NPSH Calculator – Net Positive Suction Head

Calculate the Net Positive Suction Head available (NPSHa) at a pump's suction from atmospheric pressure, static head, vapor pressure, and friction losses, then compare it to the pump's required NPSH to check for cavitation risk.

Quick Facts

NPSHa formula
NPSHa = ha + hs − hvp − hf
Atmospheric head plus static head, minus vapor-pressure head and friction losses.
Sea-level atmosphere
14.7 psi ≈ 33.9 ft (10.33 m) of water head
This is the ceiling on NPSHa for an open tank at sea level.
Cavitation margin rule
NPSHa ≥ 1.1–1.3 × NPSHr
Hydraulic Institute guidance (ANSI/HI 9.6.1) to avoid cavitation damage.
Static head sign
Positive = flooded suction, negative = suction lift
Positive when the liquid surface sits above the pump centerline.

Your Results

Calculated
NPSH Available (NPSHa)
-
Head above the liquid's vapor pressure at the suction
Atmospheric/Source Head (ha)
-
Source pressure converted to head
Vapor Pressure Head (hvp)
-
Liquid vapor pressure converted to head
Margin vs. NPSHr
-
NPSHa − NPSHr (cavitation buffer)

Ready

Enter your suction system values, then press Calculate.

How to Calculate NPSH Available (NPSHa)

Net Positive Suction Head is the excess pressure head, above the liquid's vapor pressure, that exists at a pump's suction inlet. Pump manufacturers publish a required NPSH (NPSHr) for each pump — the minimum head their impeller needs to avoid vaporizing the liquid at the eye of the impeller. Your piping system independently produces an available NPSH (NPSHa), based on the source pressure, elevation, liquid properties, and pipe friction. This calculator computes NPSHa and — if you supply the pump's NPSHr — the margin between them.

The NPSHa formula

NPSHa is calculated in head units (feet or meters of liquid) as:

NPSHa = ha + hs − hvp − hf

  • ha — atmospheric or source pressure head: the absolute pressure acting on the surface of the liquid supply (atmospheric for an open tank, or atmospheric plus any tank pressurization for a closed vessel), converted to head using ha = P / (ρg).
  • hs — static suction head: the vertical distance between the liquid surface and the pump's impeller centerline. It is positive for a flooded suction (source above the pump) and negative for a suction lift (source below the pump).
  • hvp — vapor pressure head: the liquid's vapor pressure at the pumping temperature, converted to head the same way as ha. Hotter liquids have higher vapor pressure and therefore lower NPSHa.
  • hf — friction head loss: the total head lost to pipe friction, fittings, valves, strainers, and entrance losses in the suction line between the source and the pump.

Avoiding cavitation: NPSHa vs. NPSHr

A pump cavitates when local pressure at the impeller eye falls to the liquid's vapor pressure, causing vapor bubbles to form and then collapse violently as they move into higher-pressure zones — this pits the impeller, causes vibration, and can destroy a pump within weeks. To prevent it, NPSHa must stay above NPSHr with a safety margin across the whole operating range, not just at the design point. The Hydraulic Institute standard (ANSI/HI 9.6.1) recommends a margin ratio of roughly 1.1 to 1.3 times NPSHr (or a minimum absolute margin, commonly 3-5 ft / 1-1.5 m), with a larger margin for pumps running near the end of their curve or handling hot or volatile liquids.

Practical notes

  • Keep all head terms (ha, hs, hvp, hf) in the same length unit before adding or subtracting them; this calculator converts pressure inputs to head internally using your chosen specific gravity.
  • Vapor pressure rises sharply with temperature — always look up vapor pressure at the actual pumping temperature, not at room temperature, especially for hot water or volatile solvents.
  • To raise NPSHa on an existing system: raise the liquid level, shorten or upsize the suction pipe, remove unnecessary fittings, cool the liquid, or pressurize the source tank.

Frequently Asked Questions

What is the difference between NPSHa and NPSHr?
NPSHa (available) is a property of the piping system — how much pressure head above the liquid's vapor pressure actually reaches the pump suction, based on atmospheric pressure, elevation, vapor pressure, and friction losses. NPSHr (required) is a property of the pump itself, determined by the manufacturer through testing and published on the pump curve. For safe operation, NPSHa must exceed NPSHr by an adequate margin at every point on the operating curve.
Why does the liquid's vapor pressure matter for NPSH?
If the local pressure at the impeller eye drops to or below the liquid's vapor pressure, the liquid flashes into vapor bubbles (cavitation). Those bubbles collapse violently when they reach higher-pressure regions of the impeller, causing noise, vibration, and pitting damage. Subtracting the vapor pressure head from the available pressure head is what tells you how much true margin exists before that happens.
What happens if NPSHa is less than NPSHr?
The pump will cavitate: vapor bubbles form at the impeller eye and collapse downstream, producing a crackling or gravel-like noise, reduced flow and head, vibration, and accelerated erosion of the impeller and casing. Left uncorrected, cavitation can destroy an impeller in weeks. The fix is to raise NPSHa (higher liquid level, shorter or larger suction pipe, cooler liquid) or select a pump with a lower NPSHr.
How can I increase NPSHa for a pump installation?
Raise the liquid source above the pump (or reduce suction lift), shorten the suction line and minimize elbows/valves to cut friction loss, use a larger-diameter suction pipe, lower the liquid temperature to reduce its vapor pressure, or pressurize the source tank. Any of these increases the atmospheric/static head terms or reduces the vapor-pressure and friction-loss terms in the NPSHa equation.

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