NPSH Calculator
Net Positive Suction Head Available (NPSHa) is the absolute pressure head at the pump suction, minus the fluid's vapor pressure, expressed in feet of liquid. It represents the...
Formula
Source: Engineering Toolbox, Hydraulic Institute Standards | Last reviewed: June 8, 2026
Examples
0 psi
= 40.8 ft
- P_atm = 14.7
- P_vapor = 0.5
- h_static = 10
- h_friction = 2
- SG = 1
Water at 70°F with 10 ft flooded suction = 40.8 ft NPSHa
0 psi
= 19.4 ft
- P_atm = 14.7
- P_vapor = 11.5
- h_static = 15
- h_friction = 3
- SG = 1
Water at 200°F — reduced NPSHa due to high vapor pressure
0 psi
= 29.9 ft
- P_atm = 12.2
- P_vapor = 1
- h_static = 5
- h_friction = 1
- SG = 1
High altitude (5,000 ft) installation
Where is this used?
Troubleshooting cavitation problems: if NPSHa < NPSHr, the pump will cavitate.
Hot liquid pumping: vapor pressure increases with temperature, reducing NPSHa.
High-altitude installations: lower atmospheric pressure reduces NPSHa.
Frequently Asked Questions
What is the difference between NPSHa and NPSHr?
NPSHa (available) is what the system provides — it's calculated from the installation conditions. NPSHr (required) is what the pump needs — it's a pump characteristic provided by the manufacturer. The rule is NPSHa ≥ NPSHr + margin (typically 3 ft).
Why does hot water have a lower NPSHa?
As temperature increases, the fluid's vapor pressure increases. At 200°F, water's vapor pressure is about 11.5 psia, compared to 0.5 psia at 70°F. This higher vapor pressure reduces the margin above cavitation, requiring more static head or a lower pump elevation.
How can I increase NPSHa in an existing system?
Options include: (1) lower the pump elevation relative to the suction source, (2) increase suction pipe diameter to reduce friction loss, (3) cool the fluid to lower vapor pressure, (4) pressurize the suction vessel, or (5) reduce the pump speed (which lowers NPSHr).
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.