Net Positive Suction Head Available (NPSHa) is the total head available at the pump suction flange above the vapor pressure of the liquid. It depends on system geometry, suction-side pressure, and fluid properties — not the pump itself.

What is the NPSHa formula?

NPSHa = (P_abs / ρg) + Z_s − h_f − (P_v / ρg), where P_abs is the absolute pressure at the suction source, Z_s is the static suction head (+ for flooded, − for lift), h_f is the suction-line friction/minor loss, and P_v is the vapor pressure of the liquid.

What NPSH margin is recommended?

Most pump manufacturers and the Hydraulic Institute recommend NPSHa ≥ NPSHr + 0.5 m (1.6 ft) as a minimum safety margin. For hot or volatile liquids, a margin of 1–2 m (3–6 ft) is more conservative.

What happens if NPSHa is less than NPSHr?

Cavitation occurs: vapor bubbles form in the low-pressure zone, then collapse violently as they move into higher-pressure regions. This causes noise, vibration, erosion of the impeller, and reduced pump performance.

How do I find the vapor pressure of my liquid?

Vapor pressure is a function of liquid type and temperature. For water you can use steam tables or Antoine equation. At 20 °C water's vapor pressure is ~2.34 kPa (0.34 psia); at 80 °C it rises to ~47.4 kPa (6.87 psia).

Does fluid density affect NPSHa?

NPSHa expressed in head units (meters or feet) is independent of density when using the head form of the equation. However, if you input pressures in kPa or psia, the calculator uses the density you provide to convert to head correctly.

Pump NPSH Calculator

Name: Pump NPSH Calculator
Availability: InStock
Author: Nham Vu

Calculate NPSHa for a pump system and compare it against NPSHr to confirm cavitation-free operation.

NPSH Inputs

Unit System

Absolute Pressure at Source

kPa

Use 101.325 kPa (14.696 psia) for open tanks at sea level.

Vapor Pressure of Liquid

kPa

Water at 20 °C ≈ 2.34 kPa; at 80 °C ≈ 47.4 kPa.

Fluid Density

kg/m³

Water at 20 °C ≈ 998.2 kg/m³ (62.3 lb/ft³).

Static Suction Head (Z_s)

Positive if source is above pump centerline; negative for suction lift.

Suction-Line Friction Loss (h_f)

Total head loss in suction piping (pipe friction + fittings + valves). Must be ≥ 0.

Pump NPSHr (Required) (optional)

From the pump manufacturer's curve at your operating flow point.

Enter pump system parameters and click Calculate.

Summary

Calculate NPSHa for a pump system and compare it against NPSHr to confirm cavitation-free operation.

How it works

Select your unit system: Metric (m, kPa) or Imperial (ft, psia).
Enter the absolute pressure at the suction source (atmospheric or tank pressure).
Enter the vapor pressure of the liquid at its operating temperature.
Enter the static suction head (positive if source is above pump, negative if below).
Enter the friction and minor losses in the suction piping.
Click Calculate to get NPSHa and compare against your pump's required NPSHr.

Use cases

Verify a centrifugal pump installation will not cavitate.
Size suction piping to keep friction losses below the NPSHa margin.
Check NPSH margin when pumping hot water or volatile liquids near their boiling point.
Evaluate the effect of elevation changes on pump suction performance.
Compare multiple pump placement options (flooded vs. lift configurations).
Validate pump selection against manufacturer NPSHr curves at design flow.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu