Pump NPSH Calculator

Inputs

Absolute suction pressure (P_abs)

Absolute pressure at the suction vessel/free surface or pump suction datum. Atmospheric pressure is about 1.013 bar / 14.7 psi absolute.

bar

Static suction head (H_s)

Positive when liquid level is above the pump centerline; negative for suction lift.

Fluid vapor pressure (P_v)

Use vapor pressure at the pumping temperature. Water near 20 C is about 0.023 bar absolute.

bar

Suction line head loss (h_f)

Friction and minor losses in the suction line, expressed as head.

Fluid density (rho)

Density at the operating temperature.

kg/m³

Pump NPSHr (NPSHr)

Required NPSH from the pump curve at the design flow rate.

Default result

NPSH available(NPSHa)

10.11m

Pass

Absolute pressure head plus static head, minus vapor pressure head and suction losses.

Also computed

NPSH margin(NPSHa - NPSHr)7.114m

NPSH ratio(NPSHa/NPSHr)3.371

Absolute pressure head(P_abs/(rho g))10.35m

Vapor pressure head(P_v/(rho g))0.2391m

NPSHa pressure equivalent(rho g NPSHa)0.9898bar

Method notes 3 notes

NPSHa = P_abs/(rho*g) + static suction head - P_v/(rho*g) - suction line head loss.
Compare against NPSHr from the pump curve at the same flow. Many installations need additional margin beyond simply exceeding NPSHr.
Final pump selection still needs the full system curve, inlet geometry, acceleration head, strainers, valves, temperature, altitude and manufacturer guidance.