How to use the horsepower calculator
- Choose what to calculate from: torque and rpm, force with distance and time, or a power value to convert.
- For torque, enter the torque in lb·ft or N·m and the speed in revolutions per minute.
- For work, enter the force (lbf, N or kgf), the distance moved (feet or meters) and the time taken in seconds.
- To convert, enter the power and pick its unit: mechanical hp, metric hp, electrical hp, kW or W.
- Read the power in horsepower and kilowatts, every other unit in the list, and the working.
How horsepower is calculated
Power is the rate of doing work. For something that rotates, it is torque times angular speed; for something pushed in a straight line, it is force times distance divided by time:
The number 5,252 is 33,000 ÷ 2π: James Watt defined one horsepower as 33,000 foot-pounds of work per minute, and one revolution turns a torque through 2π radians. That is also why an engine's torque and horsepower curves always cross at 5,252 rpm when torque is in lb·ft. The watt equivalents are the exact conversion factors published by NIST in its guide to SI units (SP 811).
Worked example
The defaults are 300 lb·ft of torque at 5,000 rpm:
- hp = 300 × 5,000 ÷ 5,252.1 = 285.6 hp
- In SI units: 300 lb·ft = 406.7 N·m, and 5,000 rpm = 523.6 rad/s
- Power: 406.7 × 523.6 = 212,971 W = 212.97 kW
- In metric horsepower: 212,971 ÷ 735.5 = 289.6 PS
The work mode starts with Watt's own example: lifting 1,000 lb through 33 ft in 60 seconds. That is 33,000 ft·lbf in a minute, or 550 ft·lbf per second, which is exactly 1 hp (745.7 W). In convert mode, 100 kW comes out as 134.1 hp or 136.0 PS.
Horsepower units compared
| Unit | Used for | Watts | In mechanical hp |
|---|---|---|---|
| Mechanical hp (imperial) | US and UK cars, engines, tools | 745.700 | 1 |
| Metric hp (PS, CV, ch) | European and Japanese car specs | 735.499 | 0.9863 |
| Electrical hp | Electric motor ratings | 746 | 1.0004 |
| Kilowatt | SI unit, EU spec sheets | 1,000 | 1.3410 |
A car rated at 200 PS has 197.3 mechanical hp, or 147.1 kW. The difference is small but adds up on spec sheets, so check which horsepower a brochure means. "Brake horsepower" (bhp) is mechanical horsepower measured at the crankshaft on a dynamometer.
Torque or horsepower?
Torque is the twisting force; horsepower is how fast that force does work. An engine making 300 lb·ft at 2,000 rpm produces only 114 hp, while the same torque at 6,000 rpm is 343 hp. Gearing trades one for the other, which is why power, not torque alone, sets a vehicle's top speed and acceleration at speed. To estimate an engine's output from a drag strip time instead, use the engine horsepower calculator.
Frequently asked questions
How do I convert horsepower to kW?
Multiply mechanical horsepower by 0.7457 to get kilowatts, or divide kilowatts by 0.7457 to get horsepower. A 150 hp engine is 111.9 kW. For metric horsepower (PS), multiply by 0.7355 instead.
Why do torque and horsepower cross at 5,252 rpm?
Because hp = torque × rpm ÷ 5,252, the two numbers are equal when rpm is exactly 5,252. Below that speed the torque figure is larger; above it the horsepower figure is larger. It only works with torque in lb·ft.
How much horsepower can a person produce?
A fit adult can keep up roughly 100 to 200 W, about 0.13 to 0.27 hp, for an hour of steady effort. Trained athletes can top 1 hp (746 W) in a sprint lasting a few seconds. Try it yourself in work mode with your body weight, the height of a flight of stairs and the time it takes you to climb it.
What is the difference between hp and PS?
Both describe the same idea, but mechanical hp is based on 550 foot-pounds per second, while metric hp (PS in Germany, CV in France and Italy) is based on 75 kilogram-force meters per second. One PS is about 1.4% smaller than one hp.
Related calculators
- Engine Horsepower Calculator — Estimate engine horsepower from a quarter-mile time or trap speed and the car's weight.
- Ohm's Law Calculator — Enter any two of voltage, current, resistance and power to get the other two, with the working shown.
- Speed Calculator — Speed from distance and time, distance from speed and time, or time from distance and speed, in any unit.
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