Pump power calculator
Enter the flow rate and the total head the pump must deliver, the density of the liquid and the efficiencies of the pump and its motor. The calculator gives the power given to the liquid, the power at the pump shaft and the electrical power drawn by the motor.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the flow rate in m³/h, L/s or US gallons per minute.
- Enter the total head in metres or feet.
- Enter the density of the liquid (water: 1,000 kg/m³).
- Enter the pump efficiency and the motor efficiency in %.
- Read the shaft power, hydraulic power and electrical input power in kW and hp.
- Hydraulic power: P_h = ρ × g × Q × H (W), g = 9.80665 m/s².
- Shaft power: P_shaft = P_h / η_pump.
- Electrical input: P_in = P_shaft / η_motor.
1 m³/h = 1/3,600 m³/s; 1 US gpm = 3.785 L/min; 1 ft = 0.3048 m; 1 hp = 745.7 W.
Worked example
36 m³/h of water (0.01 m³/s) against 20 m of head, pump efficiency 70 %, motor 90 %:
- P_h = 1,000 × 9.80665 × 0.01 × 20 = 1.961 kW;
- P_shaft = 1.961 ÷ 0.7 = 2.802 kW (3.76 hp);
- P_in = 2.802 ÷ 0.9 = 3.113 kW.
In US units: 100 gpm against 100 ft gives about 1.882 kW of hydraulic power; with a 65 % pump that is 2.895 kW, 3.88 hp, at the shaft.
Tips
- Efficiency comes from the pump curve at your operating point; it drops away from the best efficiency point.
- Running cost: multiply the input power by hours and the electricity price.
- Viscous liquids reduce pump efficiency and head; manufacturers give correction factors.
Frequently asked questions
How do I calculate pump power?
Hydraulic power P = ρ × g × Q × H, with ρ in kg/m³, g = 9.81 m/s², Q in m³/s and H in metres, gives watts. Divide by the pump efficiency for the shaft power and by the motor efficiency for the electrical input.
What is the difference between hydraulic and shaft power?
Hydraulic (water) power is what the liquid receives. Shaft or brake power is what the pump needs at its shaft; it is larger because of the pump's losses. The motor is sized from the shaft power, usually with some reserve.
What is total head?
The height the pump effectively lifts the liquid: the difference in level plus the friction losses in pipes and fittings, plus any pressure difference expressed as height. Use the pump curve's head at your flow rate.
Does density matter?
Yes. Power is proportional to density, so a liquid 20 % denser than water needs 20 % more power for the same flow and head. Head itself, in metres, does not depend on density.