Net positive suction head available is the amount by which the absolute pressure at a pump's suction exceeds the liquid's vapour pressure, expressed as a head of that liquid. If it falls too low the liquid boils inside the pump, the vapour bubbles collapse violently against the impeller, and the metal is eroded away. Cavitation destroys impellers, and the margin between what a system offers and what the pump needs is the whole point of the calculation.
This calculator works out NPSHa from figures you supply about your own system. It takes NPSHr — what the pump requires — as an input from the manufacturer's curve, because that is a measured property of a specific pump at a specific duty point and it is not something a web page can derive. Arb Digital built it this way on purpose: the arithmetic is straightforward and the pump data is not ours to invent.
What This NPSH Calculator Does
Enter the absolute pressure above the liquid, the liquid's vapour pressure at pumping temperature, its density, the static height difference between the liquid surface and the pump, and the friction losses in the suction line. The headline result is NPSHa in metres of liquid. The grid breaks out the surface pressure head and the vapour pressure head, then reports the margin over NPSHr both as a difference in metres and as a ratio.
The breakdown matters as much as the total, because it shows you which term is limiting. A system short on NPSHa because of friction loss has a different fix — larger suction pipe, fewer fittings — from one short because the liquid is hot, or because the pump sits too far above the sump.
What the calculator will not do is declare a system safe from cavitation. A positive margin is necessary and not sufficient: the margin required varies with the pump, the liquid, the flow relative to best efficiency point and the application, and the manufacturer sets it.
How to Use It
- Enter the absolute pressure at the liquid surface. For an open tank that is local atmospheric pressure, which is lower at altitude than the sea-level figure.
- Enter the vapour pressure at the pumping temperature, not at ambient. The vapour pressure calculator gives it for water and other liquids across a temperature range.
- Enter the density at that same temperature. The water density calculator covers the water case; hot water is measurably lighter than cold.
- Get the static head sign right. Positive when the liquid level is above the pump centreline, negative for a suction lift.
- Enter friction loss and the NPSHr from your pump curve at your actual duty point, then read the margin.
The Formula: How It's Calculated
NPSHa is an energy balance at the pump suction, written as a head of the liquid being pumped:
NPSHa = (Ps − Pv) ÷ (ρg) + Hstatic − Hfriction
where Ps is the absolute pressure on the liquid surface, Pv is the liquid's vapour pressure at pumping temperature, ρ is its density, g is gravitational acceleration, Hstatic is the height of the liquid surface above the pump centreline (negative for a lift), and Hfriction is the total loss in the suction line including fittings and the entrance. It is a direct application of the steady-flow energy balance set out in OpenStax University Physics Volume 1, section 14.6 on Bernoulli's equation.
The margin is NPSHa − NPSHr, and it is also quoted as the ratio NPSHa ÷ NPSHr. Guidance on how large that margin should be, by application, is published by the Hydraulic Institute in ANSI/HI 9.6.1, Rotodynamic Pumps Guideline for NPSH Margin; the changes in its 2024 edition, including the move from NPSH3 to manufacturer-published NPSHR as the reference metric, are summarised in the Hydraulic Institute's article on the 2024 updates to ANSI/HI 9.6.1.
A worked example matching the defaults on this page: water at 20 °C, an open sump at 101.325 kPa absolute, vapour pressure 2.34 kPa, density 998 kg/m³. The pressure term is (101,325 − 2,340) ÷ (998 × 9.80665) = 98,985 ÷ 9,787 = 10.114 m. The pump sits 2.5 m above the water, so the static head is −2.5 m, and the suction line loses 1.2 m to friction. NPSHa = 10.114 − 2.5 − 1.2 = 6.414 m. Against an NPSHr of 3.5 m from the curve, the margin is 2.914 m and the ratio is 1.83.
Why Temperature Is the Term That Catches People Out
Vapour pressure rises steeply with temperature, and it is subtracted directly from the available head. Water at 20 °C has a vapour pressure of 2.34 kPa, costing 0.24 m of head. At 80 °C it is 47.4 kPa, costing about 5.0 m. At 100 °C it equals atmospheric pressure, and the pressure term collapses to zero — which is why boiler feed and condensate systems are always arranged with a flooded suction and a substantial static head, never a lift.
Density falls at the same time, which slightly increases the head produced by a given pressure, but nothing like enough to compensate. The practical consequence is that a suction arrangement which works perfectly on cold water can cavitate the moment the process runs hot, with no change to the pipework at all. A system that will ever pump hot liquid has to be assessed at the hottest temperature it will see, not at commissioning temperature.
NPSHr Is a Pump Property, and It Moves With Flow
NPSH required is measured by the manufacturer on a test rig and plotted against flow rate on the pump curve. It rises with flow, often steeply toward the right-hand end of the curve, so a pump with comfortable margin at design flow can lose it entirely when a control valve opens or a parallel pump trips and the survivor runs out along its curve.
Historically the published figure has been NPSH3 — the suction head at which the pump's developed head has already fallen by 3% because of cavitation. That is worth sitting with: at the published NPSHr, the pump is cavitating measurably. It is a repeatable test criterion, not a threshold for cavitation-free operation, and it is precisely why a margin above NPSHr is required rather than optional. The 2024 edition of ANSI/HI 9.6.1 moves toward manufacturer-published NPSHR for this reason.
Speed matters too. NPSHr scales roughly with the square of speed, so a pump on a variable-speed drive has a different requirement at every speed, and slowing a pump down is one of the few changes that improves the suction situation on both sides of the comparison at once.
What Cavitation Actually Does
When local pressure inside the impeller eye drops below the vapour pressure, the liquid flashes to vapour. Those bubbles travel with the flow into a higher-pressure region and collapse. The collapse is not gentle: it produces a microjet and a local pressure spike intense enough to remove metal, and it repeats millions of times per hour.
The symptoms are distinctive. The pump sounds as though it is passing gravel. Discharge pressure and flow become unsteady. Vibration rises, and the damage appears as pitting on the impeller vane surfaces, concentrated where the bubbles collapse rather than where they form. Left running, it destroys impellers, wrecks mechanical seals and bearings through vibration, and can pit the casing.
Suction recirculation is a related failure that behaves differently: it occurs at low flow rather than high, damages the pressure side of the vanes, and is not fixed by adding NPSHa. Diagnosing which one you have matters, because the remedies are opposite — one wants more flow, the other less.
Where the Available Head Usually Goes Missing
Friction loss in the suction line is the term most often underestimated, because designers size suction pipe by habit rather than by calculation. Loss scales roughly with the square of velocity, so one pipe size larger on the suction is a disproportionate improvement. Fittings matter more than their length suggests: a partly closed valve, a poorly chosen foot valve, or a strainer that is fouling all consume head that then does not exist at the impeller. Run the numbers with the friction loss calculator and the pipe flow calculator rather than assuming.
Altitude quietly removes head too. Atmospheric pressure at 1,500 m is roughly 84 kPa rather than 101 kPa, which is about 1.7 m of water head gone before anything else has happened — enough on its own to turn a workable lift into a marginal one. The pressure converter and the manometer calculator help when your figures arrive in mixed units, and the power side of the duty point is covered by the pump horsepower calculator.
Arb Digital builds calculators that are precise about what they compute and honest about what they cannot, which is what makes specialists trust and link to them. Browse the library, or tell us what your audience needs.
Browse All Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Using gauge pressure for the surface term — the balance needs absolute pressure, and using gauge understates NPSHa by a full atmosphere.
- Taking vapour pressure at ambient temperature — it must be the value at the actual pumping temperature, and it rises steeply with heat.
- Getting the static head sign wrong — a lift is negative and a flooded suction is positive, and the two differ by twice the height.
- Reading NPSHr at the design point only — it rises with flow, so check it at the highest flow the pump will actually see.
- Treating a positive margin as proof of freedom from cavitation — the published requirement is usually the point at which head has already dropped 3%, so a margin above it is required.
Related Free Tools From Arb Digital
Get vapour pressure at temperature from the vapour pressure calculator and density from the water density calculator. Suction-line losses come from the friction loss calculator and the pipe flow calculator, static pressure at depth from the hydrostatic pressure calculator, and the energy balance itself from Bernoulli's equation. For the power required at the duty point see the pump horsepower calculator, and for unit work the pressure converter and the manometer calculator. Everything else is in the free online tools hub.
Frequently Asked Questions
NPSHa is what your system delivers to the pump suction, calculated from surface pressure, vapour pressure, static head and friction loss. NPSHr is what the pump needs, measured by the manufacturer and plotted on the pump curve against flow. The first is a property of the installation, the second a property of the machine.
That is set by the pump manufacturer and the application, not by a universal rule. The Hydraulic Institute publishes application-specific guidance in ANSI/HI 9.6.1, with different recommendations for process, power, water, slurry and building services duties. A positive margin is necessary but not on its own sufficient.
Yes, and often steeply. It rises as flow increases along the curve, so a pump with comfortable margin at its design point can lose it when a valve opens or a parallel pump stops. NPSHr also scales roughly with the square of pump speed, so a variable-speed installation has a different requirement at every speed.
Because vapour pressure rises steeply with temperature and is subtracted directly from the available head. Water costs about 0.24 m of head at 20 degrees Celsius and about 5.0 m at 80. At boiling point the pressure term vanishes entirely, which is why hot systems use flooded suctions rather than lifts.
It sounds like gravel passing through the pump, with unsteady discharge pressure and raised vibration. The damage shows as pitting on the impeller vanes, concentrated where the bubbles collapse rather than where they form, and it progresses to seal and bearing failure through the vibration it causes.
Yes, for any open system. Atmospheric pressure falls with elevation, so the surface pressure term shrinks. At around 1,500 metres the atmosphere provides roughly 1.7 metres less water head than at sea level, which is enough on its own to turn a workable suction lift into a marginal one.
No. The traditional published figure is NPSH3, the suction head at which the pump's developed head has already fallen by three per cent because of cavitation. It is a repeatable test criterion rather than a cavitation-free threshold, which is exactly why a margin above it is required.
This tool is provided for educational and engineering-estimate use. It computes NPSH available from the figures you supply and takes NPSH required as an input from the manufacturer's pump curve; it publishes no pump data and cannot determine that a system will operate free of cavitation. Pump selection, suction system design and the NPSH margin appropriate to an application must be established by a qualified engineer against the manufacturer's data and the applicable standard.
