Horsepower Calculator guide
Horsepower Calculator: Torque, RPM and Kilowatts
Use our horsepower calculator to work out horsepower from torque and RPM, convert kilowatts to hp and PS, or find power from work and time.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
Horsepower is the classic way to measure an engine's power. Our Horsepower Calculator works it out from torque, converts from kilowatts, or calculates it from work and time.
How to use the calculator (Horsepower Calculator)
Choose how to work it out:
- Torque and RPM: enter the torque (N·m or lb·ft) and the engine speed.
- Kilowatts: convert kW to horsepower.
- Work and time: enter the work done in joules and the time taken.
The formulas
- Power (W) = torque (N·m) × rpm × 2π ÷ 60
- In imperial units: hp = torque (lb·ft) × rpm ÷ 5,252
- 1 hp = 745.7 watts
- 1 PS (metric horsepower) = 735.5 watts
hp, bhp and PS
| Unit | Meaning |
|---|---|
| hp | Mechanical horsepower, 745.7W |
| bhp | Brake horsepower, measured at the engine's crankshaft |
| PS | Metric horsepower, about 1.4% less than hp |
| kW | The official SI unit, used on UK registration documents |
A worked example
With the calculator's default settings, an engine making 300 N·m of torque at 5,000 rpm produces 157.1kW, which is 210.6 hp or 213.6 PS.
Frequently asked questions
What's the difference between torque and power?
Torque is the turning force; power is how fast that force does work. Power = torque × speed.
Why do hp and lb·ft always cross at 5,252 rpm?
Because of the 5,252 constant in the formula: at that speed, horsepower equals torque in lb·ft.
To estimate power from a quarter-mile run, try our Engine Horsepower Calculator.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.