This horsepower calculator works out power four ways: from torque and RPM, from an electric motor’s voltage and current, and from a car’s quarter-mile trap speed or elapsed time. Results are shown in horsepower, kilowatts and metric horsepower (PS).
Horsepower formulas
- From torque: HP = torque (lb-ft) × RPM ÷ 5,252
- Electric motor: HP = volts × amps × power factor × efficiency ÷ 745.7 (multiply by √3 for three-phase)
- Trap speed: HP ≈ weight (lb) × (trap speed ÷ 234)³
- Elapsed time: HP ≈ weight (lb) ÷ (ET ÷ 5.825)³
One mechanical horsepower is 745.7 watts; one metric horsepower (PS) is 735.5 watts.
Worked examples
An engine making 300 lb-ft of torque at 5,000 RPM produces 300 × 5,000 ÷ 5,252 ≈ 285.6 hp. A 3,500 lb car with a 105 mph trap speed has roughly 316 hp at the wheels.
Why 5,252?
It comes from the definition of horsepower (33,000 ft-lb per minute) divided by 2π. Because of it, a torque curve in lb-ft and a horsepower curve always cross at 5,252 RPM.
Brake vs wheel horsepower
Makers quote brake horsepower at the crankshaft. Power at the wheels is lower because of drivetrain losses, which is why dyno and quarter-mile figures usually come in under the advertised number.
Frequently asked questions
How do I convert horsepower to kW?
Multiply horsepower by 0.7457. For example, 200 hp is about 149 kW.
What is the difference between hp and PS?
PS is metric horsepower, about 1.4% smaller than mechanical horsepower. 100 hp is about 101.4 PS.
How accurate are quarter-mile horsepower estimates?
They are rough. Traction, gearing, weather and aerodynamics all affect times, so a dyno test is more accurate.
How many amps does a 1 hp motor draw?
It depends on voltage, efficiency and power factor. Enter the details in electric mode and adjust the amps until the result shows 1 hp.
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Last reviewed: October 2026. For information only; results are estimates; check important figures with a professional.