Torque, power and rpm
Torque is the twisting force an engine produces at the crankshaft. Power is how fast that force does work: torque multiplied by rotational speed. The same torque at twice the rpm means twice the power. That is why a small high-revving motorcycle engine can make as much power as a much larger, lower-revving car engine.
The formulas
Power (kW) = torque (Nm) × rpm ÷ 9,549
Power (hp) = torque (lb-ft) × rpm ÷ 5,252
Torque (Nm) = power (kW) × 9,549 ÷ rpm
The constants come from unit conversions: 9,549 is 60,000 ÷ 2π (converting revolutions per minute to radians per second and watts to kilowatts), and 5,252 is 33,000 ÷ 2π for mechanical horsepower and lb-ft. A neat consequence: on any engine dyno chart in hp and lb-ft, the torque and power curves cross at exactly 5,252 rpm.
Power units
| Unit | Value | Where it is used |
|---|---|---|
| kilowatt (kW) | 1,000 W | Official SI unit; registration documents in many countries |
| mechanical horsepower (hp) | 745.7 W | United States, United Kingdom |
| metric horsepower (PS, CV, pk) | 735.5 W | Europe, Japan, much of Asia and Latin America |
One kW is 1.341 hp or 1.360 PS. A car advertised with 150 PS has about 110 kW or 148 hp.
Worked examples
Power from torque
An engine produces 300 Nm at 4,000 rpm. Power = 300 × 4,000 ÷ 9,549 = 125.7 kW, which is 168.5 hp or 170.9 PS.
In imperial units
250 lb-ft at 5,252 rpm gives 250 × 5,252 ÷ 5,252 = 250 hp. 250 lb-ft is also 339 Nm.
Torque from power
A motor delivers 100 kW at 6,000 rpm. Torque = 100 × 9,549 ÷ 6,000 = 159.2 Nm.
Peak figures do not happen at the same rpm
Brochures quote peak torque and peak power, but they usually occur at different engine speeds: peak torque perhaps at 2,000–4,000 rpm, peak power near the rev limit. To calculate power at a given rpm you need the torque at that same rpm, read from a dyno chart, not the peak torque figure.
Crank versus wheel power
Manufacturers quote power at the crankshaft. A chassis dynamometer measures at the wheels, after losses in the gearbox, differential and tyres. Typical drivetrain losses are in the range of about 10–20%, depending on the vehicle, so wheel figures are always lower than brochure figures.
Torque through the gears
The gearbox multiplies torque and divides speed. In first gear the torque at the wheels can be many times the engine torque, which is why acceleration is strongest in low gears. Power, apart from losses, stays the same through the gearbox.
Common mistakes
- Mixing Nm with lb-ft. 1 lb-ft = 1.356 Nm. Use 9,549 with Nm and kW, 5,252 with lb-ft and hp.
- Using peak torque with peak-power rpm. They rarely occur together.
- Confusing hp and PS. They differ by about 1.4%, small but noticeable on large engines.
Same torque, different rpm
An engine producing a steady 300 N·m makes very different power depending on how fast it turns:
| Engine speed | Power |
|---|---|
| 2,000 rpm | 62.8 kW (84 hp) |
| 4,000 rpm | 125.7 kW (169 hp) |
| 6,000 rpm | 188.5 kW (253 hp) |
Why diesels feel strong but rev low
Turbocharged diesel engines often produce high torque at low rpm, which makes them feel strong when pulling away or towing. Because they cannot rev as high as petrol engines, their peak power is often modest for their torque. High-revving motorcycle engines show the opposite pattern: little torque but a lot of power at very high rpm. Neither is better in general; it depends on the job.
Electric motors
Electric motors can deliver their maximum torque from standstill, which is why electric cars accelerate briskly from low speeds. Their power rating describes the peak they can deliver, often only for a limited time before temperature limits reduce it.
Frequently asked questions
How do I convert torque to horsepower?
Multiply torque in lb-ft by rpm and divide by 5,252 for horsepower, or multiply torque in Nm by rpm and divide by 9,549 for kilowatts.
Why do torque and horsepower cross at 5,252 rpm?
Because hp = lb-ft × rpm ÷ 5,252, the two numbers are equal exactly when rpm equals 5,252.
What is the difference between hp and PS?
PS (metric horsepower) is 735.5 W and hp (mechanical horsepower) is 745.7 W, so 100 PS is about 98.6 hp.
How many hp is 1 kW?
1 kW is about 1.341 hp, or 1.360 PS.
Is torque or horsepower more important?
Torque describes pulling force at a given rpm; power tells you how quickly work is done. Acceleration and top speed depend on power and gearing, while low-rpm torque helps with towing and drivability.
How much power does 300 N·m make?
It depends on rpm: about 84 hp at 2,000 rpm, 169 hp at 4,000 rpm and 253 hp at 6,000 rpm.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units (horsepower and newton-metre conversions).
- SAE J1349, Engine Power Test Code (net power rating conditions).
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.