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NPSH Available Calculator

For a steady liquid supply from a large vessel or reservoir, with negligible free-surface velocity and an incompressible liquid. Elevation is positive above the pump’s NPSH datum and negative below it.

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    Formula and method

    NPSHa = (p_abs − pv_abs)/(ρg) + z − h_loss, with g = 9.80665 m/s². Convert input pressure from kPa to Pa by multiplying by 1000. If a curve NPSHr is supplied, difference = NPSHa − NPSHr. Free-surface velocity head is taken as zero.

    Worked example

    Example: surface pressure 101.325 kPa absolute, vapour pressure 2.34 kPa absolute, density 1000 kg/m³, elevation +1 m and suction loss 0.8 m. NPSHr is blank by default; use the actual pump curve if comparing.

    Assumptions and limits

    • Both pressures are absolute, not gauge readings. Use local atmospheric pressure for an open reservoir and actual gas-space absolute pressure for a pressurised vessel.
    • Supply vapour pressure and density for the actual fluid and temperature. No vapour table or automatic altitude/temperature correction is built in; defaults are example inputs.
    • Include entrance, pipe, strainer, valve and fitting losses at the operating flow, in metres of the pumped liquid. Use the same pump datum for elevation and losses.
    • NPSHr must come from a manufacturer curve matching flow, speed, impeller and fluid conditions, with its test criterion confirmed. A positive difference does not certify cavitation-free operation or an adequate design margin; no universal margin is assumed.
    • Do not substitute suction-gauge readings directly or use this model for significant surface velocity, two-phase flow or transients. Horizontal pumps often use a datum near the shaft centre; confirm the manufacturer’s datum for other arrangements.

    Frequently asked questions

    How do I enter a liquid level below the pump?

    Enter a negative elevation. For a surface 2 m below the pump datum, enter −2; suction losses remain nonnegative.

    Can I calculate without knowing NPSHr?

    Yes, system NPSHa can be calculated. Leave NPSHr blank; the difference shows “—” and no comparison is made. Unknown NPSHr is not treated as zero.

    What does a negative NPSHa mean?

    The negative result is retained and flagged. The entered head budget does not maintain the suction conditions assumed by this model. Recheck pressures, vapour pressure, elevation, losses and operating conditions.

    References

    NPSH Available Calculator practical guide and common questions

    What does NPSH Available Calculator calculate?

    For a steady liquid supply from a large vessel or reservoir, with negligible free-surface velocity and an incompressible liquid. Elevation is positive above the pump’s NPSH datum and negative below it. NPSHa = (p_abs − pv_abs)/(ρg) + z − h_loss, with g = 9.80665 m/s². Convert input pressure from kPa to Pa by multiplying by 1000. If a curve NPSHr is supplied, difference = NPSHa − NPSHr. Free-surface velocity head is taken as zero.

    When is this useful?

    Example: surface pressure 101.325 kPa absolute, vapour pressure 2.34 kPa absolute, density 1000 kg/m³, elevation +1 m and suction loss 0.8 m. NPSHr is blank by default; use the actual pump curve if comparing.

    How do I enter the inputs?

    Enter the inputs for NPSH Available Calculator: Absolute pressure at free surface, Absolute vapour pressure at operating temperature, Liquid density at operating temperature, Surface elevation above NPSH datum (may be negative), Suction-line head loss, Pump-curve NPSHr (optional; blank if unknown). Calculate, check the units and assumptions, then copy or download the results.

    How should I interpret the results?

    Both pressures are absolute, not gauge readings. Use local atmospheric pressure for an open reservoir and actual gas-space absolute pressure for a pressurised vessel. Supply vapour pressure and density for the actual fluid and temperature. No vapour table or automatic altitude/temperature correction is built in; defaults are example inputs. Include entrance, pipe, strainer, valve and fitting losses at the operating flow, in metres of the pumped liquid. Use the same pump datum for elevation and losses. NPSHr must come from a manufacturer curve matching flow, speed, impeller and fluid conditions, with its test criterion confirmed. A positive difference does not certify cavitation-free operation or an adequate design margin; no universal margin is assumed. Do not substitute suction-gauge readings directly or use this model for significant surface velocity, two-phase flow or transients. Horizontal pumps often use a datum near the shaft centre; confirm the manufacturer’s datum for other arrangements.

    Where are my inputs processed?

    NPSH Available Calculator runs in this browser. Inputs are not uploaded to a calculation server; copying and CSV download require your action. The existing site analytics and advertising still load.