What the Pump Horsepower Calculator does and when to use it
This calculator sizes the power a pump needs to move liquid at a given flow rate against a given head. It returns the water (hydraulic) horsepower, the brake horsepower the pump shaft must receive after pump losses, the same shaft power in kilowatts, and the electrical input power once motor losses are included. It is a handy first check when you are comparing pumps, choosing a motor, or estimating running cost for a sump, irrigation, transfer or circulation pump.
Enter the flow in US gallons per minute, the total dynamic head in feet, the pump efficiency from its performance curve, and optionally the liquid's specific gravity and the motor efficiency. Leave specific gravity blank for water and motor efficiency blank to assume 90%. Manufacturer curves should always have the final say, since efficiency changes with the operating point.
Formula and method
Water horsepower is the useful power delivered to the liquid: WHP = Q × H × SG / 3960. The constant 3960 comes from 33,000 foot-pounds per minute in one horsepower divided by about 8.34 pounds per gallon of water, rounded. Brake horsepower divides by pump efficiency, and electrical input divides shaft kilowatts by motor efficiency.
One mechanical horsepower equals 0.7457 kW, which converts brake horsepower to kilowatts.
- Q flow rate in US gallons per minute (gpm).
- H total dynamic head in feet: static lift plus friction losses plus any pressure requirement expressed as head.
- SG specific gravity of the liquid; 1.0 for water.
- BHP brake horsepower = WHP divided by pump efficiency as a decimal.
- kW shaft power = BHP × 0.7457; electrical input = shaft kW divided by motor efficiency.
Worked example
A pump moves 100 gpm of water against 50 ft of head. Its efficiency is 65% and the motor efficiency is 90%.
- Water horsepower: 100 × 50 × 1 / 3960 = 1.263 hp.
- Brake horsepower: 1.263 / 0.65 = 1.943 hp.
- Shaft power: 1.943 × 0.7457 = 1.449 kW.
- Electrical input: 1.449 / 0.90 = 1.609 kW.
These match the calculator's output of 1.263 hp, 1.943 hp, 1.449 kW and 1.609 kW. A motor would normally be chosen from the standard sizes above the brake horsepower, with margin as recommended by the pump maker. If the liquid were a brine with a specific gravity of 1.2, every power figure would rise by 20% for the same flow and head.
Common mistakes and how to interpret the result
- Entering pressure instead of head. Head is measured in feet of the liquid being pumped; for water, one psi is about 2.31 ft. Dividing by specific gravity converts pressure to head for other liquids.
- Forgetting friction losses. Long pipes, fittings and valves add head. Using only the vertical lift will underestimate the power needed.
- Using peak efficiency for every duty point. Pump efficiency drops away from the best efficiency point, so read the efficiency at your actual flow and head.
- Ignoring viscous or solids-laden fluids. Thick liquids reduce pump efficiency and this simple model does not correct for that.
Related calculators
- NPSH Calculator — check suction conditions to avoid cavitation.
- Flow Rate Calculator — work out flow from volume and time.
- Friction Loss Calculator — estimate pipe head loss.
- HP to Amps Calculator — translate motor power to current.