Horsepower CalculatorFree online other calculator — instant results, no sign-up
Horsepower from torque and RPM.
About the Horsepower Calculator
This calculator determines mechanical horsepower and equivalent electrical/metric power in kilowatts based on rotational torque and engine speed (RPM). It is commonly used by automotive enthusiasts, mechanics, and engine builders to analyze engine dynamometer outputs and drivetrain performance.
- Torque
- Rotational force exerted by the engine, measured in pound-feet (lb-ft). Typical automotive engines range from 100 to 800 lb-ft.
- RPM
- Rotational speed of the engine crankshaft in revolutions per minute. Typical passenger car operating ranges are between 1,000 and 7,000 RPM.
Formula and method
Horsepower = (Torque × RPM) / 5252; Kilowatts = Horsepower × 0.7457
- TorqueRotational force in pound-feet (lb-ft)
- RPMEngine crankshaft speed in revolutions per minute
- 5252Conversion constant derived from 33,000 ft-lb/min divided by 2π radians per revolution
- 0.7457Standard conversion factor from mechanical horsepower to kilowatts
The tool multiplies torque by engine RPM and divides the product by 5252 to calculate mechanical horsepower. It then multiplies that horsepower value by 0.7457 to determine the equivalent power in kilowatts. Both output figures are rounded to two decimal places.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Torque300 lb-ft
- RPM5252
Results
- Horsepower300
- Kilowatts223.71
How to use the Horsepower Calculator
- 1Multiply the measured torque in lb-ft by the engine speed in RPM.
- 2Divide that result by the constant 5252 to obtain mechanical horsepower.
- 3Multiply the horsepower figure by 0.7457 to determine power in kilowatts.
- 4Round both resulting values to two decimal places for display.
Reading your result
Horsepower indicates the rate at which the engine can do work at a specific rotational speed. At exactly 5,252 RPM, horsepower and torque (in lb-ft) will always have identical numerical values. Kilowatts represent the exact same power output expressed in metric units.
Limitations
This calculation produces power at a single operating point and does not represent peak power unless peak torque occurs at that specific RPM. It also assumes torque is provided in pound-feet; entering metric torque (Newton-meters) directly will produce inaccurate results. Furthermore, it does not account for drivetrain frictional losses between the crankshaft and the drive wheels.
Common questions
Why do torque and horsepower always cross at 5,252 RPM?+
One mechanical horsepower is defined as 33,000 foot-pounds of work per minute. Converting circular motion to linear distance involves 2π radians per revolution, yielding 33,000 / (2π) ≈ 5,252.11. Consequently, when RPM equals 5,252, RPM and the constant cancel out, making horsepower numerically equal to torque in lb-ft.
Can I use torque in Newton-meters (Nm) with this formula?+
No, the 5252 divisor applies strictly when torque is measured in pound-feet (lb-ft). If your torque is in Newton-meters, divide by 9,549 to get power in kilowatts, or convert Newton-meters to pound-feet first by dividing by 1.3558.
Does this calculate wheel horsepower or brake (crankshaft) horsepower?+
The formula applies to whichever point torque was measured. If torque is measured at the wheel hubs via a chassis dynamometer, the output is wheel horsepower (whp); if measured at the flywheel or crankshaft via an engine dyno, it reflects brake horsepower (bhp).
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.