Horsepower calculator
Engine power from torque and rpm, or estimated from a quarter-mile run, in hp and kW.
About this calculator
This horsepower calculator turns torque and engine speed into horsepower, metric horsepower and kilowatts, in lb-ft or N·m.
It can also estimate a car's power from its weight and quarter-mile time or trap speed.
How it works
Power = torque × engine speed. In US units, hp = lb-ft × rpm ÷ 5,252; in metric, kW = N·m × rpm ÷ 9,549. One mechanical horsepower is 745.7 W. The quarter-mile estimates use hp = weight ÷ (time ÷ 5.825)³ and hp = weight × (trap speed ÷ 234)³, with the weight in pounds.
Last reviewed:
Worked example
- Work out from: Torque and engine speed
- Torque unit: lb-ft
- Torque: 300
- Engine speed: 5,500 rpm
Power: 314 hp (234.3 kW)
Common questions
How do I convert torque to horsepower?
Multiply torque in lb-ft by rpm and divide by 5,252. 300 lb-ft at 5,000 rpm is about 286 hp.
What is the difference between hp and PS?
PS, or metric horsepower, is 735.5 watts, a little less than the 745.7-watt mechanical horsepower. So 300 hp is about 304 PS.
What is wheel horsepower?
Power measured at the wheels, after losses in the gearbox and drivetrain. It is usually 10% to 20% below the engine's rated power.
Sources and further reading
The story behind it
Where the idea came from: James Watt coined the horsepower in the 1780s to compare his steam engines with the horses they replaced, setting it at 33,000 foot-pounds per minute.
One mechanical horsepower is about 745.7 watts. A horse can briefly produce much more than one horsepower, but Watt's figure stuck and is still used for cars today.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
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