About the Pump Power Calculator
This pump power calculator turns a duty point — flow rate and total head — into the power a pump needs. It gives the hydraulic (water) horsepower delivered to the liquid, the brake horsepower the pump shaft needs after pump efficiency, the electrical input power after motor efficiency, and the next standard motor size. Add your operating hours and electricity price to see the annual energy use and running cost.
It is useful for sizing a pump and motor for irrigation, HVAC, process, sump or booster systems, for comparing two pumps with different efficiencies, and for estimating what an inefficient old pump is costing you. Work in gpm and feet or in m³/h and metres; enter the specific gravity for liquids other than water.
Use the total dynamic head at the duty point — static lift plus friction losses plus any pressure difference — taken from your system calculation or the pump curve. Read pump efficiency from the manufacturer’s curve at the same point; typical centrifugal pumps run between 50% and 85%. Motor sizing here is a simple next-size-up; real selections add margin for the end of the pump curve and service factor.
How to use the pump power calculator
- 1Choose imperial or metric units.
- 2Enter the flow rate and total dynamic head at the duty point.
- 3Enter the liquid’s specific gravity (1.0 for water).
- 4Enter pump efficiency from the pump curve and the motor efficiency.
- 5Add operating hours and electricity price to estimate yearly cost.
Formula and method
Hydraulic power is the rate at which the pump adds energy to the liquid: density × gravity × volumetric flow × head (ρgQH), with ρ = 1,000 kg/m³ × specific gravity. In US units this reduces to the familiar water horsepower formula Q × H × SG ÷ 3,960 with flow in gpm and head in feet (the calculator uses exact SI constants, so the exact divisor is about 3,954, so results run about 0.15% higher than the 3,960 shortcut).
Brake horsepower is the power the pump shaft needs, equal to hydraulic power divided by pump efficiency. Electrical input power divides again by motor efficiency. Annual energy is input kW × operating hours, and cost is energy × price per kWh. The suggested motor is the next NEMA horsepower or IEC kW size at or above the brake power at this duty point.
- Q
- Volumetric flow rate
- H
- Total dynamic head
- SG, ρ
- Specific gravity and density of the liquid
- g
- Standard gravity, 9.80665 m/s²
- η_pump, η_motor
- Pump and motor efficiency
Worked examples
500 gpm at 100 ft of head, 75% pump, 92% motor
Water horsepower = 500 × 100 ÷ 3,954 ≈ 12.6 hp (the rounded 3,960 shortcut gives 12.63). At 75% pump efficiency the shaft needs 16.9 hp, so a 20 hp motor is the next standard size. Dividing by 92% motor efficiency gives 13.7 kW of input; 4,000 hours a year uses about 54,700 kWh, or roughly $7,100 at $0.13/kWh.
Metric: 50 m³/h at 30 m, 70% pump, 90% motor
Hydraulic power = 1,000 × 9.81 × (50/3,600) × 30 = 4.09 kW. The shaft needs 4.09 ÷ 0.70 = 5.84 kW, so a 7.5 kW IEC motor is the next size up, drawing about 6.49 kW from the supply.
Brine (SG 1.2): 200 gpm at 60 ft, 65% pump
A heavier liquid needs proportionally more power: 200 × 60 × 1.2 ÷ 3,954 ≈ 3.64 water hp. At 65% efficiency the pump needs 5.6 brake hp, just over a 5 hp motor, so choose 7.5 hp.
Frequently asked questions
How do you calculate pump horsepower?+
Water horsepower = flow (gpm) × head (ft) × specific gravity ÷ 3,960. Divide by pump efficiency to get brake horsepower. For example, 500 gpm at 100 ft with water is 12.6 water hp, or about 16.8–16.9 brake hp at 75% efficiency (3,960 shortcut vs exact constant).
What is the difference between water horsepower and brake horsepower?+
Water (hydraulic) horsepower is the useful power actually given to the liquid. Brake horsepower is the power the pump shaft must receive, which is higher because the pump wastes some energy in friction, leakage and turbulence. BHP = WHP ÷ pump efficiency.
What is a typical pump efficiency?+
Small centrifugal pumps often reach 40–65%, mid-size end-suction pumps 65–80%, and large well-designed pumps 85–90% at their best efficiency point. Efficiency drops when a pump runs far from its best efficiency point, so read it from the curve at your duty point.
How do I calculate pump power in kW?+
Hydraulic power in kW = density (kg/m³) × 9.81 × flow (m³/s) × head (m) ÷ 1,000. For water at 50 m³/h and 30 m that is 1,000 × 9.81 × 0.01389 × 30 ÷ 1,000 ≈ 4.09 kW; divide by pump efficiency for shaft power.
Why does specific gravity matter?+
Head is the same for any liquid, but the power to lift a heavier liquid to that head is proportionally larger. A brine with SG 1.2 needs 20% more power than water at the same flow and head, which can overload a motor sized for water.
How big a motor do I need?+
At minimum, the next standard motor size above the pump’s brake horsepower at the duty point. Many designers size for the maximum power on the pump curve (end of curve) so the motor never overloads if system head falls, and stay within the motor service factor.