What Is Engine Torque?
Torque is a twisting force. Every time an engine turns its crankshaft, it applies a twist — and the size of that twist is the engine's torque. Think of loosening a stubborn bolt with a wrench: the harder you push and the longer the wrench, the more torque you apply to the bolt. An engine does exactly the same job, thousands of times per minute.
Torque matters because it is the pushing force that accelerates your car, hauls a trailer up a hill, or spins the wheels when you launch from a stop. Engine spec sheets quote it alongside power, and once you know both figures at a given speed you understand what the engine is really doing.
The Torque Formula: T = P ÷ ω
Power and torque are two views of the same thing, tied together by how fast the shaft spins:
[
T = \frac{P}{\omega} \qquad \text{where} \qquad \omega = \frac{2\pi \times \text{rpm}}{60}
]
Where:
- T is the torque in newton-meters (N·m), when power P is in watts (W)
- P is the power in watts
- ω (omega) is the angular velocity of the crankshaft in radians per second
The 2π factor converts revolutions to radians (one revolution = 2π radians), and dividing by 60 converts minutes to seconds.
For imperial units, the same relationship folds into one memorable shortcut:
[
T \text{ (lbf·ft)} = \frac{\text{hp} \times 5252}{\text{rpm}}
]
The number 5252 is not magic — it is just the bundle of conversion factors (33,000 ft·lbf per horsepower, divided by 2π) that makes hp, rpm and lbf·ft fit the same equation.
Worked Example: A 100 kW Engine at 6000 rpm
Suppose an engine produces 100 kW at 6000 rpm. Step by step:
First convert the speed to angular velocity:
[
\omega = \frac{2\pi \times 6000}{60} = 628.32 \text{ rad/s}
]
Then divide power by angular velocity (remembering 100 kW = 100,000 W):
[
T = \frac{100{,}000}{628.32} = 159.15 \text{ N·m}
]
To convert to pound-feet, divide by 1.35582: 159.15 N·m ≈ 117.39 lbf·ft.
Cross-checking with the imperial shortcut: 100 kW = 134.1 hp, so T = 134.1 × 5252 ÷ 6000 ≈ 117.4 lbf·ft — the same answer, which confirms the calculation.
Try the calculator above with these numbers: enter 100, choose kilowatts, and enter 6000. You should get 159.15 N·m.
What Does the Result Mean?
Notice something important in that example: the same engine produces different torque at different speeds. If this engine made its 100 kW at 3000 rpm instead, the torque would be 318.31 N·m — double the figure, because the shaft spins half as fast. Power stayed the same; torque changed.
This is why dyno charts show torque and power curves crossing at exactly 5252 rpm on imperial scales: below that speed the torque number is bigger than the horsepower number, above it, smaller. Neither figure is "better" — they describe the same performance from two angles.
| Speed | Torque at 100 kW | Interpretation |
|---|---|---|
| 1500 rpm | 636.62 N·m | Big pulling torque — typical of a turbocharged diesel |
| 3000 rpm | 318.31 N·m | Strong mid-range punch for overtaking |
| 6000 rpm | 159.15 N·m | Peak-power scream — typical of a petrol engine at redline |
Reference: Units and Constants
Convert your power figure to watts before dividing by ω:
| Power unit | In watts |
|---|---|
| Watt (W) | 1 |
| Kilowatt (kW) | 1,000 |
| Horsepower (hp) | 745.7 |
| Metric horsepower (PS) | 735.5 |
Handy constants worth memorising:
| Quantity | Value |
|---|---|
| Angular velocity from rpm | ω = 0.10472 × rpm rad/s |
| 1 N·m in pound-feet | 0.73756 lbf·ft |
| 1 lbf·ft in newton-meters | 1.35582 N·m |
| Imperial shortcut divisor | 5252 |
Quick Recap
- Torque is twisting force; power is torque times rotational speed: T = P ÷ ω.
- Convert rpm to rad/s with ω = 2π × rpm ÷ 60, and power to watts, before dividing.
- Imperial shortcut: lbf·ft = hp × 5252 ÷ rpm — valid at any speed.
- The same power at lower rpm always means more torque, which is why diesels pull and race engines rev.
If you'd like to run the relationship the other way around, the RPM to torque calculator works from speed and torque back to power — a good way to sanity-check your results here.