Inputs

All pressure inputs are absolute. For an open tank, surface pressure is local atmospheric pressure, not 0 psig.

Liquid density

Density unit

Pressure at liquid surface, absolute

Surface pressure unit

Liquid vapor pressure, absolute

Vapor pressure unit

Liquid-surface elevation relative to pump centerline

Elevation unit

Positive if the liquid surface is above the pump; negative for suction lift.

Suction-line friction loss

Loss unit

NPSH required (optional)

NPSHr unit

Liquid-water properties reference ↗ · Pure water, 0–99°C at 100 kPa absolute.

Results

NPSH available

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NPSH available

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Margin above NPSHr

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Equation and assumptions

NPSHₐ = (Psurface,abs − Pvapor)/(ρg) + z − h_f

This reservoir-to-pump form assumes negligible velocity at the supply free surface and that the entered suction friction loss includes the entrance, pipe, and fittings up to the pump suction flange. NPSH required is pump-specific and must come from the pump manufacturer at the operating flow and speed.

What NPSH available means

Net positive suction head available, NPSHₐ, is a property of the system. It expresses how far the liquid's total suction head at the pump inlet is above the liquid vapor-pressure head. Low NPSH can allow local pressure in the pump inlet or impeller eye to fall far enough for vapor bubbles to form and collapse, which can reduce performance and damage equipment.

The equation on this page is the reservoir-to-pump form. It starts with the absolute pressure acting on the supply liquid, subtracts vapor pressure, adds or subtracts the liquid-level elevation relative to the pump, and subtracts suction-line losses.

When to use it

Centrifugal and other rotodynamic pump suction checks.
Comparing pump elevation or supply-tank level alternatives.
Evaluating the effect of higher liquid temperature, which raises vapor pressure.
Evaluating suction-pipe sizing, strainer loss, or other suction-side restrictions.

Assumptions

Surface pressure and vapor pressure are entered as absolute pressures.
The supply free-surface velocity is negligible.
The elevation is measured from the liquid surface to the pump reference elevation using the sign shown in the input.
Suction loss includes entrance, straight pipe, fittings, valves, strainers, and other losses up to the pump suction reference point.
Density and vapor pressure represent the liquid at the suction temperature.

Common mistakes

Entering 0 psig for an open tank. The equation needs absolute pressure. For an open tank, use local atmospheric pressure.
Ignoring altitude. Atmospheric pressure falls with elevation, reducing NPSH available.
Using vapor pressure at the wrong temperature. This can be a major error for hot liquids or volatile solvents.
Forgetting suction fittings or dirty strainers. Every suction-side loss reduces NPSH available.
Assuming NPSHₐ = NPSHᵣ is a good design target. Pump NPSH required is a manufacturer/test characteristic, and an engineering margin above it is normally needed for reliable operation.
Using an NPSH required value from the wrong flow or speed. NPSH required changes across the pump operating range.

NPSH margin and pump data

Manufacturer NPSH required data are pump-specific. Industry pump testing commonly defines NPSH3 around a 3% head-reduction criterion, which is not the same as a guarantee of zero cavitation. Final design should use the pump manufacturer's curve and an appropriate NPSH margin for the application.

Use the Pipe Friction Loss Calculator to estimate suction-line friction and the Pump Power Calculator for hydraulic and shaft power.

Reference background: Hydraulic Institute guidance describes NPSH available as a system characteristic and NPSH required as a pump characteristic. Hydraulic Institute Pump FAQs.

Equation values and input guide

Use absolute surface pressure and vapor pressure at the pumping temperature. Set elevation relative to the pump and include all suction losses. Compare the calculated available head with manufacturer NPSH required plus an appropriate design margin.

Psurface,abs: Absolute pressure above the supply liquid, in Pa after conversion. For an open tank use local atmospheric pressure.
Pvapor: Absolute vapor pressure at the fluid temperature, using the same pressure units as surface pressure in a hand calculation.
ρ, g: Liquid density in kg/m³ and gravitational acceleration 9.80665 m/s².
z: Liquid surface elevation minus pump reference elevation, in m. Positive for a surface above the pump; negative for suction lift.
hf: Positive suction loss head in m of the pumped liquid, including pipe, entrance, fittings and equipment up to the pump.
NPSHₐ, NPSHᵣ: Available and required suction head, in length units of the pumped liquid. Available head is calculated; required head comes from the pump curve at the operating flow and speed.

Use the unit selectors beside the inputs. Units shown in the equations must be consistent when checking the math by hand.