Why So Many Units for the Same Thing
Engine power and torque get measured in half a dozen different units depending on where the spec sheet came from. American sources use horsepower (hp) and pound-feet (lb-ft). German and Japanese sources use kilowatts (kW) and Newton-meters (Nm). Italian and French manufacturers historically used PS (Pferdestaerke, literally “horse strength”). British publications use brake horsepower (bhp). All of these describe the same physical quantities — power and torque — but in different unit systems, with slightly different definitions, which is why direct comparisons between a Ferrari spec sheet, a BMW spec sheet, and a Corvette spec sheet require unit conversion.
The conversions are exact mathematical constants, not approximations. 1 horsepower (mechanical) equals 0.7457 kilowatts. 1 PS equals 0.7355 kilowatts. 1 bhp equals 0.7457 kilowatts (same as mechanical hp, with a slightly different historical definition). 1 lb-ft equals 1.3558 Newton-meters. The calculator above runs these conversions instantly in both directions, so you can compare a 350 hp American spec to a 261 kW European spec and confirm they are the same engine, not a translation error.
Horsepower vs Brake Horsepower vs PS
Horsepower (hp) — Mechanical or SAE. The standard American unit. Defined as 33,000 ft-lb of work per minute, or roughly 745.7 watts. Modern American horsepower specs are measured per SAE J1349 (the same correction standard discussed on the dyno correction page). One mechanical hp is exactly 745.6999 watts. Almost every US car spec since the 1970s uses this number.
Brake horsepower (bhp). Historically meant horsepower measured at the engine output shaft using a “brake” (Prony brake or dynamometer brake), before any drivetrain losses. The number is mathematically identical to mechanical horsepower (745.7 watts), but the term emphasizes that the measurement was taken at the engine flywheel, not at the wheels. British publications still use bhp routinely. American publications dropped the term decades ago because all OEM horsepower specs are measured at the flywheel by convention.
PS (Pferdestaerke or DIN horsepower). The metric horsepower used in Germany, Japan, France, and Italy. One PS is exactly 75 meter-kilograms of work per second, which works out to 735.5 watts — about 1.4 percent smaller than mechanical hp. A 300 PS engine is therefore about 296 hp in American units, or 220.6 kW in pure SI. Spec sheets that quote PS often also quote kW for the same engine, but rarely both PS and SAE hp on the same page.
Kilowatts (kW). The official SI unit for power. Used in legal vehicle documentation throughout Europe and increasingly worldwide. 1 kW equals 1.341 mechanical hp or 1.36 PS. The European Union legally requires kW on registration documents; the hp or PS figures are just for the marketing department.
Why Torque Units Trip People Up
Torque is force times distance, and the units used to measure it are not interchangeable without conversion. Pound-feet (lb-ft) puts the force unit first and the distance unit second — pounds of force at a one-foot lever arm. Newton-meters (Nm) does the same thing in SI units — newtons of force at a one-meter lever arm. The conversion factor is 1 lb-ft equals exactly 1.35582 Nm.
The trap is the confusingly similar term “foot-pounds” (ft-lb), which is technically an energy unit (work done), not a torque unit, although in practice the two are used interchangeably in American engineering. The numerical values are identical for the engine torque case. Just do not confuse “foot-pounds force” (energy/work) with “pound-feet of torque” if you are reading physics textbooks — for cars, treat them as the same and move on.
When converting torque on a tightening spec, always check whether the manual is giving lb-ft (most US OEMs) or in-lb (used for very small fasteners — 12 in-lb equals 1 lb-ft). Some Asian manuals use kg-m which is yet another option. The calculator above handles all of these.
Frequently Asked Questions
How do I convert horsepower to kilowatts?
Multiply horsepower by 0.7457 to get kilowatts. A 300 hp engine is 224 kW. The other direction is multiply kilowatts by 1.341 to get horsepower. A 250 kW engine is 335 hp. These factors are exact, not rounded estimates.
Is PS the same as horsepower?
Close but not identical. PS (metric horsepower) is about 1.4 percent smaller than mechanical horsepower. A 400 PS engine is approximately 394 hp in American units. The difference comes from the historical definitions — mechanical hp uses 550 ft-lb per second, PS uses 75 kg-m per second, and the underlying physical conversion does not work out to the same number. The two are often used loosely, but for precise spec comparisons, convert carefully.
What is the relationship between horsepower and torque?
Horsepower equals torque times RPM divided by 5,252 (for lb-ft and SAE hp), or torque times RPM divided by 9,549 (for Nm and kW). At 5,252 RPM, horsepower numerically equals torque in lb-ft for a moment — this is why dyno graphs always cross at 5,252 RPM. The two are not the same physical quantity. Torque is how hard the engine twists, horsepower is how fast it does so.
Why do European cars often quote PS instead of hp?
Historical convention. The metric system spread through continental Europe in the early 20th century, and engineers chose to use a metric horsepower (PS or CV) instead of the imperial unit. Even today, German, Italian, French, and Japanese OEM specifications quote PS as the primary marketing number, with kW shown for legal compliance and SAE hp shown only for export markets that demand it.
What does “wheel horsepower” mean?
Power measured at the driven wheels on a chassis dynamometer, after drivetrain losses. Engine horsepower (the marketed number) is measured at the flywheel and is higher than wheel horsepower by roughly 15 to 25 percent — front-wheel-drive vehicles tend to lose around 15 percent, all-wheel-drive vehicles around 22 to 25 percent, rear-wheel-drive vehicles in between. A 400 hp engine spec typically corresponds to 300 to 340 wheel hp on a chassis dyno depending on drivetrain. The two numbers are not interchangeable.
How do I convert lb-ft to Nm?
Multiply lb-ft by 1.3558 to get Nm. A 400 lb-ft torque spec is 542 Nm. The other direction is multiply Nm by 0.7376 to get lb-ft. A 600 Nm engine produces 442 lb-ft. Always check the original spec carefully — torque numbers in metric and imperial look very different at a glance and confusing them can wreck a fastener or a build.
What is the difference between gross and net horsepower?
Pre-1972 American engines were rated in “gross” horsepower, measured without accessories (alternator, water pump, exhaust, air filter). Modern engines are rated in “net” horsepower, measured as installed with full exhaust, accessories, and air intake. Gross numbers are 15 to 30 percent higher than net for the same engine, which is why 1960s muscle cars sometimes look outrageously overpowered on paper. A “375 hp gross” 1969 engine might rate around 290 to 320 hp net by today’s standard.
How accurate are dyno-to-flywheel correction factors?
Not very. Standard rules of thumb (15 percent FWD, 18 percent RWD, 25 percent AWD) are approximations that can be off by several percent on any specific drivetrain. Tire pressure, ambient temperature, gear selected, and dyno calibration all affect the reading. Take dyno-to-flywheel “correction” with a grain of salt — wheel horsepower is the directly measured number on a chassis dyno, and the flywheel calculation is always an estimate.
Why is 1 hp defined as 745.7 watts?
The mechanical horsepower definition traces back to James Watt’s experiments with mine pump horses in the 1780s. Watt measured that an average horse could lift 33,000 pounds one foot in one minute, and he used that as the conversion benchmark to sell his steam engines as horse replacements. The math works out to 550 ft-lb per second, or about 746 watts. The PS metric horsepower was defined later using 75 kg-m per second, which is close but not identical, yielding 735.5 watts. Both are essentially marketing units rooted in the actual capability of a working horse in the industrial revolution.
Why We Built This
Comparing power specs across manufacturers is a constant source of confusion, especially when shopping a 400 PS BMW against a 400 hp Mustang against a 290 kW Audi. They are not all the same. The conversions are simple but you have to do them right, and you have to know which unit you are starting from. This calculator handles every common pairing in one place, including torque conversions for spec sheets and torque wrench settings. You can be the mechanic.
Help Us Make This Tool Better
Want kg-m or in-lb conversions added, or specific OEM correction factors for SAE J1349 vs DIN 70020? Send us a note and we will look at every message. Tools improve when the people using them tell us what is missing.