# Pump NPSH (Net Positive Suction Head) Calculator & Cavitation Check

Compute the available Net Positive Suction Head (NPSH_a) of a pump and compare it against the required NPSH (NPSH_r) from the pump curve. Head-balance form: NPSH_a = (p_surface_abs - p_vapor) / (ρ·g) + H_static - h_friction. Surface and vapor pressures are absolute; H_static is positive for flooded suction, negative for suction lift. Returns margin (NPSH_a - NPSH_r), ratio (NPSH_a/NPSH_r) and a classification: safe, marginal (margin < 0.5 m), or cavitation likely. If NPSH_r = 0, only NPSH_a is reported. Pressure in Pa/kPa/bar/atm/psi, length in m/ft, density in kg/m³/g/cm³/lb/ft³.

> Canonical page: https://elysiatools.com/en/tools/pump-npsh-calculator

- **Category:** Math & Numbers

- **Keywords:** NPSH, net positive suction head, cavitation, pump suction, vapor pressure, suction lift, flooded suction, centrifugal pump, fluid mechanics, NPSH_a, NPSH_r, pump curve, hydraulics, NPSH available

## Overview

Calculate a pump’s available Net Positive Suction Head (NPSH_a) from absolute surface pressure, vapor pressure, static head, friction loss, liquid density, and gravity, then compare it with the pump’s required NPSH (NPSH_r) to assess cavitation risk.

## Inputs

- **Surface Pressure p_surface (absolute)** (number): Absolute pressure at the liquid surface (p_surface). Enter in the selected Pressure Unit.
- **Pressure Unit** (select)
- **Vapor Pressure p_vapor (absolute)** (number): Absolute vapor pressure of the liquid at pumping temperature (p_vapor). Enter in the selected Vapor Pressure Unit.
- **Vapor Pressure Unit** (select)
- **Static Head H_static** (number): Elevation of liquid surface above pump centerline (H_static). Positive if liquid level is above the pump (flooded suction), negative for suction lift. Enter in the selected Static Head Unit.
- **Static Head Unit** (select)
- **Friction Loss h_friction** (number): Suction-line friction head loss (h_friction), already computed. Enter in the selected Friction Loss Unit.
- **Friction Loss Unit** (select)
- **Density ρ** (number): Liquid density (ρ). Enter in the selected Density Unit.
- **Density Unit** (select)
- **Gravity g (m/s²)** (number): Gravitational acceleration g in m/s².
- **NPSH Required (NPSH_r)** (number): The pump's required NPSH (NPSH_r) from its curve. Enter in the selected NPSH Unit. If 0, the cavitation comparison is skipped and only NPSH_a is reported.
- **NPSH Unit** (select)
- **Decimal Places** (number)

## When to use

- Check whether a pump has enough suction head for a selected operating condition.
- Compare calculated NPSH_a with NPSH_r from a pump curve.
- Evaluate how suction lift, vapor pressure, or friction loss affects cavitation margin.

## How it works

- Enter absolute surface pressure and absolute vapor pressure, selecting Pa, kPa, bar, atm, or psi.
- Provide static head, where a positive value represents flooded suction and a negative value represents suction lift.
- Enter suction-line friction loss, liquid density, gravity, and optionally the pump’s required NPSH.
- The calculator returns NPSH_a, margin, ratio, and a classification of safe, marginal, or cavitation likely when NPSH_r is provided.

## Use cases

- Pump selection and preliminary hydraulic design for centrifugal pump systems.
- Suction-piping checks for flooded tanks and suction-lift arrangements.
- Troubleshooting cavitation risk by testing pressure, temperature-related vapor pressure, elevation, and friction-loss changes.

## Frequently asked questions

### What is the NPSH_a formula?

NPSH_a = (p_surface_abs - p_vapor) / (ρ·g) + H_static - h_friction.

### Should surface and vapor pressure be absolute?

Yes. Both surface pressure and vapor pressure must be entered as absolute pressures.

### How do I enter suction lift?

Enter static head as a negative value when the liquid level is below the pump centerline.

### What does the margin represent?

Margin is NPSH_a - NPSH_r. A positive margin means available NPSH exceeds the required value.

### What happens if NPSH_r is zero?

The calculator reports NPSH_a only and skips the cavitation comparison.

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