Cavitation Number Calculator

What is the Cavitation Number?

The cavitation number (σ) is a dimensionless parameter in fluid mechanics that measures how close a flowing liquid is to forming vapor bubbles — a destructive phenomenon known as cavitation. It compares the pressure margin above the liquid's vapor pressure to the dynamic pressure of the flow.

The cavitation number is mathematically expressed as:

σ = 2(p - pᵥ) / (ρV²)

Where σ is the cavitation number (dimensionless), p is the local absolute pressure in Pa, pᵥ is the vapor pressure of the liquid in Pa, ρ is the fluid density in kg/m³, and V is the flow velocity in m/s. You can use our Pressure Converter to work in different pressure units. A higher cavitation number indicates a lower risk of cavitation, while values near or below zero suggest cavitation is likely.

Applications of the Cavitation Number

Engineers use the cavitation number extensively in the design of pumps, propellers, turbines, valves, and hydraulic systems. When the local pressure in a liquid drops below its vapor pressure, vapor bubbles form and subsequently collapse with tremendous force, causing erosion, noise, vibration, and reduced efficiency. Predicting the cavitation number helps engineers design systems that operate safely above the critical cavitation threshold. The Reynolds Number Calculator is another key dimensionless parameter used alongside the cavitation number for comprehensive flow analysis.

The cavitation number is especially critical in marine propulsion (ship propellers), pump suction design, hydrofoil design, and fuel injection systems where high velocities and pressure variations can trigger cavitation.

How to Use This Calculator

This calculator solves for any of the five variables in the cavitation number equation:

• Cavitation Number — Calculate σ when you know pressure, vapor pressure, density, and velocity
• Local Pressure — Determine the required pressure for a given cavitation number
• Vapor Pressure — Find the maximum allowable vapor pressure for safe operation
• Fluid Density — Compute density from cavitation number and flow conditions
• Flow Velocity — Calculate the maximum velocity before cavitation onset

Interpreting Cavitation Number Results

General guidelines for cavitation number interpretation:

• σ > 1 — Cavitation is unlikely; the system operates in a safe regime
• 0.5 < σ < 1 — Moderate risk; cavitation may occur under certain conditions
• σ < 0.5 — High cavitation risk; design changes should be considered
• σ ≤ 0 — Cavitation is almost certain; immediate design changes required

Note that the critical cavitation number varies with system geometry and fluid properties. Always consult manufacturer specifications for pumps and propellers to determine acceptable cavitation number ranges.