Torque conversion and power reference table
The first rows give the unit conversion factors; the remaining rows show torque computed from typical power/speed combinations with T = 9,549.3 × kW ÷ rpm.
| Input | Torque (N·m) | Torque (lb·ft) |
|---|---|---|
| 1 N·m | 1 | 0.7376 |
| 1 lb·ft | 1.3558 | 1 |
| 1 kgf·m | 9.8067 | 7.233 |
| 50 kW @ 1,500 rpm | 318.3 | 234.8 |
| 100 kW @ 3,000 rpm | 318.3 | 234.8 |
| 100 kW @ 6,000 rpm | 159.2 | 117.4 |
| 200 kW @ 4,000 rpm | 477.5 | 352.2 |
- The formula gives the torque at the shaft for the stated power and speed; drivetrain losses mean wheel torque is lower than engine torque at the equivalent ratio.
- Engine dynamometer figures are curves, not single numbers — peak torque and peak power occur at different RPM, and the formula connects the two only at the same operating point.
- The kgf·m result appears in conversion mode; it is a non-SI unit still found in older and some Asian-market vehicle specifications.
What is torque and how does it relate to power?
Torque is the rotational analogue of force — the turning effort applied about an axis, measured in newton-metres (N·m) in SI. A torque of 1 N·m corresponds to a force of one newton applied at a lever arm of one metre. Imperial practice uses the pound-foot (1 lb·ft = 1.3558179 N·m) and older metric practice the kilogram-force-metre (1 kgf·m = 9.80665 N·m).
Power and torque are linked through rotational speed: P = T × ω, where ω is the angular velocity in radians per second. An engine that produces the same torque at twice the speed delivers twice the power. This is why an engine's peak-power and peak-torque figures occur at different RPM, and why gearing trades speed for torque without changing power (minus losses).
Converting the physics into workshop units gives the practical formula. Since ω = 2π × rpm ÷ 60, torque in N·m equals power in watts divided by that angular velocity, which simplifies to T = 9,549.3 × P(kW) ÷ rpm. The constant 9,549.3 is just 60,000 ÷ 2π.
How to use this torque calculator
- Choose the mode: compute torque from power and RPM, or convert an existing torque value.
- In power mode, enter the power in kilowatts and the rotational speed in RPM to get the torque in N·m and lb·ft.
- In convert mode, enter the torque in N·m to see its equivalents in lb·ft and kgf·m.
- For engine figures quoted in hp or PS, convert to kW first (1 hp = 0.7457 kW; 1 PS = 0.7355 kW) before using power mode.
The formula behind the torque calculation
Rotational power is P = T × ω. Angular velocity from RPM is ω = 2π × N ÷ 60 rad/s. Solving for torque: T = P ÷ ω = 60 × P ÷ (2π × N). With P in kilowatts (multiply by 1,000) this becomes T = 60,000 × kW ÷ (2π × rpm) = 9,549.3 × kW ÷ rpm.
Worked example: a motor delivering 100 kW at 3,000 rpm. T = 9,549.3 × 100 ÷ 3,000 = 318.3 N·m, which is 318.3 ÷ 1.3558179 = 234.8 lb·ft. Conversion example: 300 N·m equals 300 ÷ 1.3558179 = 221.3 lb·ft and 300 ÷ 9.80665 = 30.59 kgf·m.
Common mistakes
- Using horsepower directly in the kW formula — convert first (1 hp = 0.7457 kW, 1 PS = 0.7355 kW) or the torque comes out about 34% too high.
- Comparing peak power and peak torque at the same RPM: they occur at different engine speeds, and T = 9,549.3 × kW ÷ rpm links them only point-by-point on the curve.
- Confusing lb·ft (torque) with ft·lb conventions or with lbf (force) — torque needs both the force and the lever arm.
- Mixing up N·m and kgf·m: they differ by a factor of 9.80665, so a spec misread costs almost 10×.
자주 묻는 질문
How do I calculate torque from horsepower and RPM?
Convert the power to kilowatts (1 hp = 0.7457 kW), then apply T = 9,549.3 × kW ÷ rpm. Example: 300 hp = 223.7 kW at 5,000 rpm gives 9,549.3 × 223.7 ÷ 5,000 = 427.2 N·m, or about 315 lb·ft. The imperial shortcut T (lb·ft) = 5,252 × hp ÷ rpm gives the same result directly from horsepower.
Where does the constant 9,549.3 come from?
From converting RPM to radians per second. Power P = T × ω and ω = 2π × rpm ÷ 60, so T = P ÷ ω = 60 × P ÷ (2π × rpm). Expressing P in kilowatts multiplies the numerator by 1,000, giving T = 60,000 ÷ (2π) × kW ÷ rpm = 9,549.3 × kW ÷ rpm.
How many N·m is 1 lb·ft?
One pound-foot equals 1.3558179 newton-metres — one pound-force (4.4482216 N) acting at a lever arm of one foot (0.3048 m). Conversely, 1 N·m is about 0.7376 lb·ft. A 400 N·m engine therefore produces about 295 lb·ft.
Why do peak torque and peak power occur at different RPM?
Because power is torque multiplied by rotational speed. An engine's torque curve typically peaks in the mid range and falls at high RPM; power keeps rising as long as speed increases faster than torque falls, so peak power arrives at a higher RPM than peak torque. The two figures describe the same curve at different points.
What is a kgf·m and is it still used?
The kilogram-force-metre is torque expressed with the kilogram-force: 1 kgf·m = 9.80665 N·m, so a 30 kgf·m engine produces about 294 N·m. It is not an SI unit but still appears in older documentation and some Asian-market vehicle specifications, usually written kg·m or kgm.
Does gearing change torque or power?
Gearing multiplies torque and divides speed by the same ratio, leaving power unchanged apart from frictional losses. A 3:1 reduction triples the output torque at one third of the RPM. This is why wheel torque in first gear far exceeds the engine's rated torque while the transmitted power stays the same.
참고 자료
- NIST Special Publication 811 (Thompson & Taylor). Guide for the Use of the International System of Units (SI), 2008 — Appendix B torque conversion factors (lb·ft, kgf·m).
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019 — the newton-metre and radian.