Brake horsepower vs shaft horsepower (and wheel hp, PS and kW)
Brake horsepower, shaft horsepower and wheel horsepower all use the same unit. What changes is where the power is measured, and that can move the number by 20% or more on the same vehicle.
The short version: 1 hp (whether "brake" or "shaft") = 745.7 watts = 0.7457 kW. Car makers quote brake horsepower, measured at the crankshaft. Wheel horsepower is what reaches the road and is always lower. Shaft horsepower is mostly used for ships, helicopters and turboprop aircraft.
Brake horsepower (bhp)
Brake horsepower is the power an engine produces at its crankshaft, measured on an engine dynamometer with the engine out of the car or before the gearbox. The name comes from the 19th-century Prony brake, a friction brake clamped to the shaft: engineers measured how much braking force it took to hold the engine at a given speed and worked out the power from that.
When a brochure says a car has "150 hp", it almost always means brake horsepower. In the US, figures have been quoted as SAE net horsepower since 1972, which measures the engine with its production accessories (alternator, water pump, exhaust) fitted. Older "gross" ratings were taken without them, which is why some 1960s muscle-car figures look high next to modern numbers.
Shaft horsepower (shp)
Shaft horsepower is power delivered to an output shaft, typically after a reduction gearbox. It's the standard figure for turboprop and helicopter turbines and for ship propulsion, where what matters is the power turning the propeller or rotor. For cars the term is rarely used; a car's equivalent is the power at the gearbox output, which nobody quotes.
Wheel horsepower (whp)
Wheel horsepower is measured on a chassis dynamometer ("rolling road") with the car driving its wheels against rollers. It's lower than brake horsepower because the gearbox, differential, driveshafts and tyres absorb some power on the way. Losses of roughly 10–20% are commonly quoted, with all-wheel-drive cars and automatic gearboxes at the higher end, but the real figure depends on the car, the dyno and how it's run.
That's why tuners and owners' forums often talk in whp: it's what they can measure without removing the engine. Don't compare a whp figure with a manufacturer's bhp figure directly.
PS, CV and kW: the metric units
European and Japanese makers often quote PS (Pferdestärke, also written CV or ch in France, pk in the Netherlands), which is metric horsepower: 735.5 watts, about 1.4% less than an imperial horsepower. Official documents in most of the world use kilowatts.
| Unit | Where you'll see it | Watts |
|---|---|---|
| hp / bhp | US and UK brochures, magazines | 745.7 |
| PS / CV / pk | German, French, Italian, Dutch and Japanese brochures | 735.5 |
| kW | Registration documents, EU type approval, most of the world | 1,000 |
A worked example: a common 1.0-litre turbo engine is rated at 66 kW. That's 90 PS in a German brochure and 89 hp in a British one. Same engine, three numbers. Our horsepower to kW converter does the arithmetic for any figure.
Which number should you compare?
- Compare like with like: two manufacturers' bhp (or kW) figures are directly comparable. A wheel-horsepower dyno sheet isn't comparable with either.
- Watch for PS figures labelled "hp". Some sources print the metric number with an imperial label, overstating power by about 1.4%. Checking against the kW figure settles it: kW × 1.341 = hp, kW × 1.360 = PS.
- For electric cars, check whether a figure is peak or continuous power. EU type-approval records list a 30-minute rated power that can be far below the peak output in the brochure.
Every engine on our car pages is shown in both hp and kW, from official type-approval records and manufacturer specifications. Start with all cars, or compare two in the comparison tool.