Cavitation Number Calculator

Dimensionless ratio of pressure margin to dynamic pressure. σ = (P − P_v) / (½ρV²). Predicts cavitation risk in pumps, valves, and hydraulic devices.

Fluid Mechanics II · Specialized

Water vapor pressure by temperature — click to auto-fill P_v and ρ

Inputs

Local absolute pressure P (kPa)

Always use absolute pressure — cavitation occurs when P reaches P_v.

Vapor pressure Pv (Pa)

Use the preset buttons above to auto-fill P_v for water at common temperatures.

Reference velocity V (m/s)

Fluid density ρ (kg/m³)

Theory

Cavitation number σ (also called cavitation index):

σ = (P − P_v) / (½ρV²)

NPSH = (P − P_v) / (ρg) [m]

Cavitation occurs when local pressure drops to P_v, i.e. when σ → 0. P must be absolute pressure.

Risk classification:

σ range	Risk
σ ≥ 2	Safe — no cavitation expected for most geometries
0.5 ≤ σ < 2	Caution — verify against device-specific critical σ
0 < σ < 0.5	Critical — cavitation inception likely
σ ≤ 0	Cavitating — P ≤ Pv, bubbles forming

Critical σ for common devices:

Device	σ_c
Sharp-edged orifice	0.5 – 2.0
Globe valve (full open)	0.5 – 1.5
Butterfly valve	1.0 – 3.0
Centrifugal pump	per NPSH curve
Venturi meter	0.1 – 0.3
Hydrofoil / propeller	0.2 – 0.5

Cavitation occurs when σ < σ_c. These are indicative ranges — always consult manufacturer data.

Water vapor pressure P_v:

Temperature	P_v (Pa)
Water at 5°C	872
Water at 10°C	1228
Water at 20°C	2338
Water at 30°C	4243
Water at 40°C	7384
Water at 60°C	19940
Water at 80°C	47390
Water at 100°C	101325

Where:

σ = cavitation number (dimensionless)
P = local absolute pressure [Pa]
P_v = fluid vapor pressure at operating temperature [Pa]
ρ = fluid density [kg/m³]
V = reference velocity [m/s]
NPSH = net positive suction head [m]

Inputs

Local absolute pressure P (kPa)

Always use absolute pressure — cavitation occurs when P reaches P_v.

Vapor pressure Pv (Pa)

Use the preset buttons above to auto-fill P_v for water at common temperatures.

Reference velocity V (m/s)

Fluid density ρ (kg/m³)

Theory

Cavitation number σ (also called cavitation index):

σ = (P − P_v) / (½ρV²)

NPSH = (P − P_v) / (ρg) [m]

Cavitation occurs when local pressure drops to P_v, i.e. when σ → 0. P must be absolute pressure.

Risk classification:

σ range	Risk
σ ≥ 2	Safe — no cavitation expected for most geometries
0.5 ≤ σ < 2	Caution — verify against device-specific critical σ
0 < σ < 0.5	Critical — cavitation inception likely
σ ≤ 0	Cavitating — P ≤ Pv, bubbles forming

Critical σ for common devices:

Device	σ_c
Sharp-edged orifice	0.5 – 2.0
Globe valve (full open)	0.5 – 1.5
Butterfly valve	1.0 – 3.0
Centrifugal pump	per NPSH curve
Venturi meter	0.1 – 0.3
Hydrofoil / propeller	0.2 – 0.5

Cavitation occurs when σ < σ_c. These are indicative ranges — always consult manufacturer data.

Water vapor pressure P_v:

Temperature	P_v (Pa)
Water at 5°C	872
Water at 10°C	1228
Water at 20°C	2338
Water at 30°C	4243
Water at 40°C	7384
Water at 60°C	19940
Water at 80°C	47390
Water at 100°C	101325

Where:

σ = cavitation number (dimensionless)
P = local absolute pressure [Pa]
P_v = fluid vapor pressure at operating temperature [Pa]
ρ = fluid density [kg/m³]
V = reference velocity [m/s]
NPSH = net positive suction head [m]

Cavitation Number Calculator

Water vapor pressure by temperature — click to auto-fill Pv and ρ

Inputs

Theory

Cavitation Number Calculator

Water vapor pressure by temperature — click to auto-fill Pv and ρ

Inputs

Theory

Water vapor pressure by temperature — click to auto-fill P_v and ρ

Water vapor pressure by temperature — click to auto-fill P_v and ρ