Horsepower Calculator

Why 5252 keeps appearing

Mechanical horsepower was defined by James Watt as 33,000 foot-pounds of work per minute. Convert rotational work — torque times angle — into that unit and the constant falls out: HP = torque × RPM ÷ 5252, where 5252 is 33,000 divided by 2π. It is not a fudge factor and it should not be rounded to 5250. One consequence is that every engine's horsepower and torque curves cross at exactly 5252 RPM when both are plotted in imperial units, which is a property of the arithmetic rather than anything about the engine.

Three different horsepowers

Mechanical horsepower is 745.7 watts. Metric horsepower — PS, or cheval vapeur — is 735.5 watts, about 1.4% smaller, which is why a European brochure figure looks slightly larger than the same engine quoted in imperial units. Electrical horsepower is defined as exactly 746 watts. The differences are small but they are not noise: a 500 PS car is 493 hp, and a spec sheet that mixes the two is misleading rather than wrong.

Estimating from the drag strip

The elapsed-time and trap-speed methods are empirical rules of thumb, not physics. Both are anchored to vehicle weight, because acceleration depends on power-to-weight. The trap-speed method is generally the more reliable of the two, because terminal speed is set mostly by power against drag, while elapsed time is heavily influenced by how well the car launches. A car that spins its tyres for the first sixty feet will show a low estimate from elapsed time and a much higher one from trap speed — and the trap number will be closer to the truth.

At the crank or at the wheels

Manufacturer figures are usually measured at the crankshaft on an engine dynamometer. A chassis dynamometer measures at the wheels, after the gearbox, driveshafts and differential have each taken their cut — typically 10 to 15% for a manual, more for an automatic or all-wheel drive. Drag-strip estimates like the ones here produce a crank-equivalent figure, because vehicle weight already includes everything the drivetrain does. Comparing a wheel-measured number against a brochure crank number and concluding the engine is down on power is one of the most common mistakes in the hobby.

Power-to-weight tells you more

Horsepower on its own predicts very little about how a vehicle feels. The ratio of power to mass is what governs acceleration, which is why a 200 hp motorcycle out-accelerates a 500 hp saloon. The power-to-weight line above is given in horsepower per ton so it can be compared directly across vehicles of different sizes.

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