Dyno sheets and spec sheets often give an engine's torque at a certain rpm but leave the horsepower to be worked out. The two are linked by a simple relationship: power is torque times rotational speed. This calculator converts a torque reading (in pound-feet or newton-metres) and an engine speed into horsepower, kilowatts and metric horsepower, so you can compare engines quoted in different units.
How to use the torque to horsepower calculator
- Enter the torque and choose lb-ft or N·m.
- Enter the engine speed in rpm at which that torque is produced.
- Read the power in horsepower, kilowatts and PS.
To go the other way, use the horsepower to torque calculator. To estimate how fast a car with a given power will run the quarter mile, try the quarter mile calculator.
Torque to horsepower formula
Power (W) = torque (N·m) × 2π × rpm ÷ 60
1 hp = 745.70 W; 1 PS = 735.50 W; 1 lb-ft = 1.35582 N·m
- 5,252.11 = 33,000 ft·lbf/min ÷ 2π, the constant that ties one horsepower to one revolution per minute at 1 lb-ft
- 2π ÷ 60 converts rpm to radians per second
Worked example: 400 N·m at 5,000 rpm
- Angular speed: 5,000 × 2π ÷ 60 = 523.6 rad/s.
- Power: 400 × 523.6 = 209,440 W = 209.4 kW.
- Horsepower: 209,440 ÷ 745.7 = 280.9 hp.
- Metric horsepower: 209,440 ÷ 735.5 = 284.8 PS.
Power at different engine speeds
With constant torque, power is proportional to rpm. Every value was calculated with the formula above.
| Engine speed (rpm) | Power at 300 lb-ft | Power at 400 N·m |
|---|---|---|
| 2,000 | 114.2 hp | 112.3 hp |
| 3,000 | 171.4 hp | 168.5 hp |
| 4,000 | 228.5 hp | 224.7 hp |
| 5,000 | 285.6 hp | 280.9 hp |
| 5,252 | 300.0 hp | 295.0 hp |
| 6,000 | 342.7 hp | 337.0 hp |
| 7,000 | 399.8 hp | 393.2 hp |
Torque, power and how an engine feels
Torque is the twisting force the engine puts out; horsepower is how fast it can deliver that force. Two engines with the same peak torque can feel very different if one makes it at 2,000 rpm and the other at 5,000 rpm, because the second delivers much more power at the top end. Real engines do not hold constant torque, so the calculator only gives power at the single rpm you enter. To find peak power, enter the torque at several rpm values from a dyno chart or the manufacturer's curve and take the highest result.
The conversion also explains why diesels and turbocharged engines often feel strong at low speed despite modest horsepower: they make high torque at low rpm. Gearing multiplies that torque at the wheels — see the gear ratio calculator and the engine RPM calculator — so what you feel on the road depends on torque, gearing and weight together.
Figures quoted at the crankshaft (brake horsepower) are higher than at the wheels, where drivetrain losses reduce them. The formula here applies equally to either, as long as both torque and rpm are measured at the same place.
Sources
The relationship power = torque × angular speed is classical mechanics; one horsepower is defined as 33,000 foot-pounds per minute (745.6999 W), and 1 lb-ft = 1.3558179 N·m (NIST). Metric horsepower is defined as 735.49875 W (DIN 66036). Table values were computed from the formula above.
Frequently asked questions
What is the formula to convert torque to horsepower?
Horsepower = torque (lb-ft) × rpm ÷ 5,252. In SI, power (W) = torque (N·m) × 2π × rpm ÷ 60.
Why does the number 5,252 appear?
One horsepower is 33,000 ft·lbf per minute. Dividing 33,000 by 2π (one revolution) gives 5,252.1, so at that engine speed the horsepower and lb-ft figures are equal.
Why do dyno graphs cross at 5,252 rpm?
Because horsepower = torque × rpm ÷ 5,252, the two curves must meet where rpm = 5,252 when both are plotted in hp and lb-ft.
What is PS?
PS (Pferdestärke) is metric horsepower, defined as 735.49875 W. It is about 1.4% smaller than the mechanical horsepower of 745.7 W.
Does the result tell me peak power?
No. It gives the power at the one rpm you enter. Peak power is found by calculating at several rpm values using the torque the engine makes at each.