NPSH Calculator (net positive suction head available vs required)

Understanding the NPSH calculator — concept and principle

This calculator finds the net positive suction head available (NPSHa) at a pump's inlet and compares it with the NPSH the pump requires, to check the risk of cavitation. It takes the surface pressure, vapour pressure, liquid density, suction height and suction losses. Pump engineers and plant operators use it when laying out suction lines and selecting pumps.

Atmospheric head minus lift, losses and vapour pressure is what the pump has; it needs 3.5 m.

How it is calculated

NPSHa = H_p ± H_z − H_f − H_vp

H_p = p_surface ÷ (ρ g); H_vp = p_vapour ÷ (ρ g)

H_z is the liquid level above (+, flooded suction) or below (−, suction lift) the pump centreline, H_f the friction and entrance losses in the suction line at the operating flow.
Pressures are absolute, in kPa; g = 9.80665 m/s². Margin = NPSHa − NPSHr.

The principle

Liquid boils when its pressure falls to its vapour pressure. At a pump's impeller eye the pressure is lowest, so if it drops too far, vapour bubbles form and then collapse violently further in, damaging the impeller and reducing performance; this is cavitation. NPSHa is how far the pressure at the inlet stays above the vapour pressure, expressed as a head of liquid. It must exceed the NPSH required on the pump's curve, with a margin.

Worked example

Default inputs: surface pressure 101.325 kPa, vapour pressure 2.34 kPa (water near 20 °C), ρ = 998 kg/m³, level −3 m, losses 0.8 m, NPSHr 3.5 m.

H_p = 101,325 ÷ (998 × 9.80665) = 10.35 m.
H_vp = 2,340 ÷ (998 × 9.80665) = 0.24 m.
NPSHa = 10.35 − 3 − 0.8 − 0.24 = 6.31 m = 20.7 ft.
Margin = 6.31 − 3.5 = 2.81 m; ratio = 1.8; check: meets the pump requirement.

Each point is the calculator re-run with only “Liquid density (kg/m³)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Liquid density (kg/m³)	NPSH available (m)
250	36.66
499	16.43
748	9.69
998	6.31
1,248	4.29
1,497	2.94
1,996	1.26

How to read the result

Hot liquids have much higher vapour pressure and therefore much lower NPSHa; at temperatures near boiling, a flooded suction may be required. High altitude lowers atmospheric pressure and also reduces NPSHa. Designers usually add a safety margin above NPSHr, because NPSHr on pump curves is typically defined at a point where cavitation already reduces head.

Common mistakes

Using gauge pressure instead of absolute pressure.
Ignoring the rise of vapour pressure with temperature.
Calculating losses at the wrong flow rate.
Treating a suction lift as positive.

Keep in mind: Pump selection should use the manufacturer's NPSHr curve and appropriate margins, and be checked by a qualified engineer.

Frequently asked questions

What happens when NPSHa is too low?

The pump cavitates: noise, vibration, loss of head and impeller damage.

Why does hot water cause cavitation more easily?

Its vapour pressure is much higher, so less margin remains.

Does altitude matter?

Yes. Lower atmospheric pressure reduces the pressure head on an open tank.

How can I raise NPSHa?

Raise the liquid level, shorten or enlarge the suction pipe, or cool the liquid.

Sources: Wilo technical note "Net Positive Suction Head (NPSH)"

Checked 9 Nov 2026. Wilo technical note “Net Positive Suction Head (NPSH)”: H_sv = H_p ± H_z − H_f − H_vp (absolute pressure on the liquid surface, static elevation above or below the impeller centreline, suction friction and entrance losses, vapour pressure of the liquid), and the rule that available NPSH must be at least the pump’s required NPSH. Head = pressure ÷ (ρg) with standard gravity 9.80665 m/s². Water vapour pressures in the hint are standard table values.

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