Altitude to pressure | Patm = 101.325 (1 − 2.25577 × 10⁻⁵ z)^5.2559 | z altitude in m, result in kPa. ASHRAE Fundamentals 2017, chapter 1, eq. 3 (same as the calculator engine) |
| Vapour pressure, density | Pv = psat(t), ρ = 1 / vf(t) | IAPWS-IF97 region 4 saturation pressure, region 1 saturated liquid volume. Same as the steam table tool |
| Margin and ratio | ΔNPSH = NPSHa − NPSHr, NPSHa / NPSHr | A margin of 0 or less means cavitation risk. Check the needed margin with the maker and ANSI/HI 9.6.1 |
Sources
- IAPWS R7-97(2012), Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97): saturation pressure and saturated liquid volume
- ANSI/HI 9.6.1, Rotodynamic Pumps Guideline for NPSH Margin (check the values in the standard itself)
- ASHRAE Handbook Fundamentals 2017, chapter 1 Psychrometrics, eq. 3 (altitude to pressure, calculator engine)
Last updated 2026-11-09.