Horsepower Calculator: Torque, RPM, Wheel and Crank HP
Horsepower, torque and rpm are tied together by one relationship. This calculator converts between them, estimates wheel or crank horsepower using an assumed drivetrain loss, and converts horsepower to kilowatts and metric horsepower (PS).
Horsepower and torque
Uses the standard definition where horsepower and lb-ft of torque cross at 5,252 rpm.
- Power (kW)
- 319.5 kW
- Power (PS)
- 434.4 PS
- Torque
- 450.0 lb-ft
- Engine speed
- 5,000 rpm
How the numbers relate
Power is the rate of doing work, torque is a turning force, and rpm is how fast the engine turns. Measured in pound-feet, the standard relationship is horsepower = torque (lb-ft) x rpm / 5,252.
The constant 5,252 comes from the original definition of mechanical horsepower, 33,000 pound-feet per minute, divided by 2 x pi. That is why a torque curve and a horsepower curve cross at exactly 5,252 rpm when torque is quoted in lb-ft.
In metric units the same idea is written kW = torque (N-m) x rpm x 2 x pi / 60,000. PS is a closely related metric unit that is slightly smaller than mechanical horsepower, so a figure quoted in PS is always a little higher than the same figure in hp.
Drivetrain loss is the share of engine power lost between the crankshaft and the wheels through the gearbox, driveshaft, differential and tyres. It is normally quoted as a rule-of-thumb percentage that varies with drivetrain layout, so any wheel to crank conversion can only ever be approximate.
Worked example
Three quick checks using the same engine: 450 lb-ft of torque at 5,000 rpm, 400 hp at 6,000 rpm, and 340 wheel horsepower at an assumed 15 percent loss.
- Torque at 5,000 rpm
- 450.0 lb-ft
- Resulting power
- 428.4 hp
- Resulting power
- 319.5 kW
- Power at 6,000 rpm
- 400.0 hp
- Resulting torque
- 350.1 lb-ft
- Wheel horsepower
- 340.0 hp
- Estimated crank horsepower
- 400.0 hp
Frequently asked questions
Why do horsepower and torque curves cross at 5,252 rpm?
Because the constant in the relationship is 5,252 when torque is in lb-ft. Below that engine speed torque is numerically larger than horsepower, and above it horsepower is larger. The crossing point is a consequence of the units, not the engine itself.
What is the difference between wheel and crank horsepower?
Crank horsepower is measured at the engine, usually on an engine dyno. Wheel horsepower is measured at the driven wheels on a chassis dyno and is lower because the drivetrain absorbs some power. The gap is the drivetrain loss.
How much power does a drivetrain lose?
There is no single correct number. Commonly quoted rules of thumb range from around ten percent for a well sorted manual to noticeably more for four wheel drive, but the true figure depends on the specific car and the dyno. Use the loss figure as an adjustable estimate.
How do I convert horsepower to kilowatts?
Multiply horsepower by 0.7457 to get kilowatts, or divide kilowatts by 0.7457 to get horsepower. The convert option in this calculator does it in both directions and also shows PS.
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Last updated: October 11, 2026