NPSH Available Calculator

Cavitation destroys pumps quietly until it doesn’t. Enter your suction conditions to get NPSH available — then confirm it beats the pump’s required NPSH with margin to spare.

Site elevation (ft)Sets atmospheric pressure via the standard atmosphere. Default 0 (sea level).

Water temperature

Temperature unit

Static suction head (ft)Liquid level above (+) or below (−) the pump centerline. Suction lift is negative.

Suction-line friction + minor losses (ft)Total head loss from the source to the pump inlet at design flow.

Net Positive Suction Head Available

Item	Value

How the calculation works

NPSH available
NPSH_a = (P_atm − P_vapor) / γ + h_s − h_L
P_atm = 14.696 · (1 − 6.8753×10⁻⁶ · elevation)^5.2559 psia (standard atmosphere)
P_vapor from the Antoine equation: log₁₀(P) = 8.07131 − 1730.63 / (233.426 + T_°C), P in mmHg
γ = 62.4 lb/ft³; ft of head = psi × 2.3067
h_s is positive for flooded suction, negative for suction lift.

Water temp.	Vapor pressure	As head
60 °F	0.26 psia	0.6 ft
68 °F	0.34 psia	0.8 ft
80 °F	0.51 psia	1.2 ft
100 °F	0.95 psia	2.2 ft
140 °F	2.89 psia	6.7 ft
200 °F	11.53 psia	26.6 ft

Notes & limitations

NPSH_a is a system property; NPSH_r (required) comes from the pump manufacturer’s curve at your operating point. Keep NPSH_a above NPSH_r with margin — the Hydraulic Institute recommends margin for critical services, commonly 2–3 ft or more depending on the application.

Vapor pressure climbs fast with temperature — hot-water and condensate pumps need the most NPSH attention.
Suction-line losses must be evaluated at the actual (maximum) flow, not average flow.
Entrained air, dissolved gases, and vortexing at the intake reduce effective NPSHa — this calculation assumes clean, air-free water.
For suction lift installations, also check priming and the practical lift limit (about 25 ft theoretical maximum at sea level).

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