What Horsepower Measures
Power is the rate of doing work. Torque is the twisting force an engine produces; horsepower is how quickly that force can do work, which is why it depends on engine speed as well as torque.
Horsepower = Torque (lb-ft) × RPM ÷ 5,252
The constant 5,252 comes from the definition of horsepower itself: 33,000 ft-lb per minute, divided by 2π radians per revolution. It has a well-known consequence — torque and horsepower curves always cross at exactly 5,252 rpm, on every engine ever built. If a dyno chart shows them crossing anywhere else, the chart is wrong.
Where the Unit Came From
James Watt needed to sell steam engines to mine owners who measured output in horses. He observed a mill horse turning a wheel and calculated that it did about 33,000 foot-pounds of work per minute — lifting 330 pounds by 100 feet in a minute. That figure became one horsepower, and it was generous: a real horse sustains roughly 0.7 hp over a working day, though it can peak near 15 hp briefly.
The Different Horsepowers
| Unit | Watts | Used in |
|---|---|---|
| Mechanical horsepower (hp) | 745.700 | United States, United Kingdom |
| Metric horsepower (PS, cv, ch) | 735.499 | Continental Europe, Japan |
| Kilowatt (kW) | 1,000 | SI standard, EU legal figure |
| Boiler horsepower | 9,809.5 | Steam boilers only |
Metric horsepower is about 1.4% smaller than mechanical, which is why a car quoted at 300 PS in Germany is roughly 296 hp in America. Manufacturers are not always careful about which they cite.
Common Conversions
| Horsepower | Kilowatts | PS |
|---|---|---|
| 100 | 74.6 | 101.4 |
| 150 | 111.9 | 152.1 |
| 200 | 149.1 | 202.8 |
| 300 | 223.7 | 304.2 |
| 400 | 298.3 | 405.5 |
| 500 | 372.8 | 506.9 |
| 700 | 522.0 | 709.7 |
| 1,000 | 745.7 | 1,013.9 |
Brake, Wheel and Crank Horsepower
| Figure | Measured at | Typical relationship |
|---|---|---|
| Crank / brake horsepower (bhp) | The engine's output shaft | The quoted number |
| Wheel horsepower (whp) | The driven wheels on a dyno | 10–15% lower (front-wheel drive), 15–20% (rear), 20–25% (all-wheel) |
| SAE net | Engine with all accessories fitted | The modern U.S. standard |
| SAE gross | Bare engine, no accessories | Used before 1972; inflated figures by 15–25% |
The 1972 switch from gross to net ratings is why muscle-car figures appear to collapse overnight in that model year. The engines barely changed; the measuring standard did.
Torque or Horsepower?
The short answer: torque determines how hard the car pushes at a given engine speed; horsepower determines how fast it can keep doing that. Acceleration at any moment is set by torque at the wheels, which gearing multiplies — and gearing is why a high-revving engine with modest torque can out-accelerate a low-revving one with more, by staying in a lower gear.
For top speed and sustained acceleration, horsepower is what matters. For towing and low-speed pulling, torque at low rpm matters more. Electric motors deliver near-peak torque from zero rpm, which is why an EV feels quick from a standstill even when its horsepower figure looks unremarkable.
Power to Weight
Acceleration depends on power relative to mass, not power alone.
| Vehicle type | Typical hp per ton | 0–60 mph |
|---|---|---|
| Economy car | 70–100 | 9–12 s |
| Family sedan | 100–140 | 7–9 s |
| Hot hatch | 150–200 | 5.5–7 s |
| Sports car | 200–300 | 4–5.5 s |
| Supercar | 300–500 | 2.5–3.5 s |
Frequently Asked Questions
Why do torque and horsepower always cross at 5,252 rpm?
Because horsepower = torque × rpm ÷ 5,252. When rpm equals 5,252 the multiplier is exactly 1, so the two values are numerically identical regardless of the engine.
Is 1 hp the same as 1 PS?
No. One PS is 735.5 W and one hp is 745.7 W, so PS figures run about 1.4% higher for the same engine.
Why is my dyno figure lower than the advertised power?
A chassis dyno measures at the wheels, after losses through the gearbox, driveshafts and differential. Expect 10–25% below the crank figure depending on drivetrain layout.
Does more horsepower mean a faster car?
Only relative to weight, gearing and traction. A light car with 200 hp routinely out-accelerates a heavy one with 300.