How to use the horsepower from torque calculator
- Choose whether to find horsepower from torque or torque from horsepower.
- Enter the torque in pound-feet (or the horsepower) and the engine speed in revolutions per minute.
- Read the result in horsepower and kilowatts. Torque is also shown in newton meters.
- The table beneath repeats your figure at other engine speeds to show how the two quantities relate.
Formula
Horsepower = torque (lb-ft) x rpm / 5252
Torque (lb-ft) = horsepower x 5252 / rpm
1 hp = 0.745699872 kW and 1 lb-ft = 1.3558179483 N-m
The constant 5252 comes from the definition of horsepower as 33,000 foot-pounds per minute. One turn is 2 x pi radians, so 33,000 / (2 x pi) is 5,252.11, which is usually rounded to 5252.
300 lb-ft at two engine speeds
At 5,252 rpm, 300 lb-ft makes 300 x 5,252 / 5,252 = 300 hp, because the speed cancels the constant. This is why a torque curve and a horsepower curve cross at 5,252 rpm.
At 4,000 rpm the same torque makes 300 x 4,000 / 5,252 = 228.5 hp, which is 170.4 kW. Going the other way, 300 hp at 4,000 rpm is 300 x 5,252 / 4,000 = 393.9 lb-ft.
Why 5252
Power is torque times rotational speed. One horsepower is defined as 33,000 foot-pounds of work per minute. Each revolution moves a force through 2 x pi radians, so power in horsepower is torque in pound-feet times rpm times 2 x pi, divided by 33,000. Simplified, that is torque times rpm divided by 5,252.11. The shorter figure 5252 is used in almost all published formulas, and this tool uses it too.
Because the formula includes rpm, horsepower rises with speed even if torque stays the same. That is why a high revving engine can make more power than a low revving one with the same torque.
One point at a time
The formula links torque and power at a single engine speed. A real engine makes different torque at different speeds, so one pair of numbers does not describe the whole engine. Peak horsepower and peak torque can occur at different speeds. If you enter a peak torque with the speed of peak power, the answer will not match any real measurement.
The result also does not say where the figures were measured. Crank and wheel figures differ because of drivetrain losses. For comparing cars by weight, see the power to weight ratio calculator, and for a rough estimate of acceleration see the quarter mile time estimator.
Assumptions and limits
- Torque and speed are measured at the same moment.
- The constant 5252 is the rounded value of 33,000 / (2 x pi).
- Torque is in pound-feet and power is in mechanical horsepower.
- Conversions to kilowatts and newton meters use the exact definitions of the units.
Frequently asked questions
How do I convert torque to horsepower?
Multiply torque in pound-feet by engine speed in rpm and divide by 5252. 300 lb-ft at 4,000 rpm gives 228.5 hp.
Why do the torque and horsepower curves cross at 5,252 rpm?
At 5,252 rpm the formula reduces to horsepower equals torque, because the rpm and the constant cancel. Below that speed horsepower is smaller than torque in the same units, and above it horsepower is larger.
Can I use newton meters?
The form takes pound-feet. To convert, divide newton meters by 1.3558179483 first. The result panel shows the newton meter equivalent of your pound-feet.
Is the horsepower from this formula the same as a dyno reading?
Only if the torque reading came from the same test and speed. Dyno figures can be measured at the crank or the wheels, so check which one you are using.