The formula
How it works
Horsepower measures how fast an engine can do work. It comes straight from two numbers you can read off a dyno chart — torque and engine speed — using a formula whose magic constant, 5252, is where every torque and power curve cross.
FAQ
Why the number 5252?
It comes from converting rotational work into horsepower: 5252 is 33,000 (the definition of one horsepower in foot-pounds per minute) divided by 2π. It is also the RPM where the torque and horsepower curves always meet.
How do I convert horsepower to kilowatts?
Multiply by 0.7457. So 300 horsepower is about 224 kW. The calculator shows the kilowatt figure alongside the result.
What is the difference between horsepower and torque?
Torque is the twisting force an engine produces right now, while horsepower measures how quickly that force does work as the engine spins. A low-revving engine can have huge torque but modest horsepower if it does not spin fast.
Why does horsepower rise with RPM even if torque stays flat?
Horsepower is torque multiplied by engine speed, divided by the constant. So if torque holds steady while RPM climbs, the horsepower figure climbs right along with it.
What is brake horsepower?
Brake horsepower is the power measured directly at the engine’s crankshaft on a dynamometer, before losses in the transmission and drivetrain. It is usually a bit higher than the power that actually reaches the wheels.
Can I use this formula for electric motors?
Yes — the same torque-times-speed relationship applies to any rotating shaft, including electric motors. Just plug in the motor’s torque and rotational speed the same way.
Does the formula work in metric units?
The 5252 constant is specific to pound-feet and rpm. For torque in newton-metres, the equivalent formula is kW = (T × RPM) / 9550, which this calculator’s kilowatt output is consistent with.
About the horsepower calculator
This calculator works out an engine’s horsepower from its torque and the speed it is turning at. Torque is the twisting force the engine produces, and power is how quickly that force does work — combine them with engine speed and you get horsepower, the number most people use to describe how strong an engine is. It is the same calculation a dynamometer performs when it measures an engine.
How to use it
Enter the torque in pound-feet and the engine speed in revolutions per minute. The calculator returns the horsepower, and the equivalent in kilowatts. For example, an engine making 300 lb-ft of torque at 5,252 rpm produces exactly 300 horsepower — a neat result that follows from the formula’s constant. Increase the RPM and the horsepower rises even if torque stays the same, which is why engines make peak power high in the rev range.
The formula
Horsepower from torque is , where is the torque in pound-feet and RPM is the rotational speed. The constant 5252 converts the twisting work per minute into the horsepower unit. A useful consequence is that when RPM equals 5252, horsepower equals torque exactly — which is why those two curves always cross at that point on a dyno graph. To convert to metric, one horsepower is about 0.7457 kilowatts.
Where it is used
Horsepower is the headline number for cars, motorcycles, boats and machinery, used by manufacturers to advertise performance and by buyers to compare models. Tuners and engine builders use the formula to turn dyno torque readings into power figures, and to see where in the rev range an engine is strongest. The same idea scales to electric motors, generators and industrial equipment, wherever rotating machinery does work.