NPSH Calculator — Net Positive Suction Head
Net Positive Suction Head (NPSH) determines whether a pump will cavitate. Enter the surface pressure, liquid vapour pressure, density, static suction head, friction losses and the pump's NPSH required to find NPSH available and the safety margin.
Pa
Pa
kg/m³
m
m
m
Safe — NPSH available exceeds NPSH required
- 1
Pressure head (Pa − Pv) ÷ ρg
(101,325 − 2,338) ÷ (998 × 9.81) = 10.111Net pressure above vapour pressure converted to metres of liquid head. - 2
NPSH available = pressure head + Hs − Hf
10.111 + -2 − 0.5 = 7.611
How does this calculator work?
NPSH available = (P_surface − P_vapour) / (ρg) + static head − friction loss. Cavitation risk is eliminated when NPSH_a exceeds the pump's NPSH required from its curve. Default values represent water at 20 °C pumped from 2 m below the pump with 0.5 m of suction-line friction losses.
Formula
How this is calculated
Cavitation occurs when the absolute pressure in the pump suction falls below the liquid's vapour pressure, causing bubbles to form and collapse violently — damaging impellers and reducing flow. NPSH Available (NPSH_a) is the energy margin above vapour pressure expressed as metres of liquid head.
NPSH_a is calculated from four contributions: the pressure head term (P_a − P_v) / (ρg) represents how far the surface pressure exceeds vapour pressure; H_s is the static suction head (positive if the liquid surface is above the pump, negative if the pump sits above the sump — negative values directly subtract from the margin); H_f is the head lost to friction in the suction pipe and fittings. Increasing pipe diameter, shortening the suction line and minimising bends all reduce H_f and improve margin.
The pump manufacturer specifies NPSH Required (NPSH_r) on the performance curve at each flow rate — it rises at higher flows. For safe operation NPSH_a must exceed NPSH_r; most engineers add a safety buffer of at least 0.5–1 m (sometimes 10–20% of NPSH_r). Vapour pressure rises sharply with temperature — hot water near 100 °C has P_v close to atmospheric, leaving almost no margin — so temperature is one of the most critical design variables.
Frequently asked questions
NPSH available (NPSH_a) is a property of the system — determined by the installation geometry, pressures and liquid properties. NPSH required (NPSH_r) is a property of the pump — published on the pump's performance curve and stating the minimum head needed to avoid cavitation. Safe operation requires NPSH_a > NPSH_r, usually with a margin of at least 0.5 m.
Cavitation collapses vapour bubbles near the impeller at very high local pressures (thousands of bar), eroding metal, causing noise (sounds like gravel in the pump), reducing flow and head, and ultimately causing impeller failure. Even brief cavitation can cause permanent damage to stainless steel and cast iron impellers.
Lower the pump (increase H_s positively), raise the liquid level, use a flooded suction, cool the liquid (lowers vapour pressure), enlarge the suction pipe diameter (reduces friction head H_f), or install an inducer upstream of the impeller. Reducing pump speed also reduces NPSH_r, giving more margin at the same installation.
Also known as
TG we-Calculate Editorial Team. (2026). NPSH Calculator — Net Positive Suction Head [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/npsh-net-positive-suction-head-calculator
TG we-Calculate Editorial Team. "NPSH Calculator — Net Positive Suction Head." TG we-Calculate. 2026. https://we-calculate.com/calculator/npsh-net-positive-suction-head-calculator.
TG we-Calculate Editorial Team, "NPSH Calculator — Net Positive Suction Head," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/npsh-net-positive-suction-head-calculator
@misc{wecalculate_npsh_net_positive_suction_head_calculator, title = {NPSH Calculator — Net Positive Suction Head}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/npsh-net-positive-suction-head-calculator}}, year = {2026}, note = {TG we-Calculate} }
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