Calculate net positive suction head available from altitude or vessel pressure, liquid temperature, static head, and friction — with built-in atmospheric-pressure and vapor-pressure lookups and a cavitation-risk verdict.
US + metric
Formula shown
Shareable results
NPSH Available
ft
psia
°F
psia
SG
ft
ft
ft
ft
Atmospheric / surface pressure14.7 psia
Vapor pressure at 68 °F0.339 psia
NPSH Available23.2ft(7.06 m)
NPSH margin: 11.2 ftMargin ratio: 1.93
Adequate margin (ratio 1.93). HI guidance is service-dependent (often 1.1–1.5×).
K = 2.31 ft/psi (US) or 10.2 m/bar (SI). Atmospheric pressure is looked up from altitude via the ISA barometric formula; water vapor pressure is interpolated from a saturation table by temperature.
For preliminary sizing and educational use. Verify final design against manufacturer NPSHr curves and applicable codes; final designs should be reviewed by a licensed professional engineer.
NPSH available is the absolute pressure at the pump suction, above the liquid’s vapor pressure, expressed as head. It must comfortably exceed the pump’s NPSH required, or the liquid flashes to vapor and the pump cavitates. NPSH available equals atmospheric (or vessel) pressure head minus vapor-pressure head, plus static suction head, minus suction friction loss, measured in feet or metres.
Worked example
An open tank at sea level supplies water at 68 °F (20 °C). The pump sits 8 ft above the water surface (a suction lift), and the suction line loses 2 ft to friction. Atmospheric pressure is 14.7 psia and the vapor pressure of water at 68 °F is about 0.34 psia:
Static suction head: −8 ft (lift), friction: −2 ft
NPSHa = 33.2 − 8 − 2 = 23.2 ft (7.06 m)
Against an NPSHr of 12 ft: margin 11.2 ft, ratio 1.93 — adequate
Frequently asked questions
What is the difference between NPSHa and NPSHr?
NPSH available is a property of your system — how much suction head the installation provides. NPSH required is a property of the pump, published by the manufacturer as the head needed at the impeller eye to avoid cavitation at a given flow. Cavitation is avoided when NPSHa exceeds NPSHr by a margin; the Hydraulic Institute suggests roughly 1.1–1.5× depending on service and energy level, so treat the ratio as service-dependent, not a universal number.
Why does temperature matter so much?
Vapor pressure rises steeply with temperature. Water’s vapor pressure is 0.34 psia at 68 °F but about 14.7 psia at 212 °F — at which point boiling water at sea level has essentially zero NPSHa. Hot-water and condensate services are where NPSH problems bite hardest, which is why this calculator looks vapor pressure up from temperature for you.
Why does altitude reduce NPSHa?
On an open system the driving pressure is atmospheric, and atmosphere thins with elevation: about 14.7 psia at sea level but near 12.2 psia at 5,000 ft. That lost ~2.5 psi is roughly 5.8 ft of water head removed straight off your NPSHa — often the difference between a pump that primes and one that cavitates in mountain installations.
Variables
Symbol
Meaning
US unit
SI unit
P_atm
Absolute pressure on the liquid surface (atmospheric for an open tank)
psia
bar a
P_vap
Vapor pressure of the liquid at pumping temperature
psia
bar a
h_static
Static suction head (positive if source above pump, negative if a lift)
