Power is torque times rotational speed — that one idea is the whole conversion. In imperial units it's HP = torque(lb·ft) × RPM / 5252; in SI it's kW = torque(N·m) × RPM / 9549. The constants 5252 and 9549 just fold in the unit conversions (the famous 5252 is 33,000 ÷ 2π). The key takeaway: an engine's horsepower depends on how fast it spins as well as how hard it twists, which is why torque alone never tells the full story.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: P = T·ω with HP/lb·ft and kW/N·m unit constants.
The torque–power equations
Power equals torque times angular speed; the constants simply package the unit conversions so you can work in RPM directly. Use 5252 with pound-feet to get horsepower, or 9549 with newton-metres to get kilowatts. To recover torque from a power rating, divide by the speed and multiply by the constant. The calculator returns both kW and HP at once, so you can read whichever your application uses.
Worked example — a motor rating
Scenario: An electric motor produces 50 N·m of torque at 3000 RPM. What is its power output?
The motor delivers 15.71 kW, or about 21.07 HP. Halve the speed to 1500 RPM at the same torque and the power halves to ~7.85 kW — proof that horsepower scales with RPM, not torque alone. This is exactly why a high-torque, low-revving diesel and a low-torque, high-revving petrol engine can post identical peak power: it's the product T × RPM that counts, and the constant 5252 is just where the HP and lb·ft curves happen to cross on a dyno.
Frequently Asked Questions
HP = lb·ft × RPM / 5252, or kW = N·m × RPM / 9549. 50 N·m at 3000 RPM = 15.71 kW = 21.07 HP.
It's 33,000 ÷ 2π — the RPM where HP and lb·ft torque are numerically equal, so the curves cross there.
kW = N·m × RPM / 9549, or watts = T × 2π·RPM/60. Torque = 9549 × kW / RPM.
× 1.341 (1 HP = 0.7457 kW). 15.71 kW ≈ 21.07 HP. The calc shows both.
Not necessarily — HP = torque × RPM. Low-revving high torque can equal high-revving low torque.