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Pumps & Rotating Equipment

NPSH Available Calculator

Net positive suction head available (NPSHa) describes how much the absolute pressure at the pump suction exceeds the liquid vapour pressure, expressed as a head of liquid. This calculator estimates NPSHa from the suction-side conditions — absolute pressure at the liquid surface, static suction head, vapour pressure, and suction losses. It is a preliminary estimate only: NPSHa is the system side of the cavitation check and must be compared against the pump NPSHr from the vendor curve. An interactive five-case arrangement library (paraphrased from the Cameron-style hydraulic handbook treatment) helps you set up the terms correctly for the most common suction layouts: an open tank with flooded suction, an open tank with suction lift, a closed or pressurised vessel, a closed vessel under vacuum, and hot or near-boiling service. Each case explains how to treat the source-pressure and static-head terms, what to watch out for, and whether it usually raises or lowers NPSHa — then suggests reasonable, fully editable starting values. The five cases exist because NPSHa depends on four levers: the source pressure, the suction elevation (static head), the liquid vapour pressure, and the suction losses. Raising the source pressure or static head adds margin; a low absolute source pressure (vacuum) or a high vapour pressure (hot liquid) removes it sharply, because they attack the two terms that normally supply the margin. Whatever the layout, NPSHa is only useful when compared against the vendor NPSHr at the duty flow, with an appropriate margin on top — the case library is a setup aid for an estimate, never a substitute for vendor data.

TypeInteractive engineering calculator

Calculator

Suction arrangement — Cameron-style case library

Pick the arrangement that matches your suction layout. The library is a setup aid: it switches to pressure terms and suggests reasonable, fully editable starting values and sign conventions for that case. It does not replace the formula — NPSHa = Ha + Hs − Hv − Hf. Positive static head means a flooded suction; negative static head means a suction lift.

Suction source (absolute pressures)
kg/m³
kPa abs
kPa abs
m/s²

Standard gravity (editable)

Suction line
m

+ flooded suction, − suction lift

m

Estimate with Pipe Head Loss + Minor Loss

m

From the vendor pump curve, for a margin check

Absolute pressure head (Ha)10.3323 m
Vapour pressure head (Hv)0.238614 m
Static suction head (Hs)3 m
Suction losses (Hf)1.2 m
NPSH available11.8937 m

Preliminary NPSHa estimate only. Not pump selection, not NPSHr, not a cavitation guarantee, and not a transient/surge analysis. Final pump selection requires vendor data, site conditions, suction piping design, fluid properties, operating cases, and qualified engineering review.

Related: Pump Sizing · Total Dynamic Head · Head ↔ Pressure · Pipe Head Loss · Minor Loss

Formulas

Head terms
NPSHa = Ha + Hs − Hv − Hf
Pressure terms
NPSHa = (P_abs − P_vap) / (ρ·g) + z_static − h_losses
Pressure head
Ha = P_abs / (ρ·g), Hv = P_vap / (ρ·g)

Diagram

liquid surfaceP_abs (Ha)HsHf (losses)pump suctionNPSHa =Ha + Hs− Hv − Hf

Worked example

Water at 1000 kg/m³ is drawn from a source at 101.325 kPa abs with a vapour pressure of 2.34 kPa abs. The static suction head is +3.0 m (flooded) and suction losses are 1.2 m. Estimate NPSHa using the pressure-terms mode (g = 9.80665 m/s²).

  1. 01Ha = 101325 / (1000 × 9.80665) = 10.33 m
  2. 02Hv = 2340 / (1000 × 9.80665) = 0.24 m
  3. 03NPSHa = 10.33 + 3.0 − 0.24 − 1.2
  4. 04NPSHa = 11.89 m
Result

NPSHa ≈ 11.89 m. Compare against the pump NPSHr (with margin) from the vendor curve before selection.

FAQ

What is the difference between NPSHa and NPSHr?
NPSHa (available) is a property of your system — the suction-side pressure margin above vapour pressure. NPSHr (required) is a property of the pump, read from the vendor curve at the duty flow. To avoid cavitation you need NPSHa to exceed NPSHr by a margin. This calculator estimates NPSHa only.
When is static suction head negative?
Static suction head is negative for a suction lift — when the pump centreline sits above the liquid surface and the pump must lift the liquid. It is positive for a flooded suction, where the liquid surface is above the pump.
Why does hot liquid reduce NPSHa?
Vapour pressure rises sharply with temperature. As the liquid gets hotter, Hv increases and directly subtracts from NPSHa, which is why hot-liquid and near-boiling services are the most cavitation-prone. Use the vapour pressure at the actual pumping temperature.
Should I include a safety margin?
Yes. Enter the pump NPSHr if you have it to see the margin (NPSHa − NPSHr). Industry practice (e.g. API 610 / Hydraulic Institute guidance) is to keep a margin above NPSHr; a margin near zero is a cavitation risk and should be reviewed by a qualified engineer.
Does this account for suction piping design?
Only through the suction-losses input, which you estimate separately (for example with the Pipe Head Loss and Minor Loss calculators). The calculator does not size suction piping or solve a pipe network.
What is the five-case library for?
It is an interactive setup aid, paraphrased from the Cameron-style hydraulic handbook treatment, covering five common suction arrangements: open tank with flooded suction, open tank with suction lift, closed or pressurised vessel, closed vessel under vacuum, and hot or near-boiling service. Picking a case explains how to treat the source-pressure and static-head terms, what to watch out for, and whether the arrangement usually raises or lowers NPSHa, then suggests reasonable, fully editable starting values. It guides how you set up the terms — it does not replace the formula or make any hidden assumption.
Why do vacuum and hot-liquid sources reduce NPSHa so sharply?
NPSHa is built from the source-pressure head plus static head, minus the vapour-pressure head and losses. A vacuum source has a low absolute pressure, so the source-pressure term that normally supplies the margin is small — which is why vacuum services rely on a tall flooded suction leg. A hot or near-boiling liquid has a high vapour pressure that subtracts a large head, and at saturation the source pressure and vapour pressure nearly cancel, leaving NPSHa to come almost entirely from static head minus losses. Both cases attack the terms that provide margin, so NPSHa drops fast.
How much margin should NPSHa have over NPSHr?
NPSHa (system) must exceed NPSHr (pump, from the vendor curve at the duty flow) by a margin, not just match it. Industry practice (for example API 610 and Hydraulic Institute guidance) sets the margin by service — larger for hot, volatile, or high-energy duties. This calculator estimates NPSHa and can show NPSHa − NPSHr if you enter NPSHr, but the required margin and final selection must come from vendor data and a qualified engineer.

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