A pump power calculator from flow and head
The power a pump needs depends on how much liquid it moves, how high or how hard it pushes it, and how efficient the pump and motor are. This pump power calculator turns a flow rate and a total head, or a flow rate and a pressure rise, into the hydraulic power delivered to the liquid, the shaft power the pump takes, the electrical power the motor draws and the next standard motor size.
It works for water and for other liquids by density, so the same page covers a centrifugal pump on a water supply, a transfer pump on diesel or oil, and a booster on seawater. Flow can be entered in cubic metres per hour, litres per second or minute, or US gallons per minute, and head in metres or feet.
If you want the head built from a tank height and pipe friction, use the water pump HP calculator, and use the pipe flow calculator for the friction loss itself. Here you enter the total head you already have. The pump curve, NPSH and viscous corrections are outside this estimate.
How to use the Pump Power Calculator
- Enter the flow rate and choose how the duty is given: total head or pressure rise.
- Pick the fluid, or a custom density in kg/m³.
- Enter the pump efficiency from the pump curve and the motor efficiency from the motor datasheet.
- Set a margin for the motor size, the running hours per year and, if you wish, the price per kWh.
- Read the shaft power, the motor size and the yearly energy.
Pump Power Calculator features
Head or pressure
Enter the total head in m or ft, or the pressure rise in bar, kPa, psi or MPa.
Any fluid
Water, seawater, oils and diesel, or your own density.
Motor sizing
Next standard IEC motor size above the shaft power plus margin.
Running cost
Energy per year and cost at the tariff you enter.
The pump power formula
The power delivered to the liquid is Phydraulic = ρ g Q H, or equivalently Q × Δp, where ρ is the density, g = 9.80665 m/s², Q the flow in m³/s, H the total head in metres and Δp the pressure rise. The pump takes Pshaft = Phydraulic ÷ ηpump and the motor draws Pelectrical = Pshaft ÷ ηmotor.
Example: 50 m³/h of water against 30 m total head. Hydraulic power is 998.2 × 9.80665 × 0.01389 × 30 = 4.08 kW. At a pump efficiency of 70% the shaft power is 5.83 kW, and with a motor efficiency of 90% the motor draws 6.47 kW. Adding a 10% margin gives 6.4 kW, so the next standard motor is 7.5 kW. In horsepower, the 5.83 kW shaft power is about 7.8 HP.
Choosing efficiency and margin
- Read the pump efficiency from the manufacturer’s curve at your duty point; small pumps are often 40 to 60% efficient, large ones 80% or more.
- The efficiency table shows how much the answer moves if the real efficiency is lower than expected.
- The motor margin covers curve tolerance and operation off the design point. Check the maximum power on the pump curve across its whole working range, because some pumps draw more at high flow.
- Heavier or more viscous liquids need more power and a lower efficiency: use the pump maker’s correction for viscous fluids.
Pump Power Calculator FAQ
How do I calculate pump power?
Hydraulic kW = ρ g Q H ÷ 1,000 with Q in m³/s, then divide by the pump efficiency for shaft power. 50 m³/h at 30 m of water needs 4.08 kW hydraulic and 5.83 kW at 70% efficiency.
What is the difference between hydraulic, shaft and electrical power?
Hydraulic power is what the liquid receives, shaft power is what the pump takes from the motor shaft, and electrical power is what the motor takes from the supply.
How do I calculate the power of a centrifugal pump?
Use the same formula with the total dynamic head and the efficiency at the operating point on the pump curve. This calculator then picks the next standard motor.
Does the density matter?
Yes, power is proportional to density at the same flow and head. A liquid of density 1,200 kg/m³ needs 20% more power than water.
How do I convert kW to HP for a pump?
Divide kW by 0.7457 for mechanical horsepower. The motor result shows both.
What head should I enter?
The total head: static lift, plus pipe and fitting friction, plus any pressure at the outlet expressed as head, less any suction head. Use the pipe flow calculator to get the friction part.