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Horsepower Calculator

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Power, torque and engine speed are three faces of one relation, so knowing two of them fixes the third. This solves for whichever is missing and reports the answer in all three of the units that specifications use — imperial horsepower, kilowatts and metric PS — because those are 1.4% apart and quoting one as another is the most common error in a spec table.

What do you want to find?
RPM

The speed at which that torque or power figure was measured.

Choosing a drivetrain treats the figure as a wheel measurement and estimates the crank figure.

How to use the horsepower calculator

  1. 1Choose which of power, torque or engine speed you want to find.
  2. 2Enter the other two, picking the units they were quoted in.
  3. 3Leave the drivetrain as 'measured at the crank' for a manufacturer's figure.
  4. 4Choose a drivetrain to treat your figure as a rolling-road measurement and estimate what the crank was making.

Examples

A torque figure turned into power

Input
250 lb-ft at 5500 RPM
Result
262 hp, 195 kW, 265 PS

hp = lb-ft x RPM / 5252, so the same torque at 3000 RPM would be only 143 hp.

Working back to torque

Input
150 kW at 4200 RPM
Result
341 N·m, or 251.5 lb-ft

Useful when a specification quotes peak power and peak torque at different speeds and you want to compare like with like.

Wheels to crank

Input
300 hp at the wheels, manual rear-wheel drive
Result
About 353 hp at the crank

The 15% figure is a convention, not a measurement — it moves with gear, temperature and how the run was done.

About the horsepower calculator

One relation in two sets of units

In SI the statement is unadorned: power in watts equals torque in newton metres times angular velocity in radians per second. Nothing is hidden and no constant appears. The imperial version, hp = lb-ft x RPM / 5252, looks like a different formula with a magic number in it, but it is the same relation with the unit conversions folded into one divisor — 33,000 foot-pounds per minute in a horsepower, over 2 pi radians in a revolution.

Writing it out that way makes the 5252 stop being magic, and it explains the one fact everybody notices about a dyno plot. The curves cross there because at that speed the arithmetic of the units makes the two figures numerically equal, not because anything changes in the engine.

Where the figure was measured changes what it means

An engine dynamometer measures at the flywheel with the engine on a stand, which is the condition manufacturers quote. A chassis dynamometer measures at the roller under the driven wheels, with the whole transmission, differential and tyres in the path. The second number is always smaller, and the difference is not a constant — it includes gear meshing losses, oil churning that depends on temperature, and tyre deformation that depends on pressure and how hard the tyre is squashed against the drum.

It is also worth knowing that published figures are governed by standards that specify air temperature, pressure and humidity corrections, and what accessories are driven during the test. Two figures from different standards are not directly comparable, and a figure with no standard attached is a marketing number.

Using it sensibly

The most useful thing this calculator does is let you compare quoted figures that were not written in the same units. A car advertised at 265 PS and another at 262 hp are the same car; a spec sheet listing 195 kW is that car again. Converting all three to one unit before comparing takes a second and removes an entire class of confusion.

The second use is checking that a quoted pair is self-consistent. Peak power and peak torque usually occur at different engine speeds, so they do not have to satisfy the relation simultaneously — but if a figure claims a certain power at the same RPM as a stated torque, the two must agree. When they do not, one of the numbers has been rounded, converted badly, or invented.

Frequently asked questions

Why do horsepower and torque curves always cross at 5252 RPM?
Because of the units, and for no mechanical reason at all. Horsepower in the imperial system is defined as torque in pound-feet times RPM divided by 5252, and that divisor is Watt's 33,000 foot-pounds per minute divided by the 2 pi radians in a revolution. Set RPM to 5252 and the division does nothing, so the two numbers are equal and the curves meet. Plot the same engine in newton metres and kilowatts and the crossing moves somewhere else entirely, which is the clearest evidence that it means nothing.
What is the difference between hp, bhp, PS and kW?
Kilowatts are SI and unambiguous. Mechanical horsepower is 745.7 W and is what American figures mean. Metric horsepower — written PS in Germany, cv in France and Italy, pk in the Netherlands — is 735.5 W, about 1.4% smaller, and is what most older European figures meant. 'Brake horsepower' describes how the measurement was taken, at the flywheel against a brake, rather than naming a different unit; in modern usage it is normally mechanical horsepower. A car quoted at 300 PS is 296 hp, and the gap is entirely in the definitions.
Is torque or power the number that matters?
Power, if you are only allowed one. Torque is the twisting effort at a given instant, but gearing multiplies torque freely — a low first gear turns modest engine torque into a great deal at the wheels — while no gearbox creates power. Acceleration at a given road speed depends on the power available there, which is why engines that rev high can out-accelerate ones making far more torque lower down. What torque describes well is how the engine feels: a broad torque curve means fewer gearchanges to stay in the useful part of the range.
How accurate is the wheel-to-crank estimate?
Treat it as an order-of-magnitude sanity check rather than a number to quote. The percentages here are the conventions the tuning world uses — roughly 10% for a front-wheel-drive manual, 15% for rear-wheel drive, more for automatics and all-wheel drive — but a real drivetrain's loss is not a fixed percentage at all. It varies with the gear the run was done in, oil temperature, tyre pressure and the rolling resistance of the tyre against the drum. Two dynos rarely agree on the same car, which is why the useful comparison is before and after on the same machine on the same day.
Can I use this for an electric motor?
Yes, and the relation is the same one — power is torque times angular velocity whatever produces it. The shape of the curve is what differs. An internal combustion engine makes very little torque at low RPM and needs gearing to be useful; an electric motor produces close to its full torque from a standstill and holds it until it reaches base speed, after which power stays roughly constant and torque falls away. So a single quoted torque figure describes an electric motor far better than it describes an engine, and a single power figure describes neither on its own.

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