NPSH calculator: NPSH available (NPSHA) and margin to NPSHR
This NPSH calculator works out the NPSH available with the formula from the KSB pump lexicon; for water it takes the vapour pressure from the temperature using the IAPWS-IF97 steam equations. Example: an open tank with the water level 3 m below the pump, 20 °C and 0.5 m suction loss gives an NPSHA of 6.59 m.
Source: KSB Centrifugal Pump Lexicon, “NPSH”: NPSHA = (p_e + p_b − p_D)/(ρ·g) + v_e²/2g + z_e − z_s − H_v,e,s; NPSHA ≥ NPSHR, NPSHR commonly = NPSH3 (3 % head drop) (read 09.10.2026). Updated: .
How it is calculated
NPSHA = (p_e + p_b − p_D) ÷ (ρ · g) + z − H_v. This is the KSB lexicon’s definition of the net positive suction head available, written for a tank: p_e is the gauge pressure on the liquid surface (0 when open), p_b the atmospheric pressure, p_D the vapour pressure of the liquid, z the height of the liquid level above the impeller centre and H_v the head loss of the suction line. KSB also adds the velocity head v_e²/2g at the inlet section; on a tank surface it is practically zero.
Examples (water, ρ = 1000 kg/m³)
| Case | Temperature | Level z | H_v | NPSHA |
|---|---|---|---|---|
| open tank, suction lift | 20 °C | −3 m | 0.5 m | 6.59 m |
| open tank, flooded suction | 20 °C | +2 m | 1.0 m | 11.09 m |
| open tank, hot water | 80 °C | +3 m | 0.5 m | 8.00 m |
| closed tank, 1 bar gauge | 60 °C | −2 m | 0.8 m | 15.70 m |
The density is kept at 1000 kg/m³ for the comparison; for hot water enter the real, lower density. At 20 °C the air pressure alone is worth 10.09 m of water once the vapour pressure is subtracted. If the local air pressure is only 0.90 bar instead of 1.01325 bar, for example at a higher altitude, the first case drops to 5.44 m.
Vapour pressure of water (IAPWS-IF97)
The calculator uses equation (30) of the IAPWS Industrial Formulation 1997, which reproduces the official check value of 3.53658941 kPa at 300 K. Some values: 0 °C 0.61 kPa, 10 °C 1.23 kPa, 20 °C 2.34 kPa, 40 °C 7.38 kPa, 60 °C 19.95 kPa, 80 °C 47.41 kPa, 100 °C 101.42 kPa. Near the boiling point the vapour pressure eats almost the whole air pressure – hot water needs a flooded suction.
NPSHA against NPSHR
KSB: the pump runs without impermissible cavitation if NPSHA ≥ NPSHR. NPSHR is usually NPSH3, the value at which the head has already dropped by 3 %, so a margin above it is common practice. The suction loss H_v can be worked out with the pressure drop calculator, the power the pump needs with the pump power calculator, and gauge readings in psi with bar to psi.
Frequently asked questions
How do you calculate NPSH available?
NPSHA = (p_e + p_b − p_D)/(ρ·g) + z − H_v. Open tank, 20 °C water, level 3 m below the pump, 0.5 m suction loss: (101325 − 2339) Pa ÷ (1000 · 9.80665) − 3 − 0.5 = 6.59 m.
What is the difference between NPSHA and NPSHR?
NPSHA belongs to the system (tank, pipe, liquid), NPSHR to the pump and comes from the manufacturer’s curve. The pump needs NPSHA ≥ NPSHR, plus a safety margin.
Why does hot water cause cavitation sooner?
Its vapour pressure rises steeply: 2.34 kPa at 20 °C but 47.41 kPa at 80 °C. With a 3 m flooded suction the 80 °C case still reaches 8.00 m, but a suction lift would hardly work.
Is z positive or negative?
Positive when the liquid level is above the impeller centre (flooded suction), negative when it is below (suction lift). In the first example z = −3 m.
Do I need absolute pressures?
Yes. NPSH works with absolute pressure: atmospheric pressure plus the gauge pressure in the tank. A closed tank at 1 bar gauge and 60 °C gives 15.70 m in the example.
How exact is the result?
It is a calculated value from your inputs. The suction loss and density must match the real operating point; the velocity head on the tank surface is neglected.
Sources and legal basis
- KSB Centrifugal Pump Lexicon, “NPSH”: NPSHA = (p_e + p_b − p_D)/(ρ·g) + v_e²/2g + z_e − z_s − H_v,e,s; NPSHA ≥ NPSHR, NPSHR commonly = NPSH3 (3 % head drop) (read 09.10.2026)
- IAPWS R7-97(2012), Industrial Formulation 1997 for water and steam: saturation-pressure equation (29b)/(30), coefficients Table 34, check values Table 35 (300 K → 0.003 536 589 41 MPa) (read 09.10.2026)
- NIST SP 811, Appendix B.8 – standard atmosphere 101 325 Pa, standard acceleration of gravity 9.806 65 m/s², 1 bar = 10⁵ Pa (read 09.10.2026)
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