NPSH
Calculate NPSH from atmosphere, suction, friction, and vapour
Last reviewed 21 October 2026.
NPSH is the pressure margin, written as head of the pumped liquid, between what the suction side can deliver and what the liquid needs to stay liquid at the impeller eye. Available NPSH is a property of the site, the fluid, and the suction line. It is not printed on the pump curve. The curve tells you what the pump needs. This page estimates only the available side, from four numbers you already have or can measure: atmospheric pressure, suction lift or flooded head, suction friction, and vapour pressure.
The arithmetic is CurvePoint’s, not a second formula typed into the page. Atmospheric pressure is matched to the app’s site-altitude model. Vapour pressure and atmospheric pressure are turned into head with the app’s fluid-head conversion. Suction lift or flooded head uses the same centerline datum as a study. The friction head you type is placed into the suction loss by the app’s loss evaluation at the flow you name, and the available NPSH is then read back from that evaluation. Every result is an estimate.
| Elevation | Pressure | Pressure | Head | Head |
|---|---|---|---|---|
| 0 m (0 ft) | 14.70 psia | 101.33 kPa | 33.96 ft | 10.35 m |
| 300 m (984 ft) | 14.18 psia | 97.77 kPa | 32.77 ft | 9.99 m |
| 600 m (1969 ft) | 13.68 psia | 94.32 kPa | 31.61 ft | 9.64 m |
| 1000 m (3281 ft) | 13.04 psia | 89.87 kPa | 30.12 ft | 9.18 m |
| 1500 m (4921 ft) | 12.26 psia | 84.56 kPa | 28.34 ft | 8.64 m |
Illustrative Lakeshore Process Equipment figures from CurvePoint’s own evaluation. Not a sealed calculation.
| Fluid | Temperature | Pressure | Pressure | Head | Head |
|---|---|---|---|---|---|
| Water | 10°C (50°F) | 0.177 psia | 1.22 kPa | 0.41 ft | 0.12 m |
| Water | 20°C (68°F) | 0.338 psia | 2.33 kPa | 0.78 ft | 0.24 m |
| Water | 30°C (86°F) | 0.614 psia | 4.23 kPa | 1.42 ft | 0.43 m |
| Water | 40°C (104°F) | 1.067 psia | 7.36 kPa | 2.48 ft | 0.76 m |
| Water | 60°C (140°F) | 2.882 psia | 19.87 kPa | 6.76 ft | 2.06 m |
| Diesel | 20°C (68°F) | 0.044 psia | 0.30 kPa | 0.12 ft | 0.04 m |
| Diesel | 35°C (95°F) | 0.107 psia | 0.74 kPa | 0.29 ft | 0.09 m |
| Gasoline | 20°C (68°F) | 7.977 psia | 55.00 kPa | 24.91 ft | 7.59 m |
| Gasoline | 35°C (95°F) | 14.523 psia | 100.13 kPa | 45.35 ft | 13.82 m |
Illustrative Lakeshore Process Equipment figures from CurvePoint’s own evaluation. Not a sealed calculation.
| Item | US | SI |
|---|---|---|
| Operating point | 431 gpm (27.2 L/s) at 69.0 ft (21.04 m) | 21.04 m head |
| Atmospheric head | 28.51 ft | 8.69 m |
| Vapour-pressure head | 2.48 ft | 0.76 m |
| Suction friction | 10.34 ft | 3.15 m |
| NPSH available | -2.32 ft (below zero) | -0.71 m (below zero) |
Illustrative Lakeshore Process Equipment figures from CurvePoint’s own evaluation. Not a sealed calculation.
NPSH available, estimate
21.18 ft
- Atmospheric head
- + 33.97 ft
- Vapour-pressure head
- − 0.79 ft
- Static suction lift
- − 10.00 ft
- Suction friction
- − 2.00 ft
Estimate for water near 68°F. Atmospheric pressure is matched with CurvePoint’s site-altitude function, and the friction head is placed with the app’s loss evaluation at 300 gpm. Discharge losses are not part of NPSH.
Barometric head, vapour, and a hot day at elevation
The first table is CurvePoint’s altitude model, written as head of 20°C water at 0, 300, 600, 1,000, 1,500 m and the foot equivalents. Atmospheric head falls as the site rises. The second table is the app’s vapour model: water at 10, 20, 30, 40, and 60°C, then diesel and gasoline at the temperatures the fuel catalog uses. Those fuel rows are catalogue models for an illustration. They are not a laboratory assay of a particular delivery. The tanker offload guide carries them into a hose.
The hot-day figure is water at 40°C with an 18 ft lift at 1,500 m. The pump curve and the system curve still cross, so there is an operating point on head. Available NPSH at that point is below zero. A crossing on the head chart is not a usable suction when the ledger has already spent the atmosphere on elevation, vapour, lift, and suction friction. Follow your specification or local standard for the margin you keep above the curve reading. The app field for that curve reading is NPSHr.
A published mine-sump decision describes an inlet held about 30 ft (9.1 m) below the water surface. That depth is submergence. It is not NPSH, and CurvePoint does not turn it into a suction-head term.
A screen that is barely covered can vortex and feed air while a handbook NPSH number still looks comfortable. Keep the two checks separate.
The four terms, in the order the ledger uses
Start with atmospheric head. At sea level, standard air is about 14.7 psia, which is a little over 33 ft of cold water. CurvePoint does not ask you to memorize that conversion. You enter psia, the page finds the site elevation whose modeled atmosphere matches it, and the study reports atmospheric head for water at that elevation. If you type a pressure the altitude model cannot reach, the nearest modeled pressure is used and the note under the calculator says so. Altitude is the usual reason the number is not 14.7. A plant at 1,000 ft is already down around 14.2 psia, and that missing half-psi is more than a foot of water.
Subtract vapour-pressure head next. Vapour pressure is the pressure at which the liquid boils at the temperature you have, expressed here as psia absolute, not gauge. Cold water is a small subtraction, a few tenths of a foot. Warm water, condensate, and light hydrocarbons are not small. The calculator overrides the water vapour pressure with the psia you enter, then converts it with the same head function, so a hotter day or a different fluid is a new input rather than a hidden table. If you only know temperature, look up the vapour pressure for that fluid and type the absolute pressure. Do not type a gauge reading from a suction transmitter and call it vapour pressure.
Static suction comes third, and the sign matters. Lift means the free surface is below the pump centerline, so the entire lift is subtracted. Flooded head means the surface is above the centerline, so that head is added. A wet well that looks “almost flooded” because the pipe is full can still be a lift if the surface sits below the impeller. Measure to the low water level you must run, not the average. The last subtraction is suction friction: pipe plus fittings between the surface and the suction nozzle, at the flow you are checking. Discharge friction does not belong in this ledger. It raises the system curve, which is a different page.
Why friction is tied to a flow
A friction head without a flow is an incomplete statement. Loss grows with velocity squared, and velocity is flow over the inside area. This calculator does not rebuild your suction pipe. It takes the friction head you believe applies at the flow you type and places that loss in the suction evaluation, so the NPSH result uses CurvePoint’s head balance rather than a hand subtraction that can drift from the app. If the real loss was calculated at 200 gpm and the pump will run at 400 gpm, do not reuse the 200 gpm head. Recompute it, or open the system curve calculator and read suction friction at the intersection.
Suction velocity is the usual place NPSH is wasted by accident. A long, small suction line, a clogged strainer, several elbows at the pump, and a check valve on the suction side all show up as friction head. They also make priming harder on a lift. Keep suction K low. Put the control valve, the meter, and the long run on the discharge. The suction lift guide is the layout discussion; this page is the arithmetic once you have a friction number.
What the pump curve still has to say
Available NPSH is only half of the check. The pump curve publishes the head the impeller needs at each flow. In the CurvePoint app that published value is entered as NPSHr, the water-curve reading, and a margin is applied on top of it. The surplus is available NPSH minus the required value. This page stops before that comparison on purpose. A margin is a judgment about the service, the liquid, and how often the pump runs away from its best efficiency flow. Naming a standard such as ANSI/HI 9.6.1 is not the same as copying its tables into a marketing page. Use the preset inside the app, and read the curve at the flow you will actually run, not at best efficiency if you will not be there.
NPSH required climbs as you move right on most curves, while available NPSH falls because suction friction climbs. The comfortable point at half flow can be the failed point at the end of the curve. If you only check one flow, check the highest flow you will allow, not the design average. A throttle that cuts flow can improve the NPSH picture while wasting energy. A speed reduction changes both curves. Both effects are on the operating point page.
What this estimate does not know
The ledger assumes the friction head, the vapour pressure, and the atmospheric pressure you typed, and it assumes water’s density for the head conversion except for the vapour override. It does not know about air leaks, dissolved gas, a vortex at the inlet, a clogged screen, or a fluid hotter than the vapour pressure you used. It does not add velocity head a second time if that head is already inside the published NPSH of the pump. Double-counting the suction nozzle velocity is a common way to make a passing installation look like it fails.
Submersible pumps sitting in the liquid are a different problem: there is no suction pipe, and the static term is submergence. Do not force this calculator to describe that duty by typing a negative lift you invented. Use the app’s submersible pump type, which drops the suction line. Self-priming lifts, end-suction transfers, and fuel offloads are the duties this ledger is meant for.
When the estimate is close to the required value, stop treating one decimal place as a decision. Rebuild the suction line in the calculator, plot the curve, and let the study compare the two heads at the intersection. The fictional wastewater sample shows that comparison in a finished report: available head in the high teens of feet against a required value just under 10 ft. The homepage has the same sample PDF, and pricing covers the export.