Horsepower and Torque Calculator

Worked example

300 lb-ft at 5,252 rpm is exactly 300 hp — the two figures always cross at that speed, because 5,252 is what the definition of horsepower works out to in revolutions. It is 223.7 kW, or 304.2 PS in a European brochure.

How to work it out yourself

1. 1.Take the torque figure and the engine speed it was measured at. A single torque number without an rpm says nothing about power.
2. 2.Divide the product by 5,252. Below that speed an engine makes more torque than horsepower; above it, more horsepower than torque.
3. 3.Compare like with like across markets: PS is about 1.4% smaller than mechanical horsepower, so the same engine always reads slightly higher in a European brochure than in an American one.

Horsepower at 300 lb-ft, by engine speed

Torque held at 300 lb-ft.

The formula

1. Horsepower300 * 5252 / 5252
2. Kilowatts300 * 0.7456999
3. Metric horsepower (PS)223.70996 / 0.7354987
4. Torque in newton meters300 * 1.3558179

Source: NIST Special Publication 811 — Guide for the Use of the International System of Units

Questions people actually ask

Why do horsepower and torque always cross at 5,252 rpm?

Because horsepower is defined as 33,000 foot-pounds per minute, and a revolution is 2π radians. Dividing 33,000 by 2π gives 5,252.11, so the formula is torque × rpm ÷ 5,252 — and at exactly that speed the multiplier is one. Every dyno graph in imperial units crosses there, and a graph that does not is drawn wrong.

Which matters more, horsepower or torque?

Torque is the twist available now; horsepower is the rate of doing work, which is torque multiplied by how fast you can repeat it. Acceleration at a given speed follows the power at the wheels, which is why a high-revving small engine can out-accelerate a low-revving large one despite far less torque. Towing favours torque low down, because that is where the load sits.

What is the difference between hp, PS and kW?

Mechanical horsepower is 745.7 W. PS — Pferdestärke, quoted as CV in France and cv in Italy — is 735.5 W, about 1.4% less. The kilowatt is the SI unit and the only one of the three that is unambiguous. European figures are often quoted in all three on the same page.

Is this crank power or wheel power?

Whatever your torque figure was measured at. Manufacturer numbers are at the crankshaft; a chassis dyno reads at the wheels and comes out roughly 10 to 15% lower on a rear-drive car because the drivetrain absorbs the difference. The formula does not know which, so the answer inherits the input.

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