Mechanical horsepower, metric horsepower (PS), and kilowatts are three different power units, so an engine rated at 300 hp is not the same power as one rated at 300 PS. 1 mechanical horsepower equals 745.6998715822702 W and 1 metric horsepower (PS) equals exactly 735.49875 W, so 300 mechanical hp is 223.71 kW while 300 PS is only 220.65 kW — about 3.06 kW less for the identical number 300.
What is the worked comparison between 300 hp and 300 PS?
Converting 300 mechanical hp to kilowatts: 300 × 745.6998715822702 ÷ 1000 = 223.71 kW. Converting 300 metric PS to kilowatts uses a different constant: 300 × 735.49875 ÷ 1000 = 220.65 kW. The gap, 223.71 − 220.65 = 3.06 kW, comes entirely from the two definitions being about 1.4% apart, not from any measurement difference. A 120 hp (mechanical) engine is 120 × 745.6998715822702 ÷ 1000 = 89.48 kW, while a 120 PS engine is 120 × 735.49875 ÷ 1000 = 88.26 kW.
How do I convert hp, PS, and kW myself?
Multiply the hp or PS figure by its own defining constant in watts, then divide by 1000 for kilowatts: mechanical hp uses 745.6998715822702 W, and metric PS uses exactly 735.49875 W. To go the other way, divide a kilowatt figure by the same constant. The power converter keeps mechanical hp, PS, and kW as separate selectable units rather than one shared “horsepower” box, precisely because they are not interchangeable numbers.
How do I check a horsepower conversion?
A quick plausibility check is that PS is always the larger number for the same physical power, because 1 PS is a smaller unit of power than 1 mechanical hp: 100 kW converts to 134.10 mechanical hp but 135.96 PS. Round-tripping confirms the arithmetic: 300 PS → 220.649625 kW → 220.649625 × 1000 ÷ 735.49875 = 300.0 PS exactly, with no drift.
What mistakes mix up hp, PS, and kW?
The most common mistake is treating a PS figure on a European spec sheet as though it were hp and converting it with the wrong constant, or vice versa — the two numbers differ by about 1.4% for the same physical engine, small enough to look plausible but large enough to be wrong. Another is confusing this unit conversion with a claim about how the power was measured; a dynamometer reading depends on the testing standard used (see below), which this conversion does not correct for. A third is using the “electrical horsepower” constant of exactly 746 W for an automotive rating, when automotive hp figures are almost always the mechanical 745.6998715822702 W definition.
What does a power-unit conversion not correct for?
This conversion only translates a stated number from one power unit to another; it assumes the original figure was measured and reported correctly. It does not adjust for different dynamometer testing standards — SAE net, SAE gross, DIN, or ECE — which can report different power figures for the identical physical engine even before any unit conversion is applied, because they measure the engine in different states of tune and with different accessories attached.
Source for the external fact
The U.S. National Institute of Standards and Technology lists 1 horsepower (mechanical) as 745.7 W in its unit-conversion tables, distinct from the metric horsepower (PS) value of 735.49875 W, which is derived from 75 kgf·m/s using standard gravity.
A second worked example: converting a 75 kW electric motor nameplate
An electric motor nameplate reads 75 kW. In mechanical horsepower, that is 75,000 ÷ 745.6998715822702 = 100.58 hp. Using the separately defined “electrical horsepower” of exactly 746 W instead — the constant used on some North American motor nameplates — gives 75,000 ÷ 746 = 100.54 hp, a difference of only 0.04 hp at this power level. In metric PS, the same 75 kW is 75,000 ÷ 735.49875 = 101.97 PS. Three conversions of the identical 75 kW motor produce three slightly different numbers, all correct for the unit each one names.
A conversion table across common engine ratings
Holding each rating fixed in its native unit and converting to the other two shows how the gap between hp and PS grows with the power level.
| Rating | Kilowatts | Mechanical hp |
|---|---|---|
| 100 PS | 73.55 | 98.63 |
| 150 PS | 110.32 | 147.95 |
| 200 PS | 147.10 | 197.26 |
| 300 PS | 220.65 | 295.90 |
| 400 PS | 294.20 | 394.53 |
A car advertised as “200 PS” is close to but not quite 200 mechanical hp — it is 197.26 hp, a gap that widens in absolute terms as the rating climbs.
At very high power, even the electrical and mechanical horsepower definitions diverge measurably
The 0.04 hp gap between electrical and mechanical horsepower in the 75 kW example above looks negligible, but it scales with power. At 500 hp, the electrical definition (746 W) and the mechanical definition (745.6998715822702 W) differ by 500 × (746 − 745.6998715822702) = 150.06 W — about a fifth of a horsepower, small but no longer trivial for a precise industrial power budget. At 1,000 hp the same gap doubles to roughly 300.13 W. Neither definition is more “correct” than the other; they are simply two different fixed constants that happen to sit within 0.04% of each other.
Why the same displayed “hp” can still mean different things on paper
Beyond the unit-definition gap covered above, a printed horsepower figure can come from different testing conventions that this conversion does not reconcile. SAE net horsepower is measured with a production engine’s full accessories, exhaust, and intake installed; SAE gross horsepower historically was measured on a bare engine without those losses, which is one reason many pre-1972 U.S. engines carry gross-rated figures noticeably higher than a net-rated figure for comparable hardware. Converting a gross-rated hp figure into kW with the standard constant produces an arithmetically correct kilowatt number that still describes the more favorable gross test condition — the unit conversion is exact, but the two engines still are not being compared on the same basis.
How power connects to torque and engine speed, once the unit is fixed
Once a power figure is expressed in a consistent unit, it can be checked against an engine’s torque and rotational speed using P(kW) = torque (N·m) × rpm ÷ 9,549.297. An engine producing 300 N·m of torque at 4,000 rpm makes 300 × 4,000 ÷ 9,549.297 = 125.66 kW — which converts, using the same constants as the worked example above, to 125.66 × 1,000 ÷ 745.6998715822702 = 168.52 mechanical hp, or 125.66 × 1,000 ÷ 735.49875 = 170.86 PS. A diesel engine making more torque, 400 N·m, but at a much lower 2,000 rpm produces less power, not more: 400 × 2,000 ÷ 9,549.297 = 83.78 kW, well under half the first engine’s output despite the higher torque figure, because power depends on the product of torque and rotational speed together, not on torque alone.
Related calculators
Convert any of the three power units with the power converter, separate a power rating from an energy total in the power vs energy guide, and turn a kilowatt figure into a household energy cost with the watts and kWh guide.