Pump Power Calculator
Find how much power a pump needs for a given flow and head. Enter the flow, total head (or pressure rise), fluid specific gravity and the pump and motor efficiencies to get hydraulic power, shaft (brake) power and electrical input in kW and horsepower, a standard motor size, and the yearly energy cost.
Last updated:
Static lift + friction losses + pressure difference, as head
Small centrifugal pumps 40–65%, large 75–90%
Premium-efficiency motors ≈ 85–96%
How to use the Pump Power Calculator
- Enter the flow rate at the duty point.
- Enter the total dynamic head, or switch to pressure rise if you know discharge minus suction pressure.
- Choose the fluid (specific gravity).
- Enter pump and motor efficiencies from the pump curve and motor nameplate.
- Read the shaft power and motor size; add hours and price to see the running cost.
Pump power formulas
- Hydraulic power (SI): P_h (W) = ρ × g × Q × H = 9,806.65 × SG × Q (m³/s) × H (m)
- Hydraulic power (metric shortcut): P_h (kW) = Q (m³/h) × H (m) × SG ÷ 367.1
- Water horsepower (US): WHP = Q (GPM) × H (ft) × SG ÷ 3960
- Shaft / brake power: P_shaft = P_h ÷ η_pump
- Electrical input: P_in = P_shaft ÷ η_motor (overall efficiency = η_pump × η_motor)
- Pressure and head: H = ΔP ÷ (ρ g); for water 1 psi = 2.31 ft, 1 bar = 10.2 m
1 hp = 745.7 W. The calculation is for the duty point you enter; read the actual efficiency and power from the manufacturer’s pump curve for that flow.
Quick reference: shaft power for water
Shaft horsepower at 70% pump efficiency (SG 1.0).
| Flow | 50 ft head | 100 ft head | 200 ft head |
|---|---|---|---|
| 10 GPM | 0.18 hp | 0.36 hp | 0.72 hp |
| 50 GPM | 0.90 hp | 1.80 hp | 3.61 hp |
| 100 GPM | 1.80 hp | 3.61 hp | 7.22 hp |
| 250 GPM | 4.51 hp | 9.02 hp | 18.0 hp |
| 500 GPM | 9.02 hp | 18.0 hp | 36.1 hp |
Ways to cut pumping power
- Lower the head: larger pipes, fewer fittings and open (not throttled) valves reduce friction head, and power falls in proportion.
- Run near the best efficiency point of the pump curve; an oversized pump throttled back wastes energy.
- Use a variable-speed drive where flow varies: by the affinity laws, power falls roughly with the cube of speed in systems dominated by friction.
- Choose premium-efficiency motors for pumps that run many hours a year.
Frequently asked questions
How do you calculate pump power?
Hydraulic power is P_h = ρ × g × Q × H (watts, with ρ in kg/m³, Q in m³/s and H in m). Shaft or brake power is P_h divided by pump efficiency, and electrical input is shaft power divided by motor efficiency. In US units, water horsepower = GPM × head (ft) × SG ÷ 3960.
What is the difference between water horsepower and brake horsepower?
Water (hydraulic) horsepower is the useful power delivered to the liquid. Brake horsepower (BHP) is the power the pump shaft needs, which is higher because of internal pump losses: BHP = WHP ÷ pump efficiency. Motor input is higher again because of motor losses.
What is total dynamic head?
Total dynamic head (TDH) is the total energy per unit weight the pump must add: the vertical lift from the suction level to the discharge level, plus friction losses in pipes and fittings, plus any pressure difference between the source and destination, expressed in feet or meters of the liquid.
How do I convert pressure to head?
Head = pressure ÷ (ρ × g). For water, 1 psi ≈ 2.31 ft and 1 bar ≈ 10.2 m. For other liquids divide by the specific gravity: 1 psi of diesel (SG 0.84) is about 2.75 ft of head.
What pump efficiency should I use?
Use the efficiency from the pump curve at your duty point. As a rough guide, small centrifugal pumps are 40–65% efficient, medium ones 65–80% and large well-matched pumps 80–90%. Running far from the best efficiency point lowers efficiency.
How big a motor do I need?
At least the shaft power at the duty point, rounded up to the next standard size. Because flow and power can rise if system head is lower than expected, many designers size for the maximum power on the pump curve or add a 10–25% margin, and check the motor service factor.
How much does it cost to run a pump?
Multiply the electrical input in kW by the hours of operation and the price per kWh. A pump drawing 3 kW for 8 hours a day uses 24 kWh/day, about 8,760 kWh a year; at $0.17/kWh that is about $1,490 a year. Improving efficiency or reducing head lowers this directly.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.