Pump Horsepower Calculator
Size a water pump: find the hydraulic and brake horsepower needed to move a given flow against a given total head.
How it works
Water HP = GPM × head (ft) ÷ 3960 · Brake HP = Water HP ÷ efficiency · kW = HP × 0.7457
Hydraulic horsepower is the useful work of lifting and pressurizing water; the constant 3,960 comes from the definition of horsepower applied to gallons and feet of head. Real pumps waste energy to friction and slip, so the motor must supply more — the brake horsepower — found by dividing by the pump efficiency. Total head must include both the vertical lift and the friction loss in the pipes, not just the height you can see.
Worked example
To move 20 GPM against 100 ft of total head: water HP = 20 × 100 ÷ 3,960 = 0.505 HP. At 60% pump efficiency the motor must deliver 0.505 ÷ 0.60 = 0.84 brake HP, so you would fit a 1 HP motor — the next standard size up.
Frequently asked questions
What is total head?
It is the total resistance the pump works against, in feet of water: the static lift (vertical rise from source to discharge) plus friction loss in the pipe and fittings plus any pressure you need at the outlet. Leaving out friction is the most common sizing error.
Why divide by efficiency?
Hydraulic HP is the energy that actually ends up in the water. Pumps lose energy to internal friction and turbulence, so the motor (brake) HP is always larger. Centrifugal pumps typically run 50–75% efficient near their best operating point.
Should I oversize the motor?
Pick the next standard motor size above the calculated brake HP to keep a margin, but avoid gross oversizing — a pump running far from its design point is inefficient and can cavitate or wear quickly.
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This calculator is for educational purposes. Double-check important results before acting on them.