Convert between power, torque and rpm in kW, hp, PS, N·m and lb·ft. Paste or type a dyno curve to plot torque and power, find the peaks, the power band and the wheel power after drivetrain losses.
| # | rpm | N·m |
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| Unit | Power | Unit | Torque |
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| rpm | Torque | Power | Wheels | of peak |
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Power is torque times rotational speed. An engine with lots of torque at low rpm and one with less torque at high rpm can make the same power; gearing turns one into the other.
Typical drivetrain losses: 10–15 % for front- or rear-wheel drive manual cars, 15–20 % for automatics and all-wheel drive, 5–8 % for chain-drive motorcycles. Peak power always sits at a higher rpm than peak torque; the power band shown is the range where the engine makes at least 90 % of its peak power, which is where you want to keep it when accelerating.
This calculator converts between torque, rotational speed and power. Enter any two of them and it finds the third in kW, hp and PS, with torque in N·m, lb·ft or kgf·m. It also estimates power at the wheels after drivetrain losses, and from a dyno torque curve it finds peak power, peak torque and the usable power band. Mechanics, tuners, engineers, students and car and motorcycle enthusiasts use it to read dyno sheets, compare engines, size electric motors or check a spec sheet.
200 N·m at 4500 rpm is 94.3 kW, 126.4 hp or 128.1 PS at the crankshaft, and about 80 kW (107 hp) at the wheels. Any torque figure means nothing for power until you know the rpm it is made at.
Multiply torque by engine speed and divide by a constant. In US units: hp = torque (lb·ft) × rpm / 5252. In metric: kW = torque (N·m) × rpm / 9549, and hp = kW / 0.7457. Example: 300 lb·ft at 5000 rpm is 300 × 5000 / 5252 = 285.6 hp. You cannot convert torque to horsepower without the rpm, because power is the rate of doing work: the same torque at twice the speed is twice the power.
One mechanical horsepower is 33 000 ft·lbf per minute. A torque of T lb·ft turning at n rpm does T × 2π × n ft·lbf of work per minute, so hp = T × 2π × n / 33 000 = T × n / 5252. At exactly 5252 rpm the two numbers are equal, so on any dyno chart that plots hp and lb·ft on the same scale the curves cross there. It is a coincidence of units: with kW and N·m the curves cross at 9549 rpm instead.
Torque is a twisting force, measured in N·m or lb·ft. Power is how fast that force does work, torque times rotational speed. Torque at the wheels decides how hard the car pushes at a given moment, and gearing multiplies it. Power decides how much push you can have at a given road speed, because gearing trades torque for rpm but cannot create power. That is why a high-revving engine with modest torque and a slow diesel with large torque can deliver the same acceleration if they make similar power.
kW is the SI unit and is what EU registration documents use. One mechanical horsepower (hp), the US and UK unit, is 0.7457 kW. One metric horsepower (PS, CV or pk) is 0.7355 kW, so a 100 kW engine is 134 hp or 136 PS. Brake horsepower (bhp) is power measured at the crankshaft on an engine dyno, before transmission losses. Wheel horsepower (whp) is measured on a chassis dyno and is lower. SAE J1349 defines how US net power ratings are corrected to standard conditions.
Typical losses between crankshaft and wheels are 10–15 % for front- or rear-wheel-drive cars with manual gearboxes, 15–20 % for automatics and all-wheel drive, and 5–8 % for chain-drive motorcycles. So a 300 hp engine in an all-wheel-drive car with an automatic might show about 240–255 hp on a chassis dyno. Part of the loss is fixed friction rather than a percentage, and chassis dyno readings also depend on tire pressure, temperature and how the dyno is calibrated.
Power is torque times rpm. Just past the torque peak, torque falls slowly while rpm keeps rising, so their product still grows. Power only stops rising when torque falls faster in percentage terms than rpm increases. For that reason peak power always sits above peak torque in the rev range. The power band, where the engine makes at least 90 % of peak power, is the range to stay in when accelerating, and gear ratios are chosen so each upshift lands back inside it.