NPSH Calculator – Net Positive Suction Head

The NPSH calculator – net positive suction head – is a handy tool that will help you obtain the available NPSH of your system, so you can avoid having cavitation in your pump.

NPSH margin8 ftavailable exceeds required — no cavitation expected

Brake horsepower10.522 hp7.8462 kW

Hydraulic horsepower7.576 hpthe useful work done on the fluid

Power lost in the pump2.946 hp28% of the input

Hydraulic power in SI5.6492 kW

Flow in SI0.015773 m³/s946.4 litres a minute

Head in SI36.576 m

Electrical input required8.5284 kWincluding 8% motor loss

Overall wire-to-water efficiency66.24%

Running cost per 1000 hours at 0.30 a kWh2,558.53a pump's energy bill usually dwarfs its purchase price

NPSH margin ratio1.571×a margin of at least 1.3× the required figure is usual practice, not merely a positive difference

At 90% of the speed7.67 hpflow falls with speed, head with speed squared, power with speed cubed — so a 10% speed cut saves 27.1% of the power

At 80% of the speed5.387 hp48.8% less power for 20% less flow

The formula

hydraulic hp = Q × H × SG ÷ 3960; brake hp = that ÷ efficiency

Cubed, which is why variable speed pays

The affinity laws govern centrifugal pumps: flow is proportional to speed, head to speed squared, and power to speed cubed. That last exponent is the entire economic case for variable-speed drives. Slowing a pump by 20% cuts its power draw by nearly half, so throttling a valve to reduce flow — which wastes the difference as heat across the valve — is far more expensive than slowing the pump.

Net positive suction head is the other thing that decides whether an installation works. The pump needs enough pressure at its inlet to stop the liquid boiling as it accelerates into the impeller; if the available head falls below what the pump requires, vapour bubbles form and collapse violently, eroding the impeller and destroying the head. A positive margin is not sufficient — usual practice is at least 1.3 times the required figure.

The 3960 in the horsepower formula is a unit conversion, not physics: it folds together gallons, feet, minutes and the weight of water. This mode works in those customary units deliberately, so its answers can be checked against any pump catalogue.