The three horsepower definitions, with the gap between them calculated
Three different units are called horsepower. Metric horsepower (PS) is exactly 735.49875 W, mechanical horsepower (hp) is 745.69987158227022 W, and electrical horsepower is exactly 746 W. Mechanical horsepower is 1.39 percent larger than metric PS, so a car rated at 300 PS is 295.9 hp. This table sets all three definitions side by side and restates one rating in every one of them.
The three horsepower definitions
The watt column is the defined value: metric horsepower is 75 kilogram-force metres per second on the standard gravity of 9.80665 m/s2, mechanical horsepower is 550 foot pounds-force per second on the international foot and pound, and electrical horsepower was fixed at a round 746 W for motor nameplates. The kilowatt column and the gap column are calculated from it. NIST SP 811 prints the same values rounded to seven figures.
| Definition | Defined as | Watts (quoted) | Kilowatts | Gap versus metric PS |
|---|---|---|---|---|
| Metric horsepower (PS) | 75 kgf m/s, on the standard gravity 9.80665 m/s2 | 735.49875 | 0.73550 | +0.000% |
| Mechanical horsepower (hp) | 550 ft lbf/s, on the international foot and pound | 745.69987158227022 | 0.74570 | +1.387% |
| Electrical horsepower (hp(E)) | defined outright as 746 W for electric motor ratings | 746 | 0.74600 | +1.428% |
The same rating of 300 restated in every definition
Each row takes a rating of 300 quoted in one definition, converts it to watts, and expresses that power in all three. Every cell is calculated, which is why the diagonal reads 300.00 and nothing else does. This is the table to reach for when one brochure says PS and another says hp.
| A 300 unit rating | In kilowatts | In PS | In hp | In hp(E) |
|---|---|---|---|---|
| 300 PS (metric horsepower) | 220.65 | 300.00 | 295.90 | 295.78 |
| 300 hp (mechanical horsepower) | 223.71 | 304.16 | 300.00 | 299.88 |
| 300 hp(E) (electrical horsepower) | 223.80 | 304.28 | 300.12 | 300.00 |
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Why one name covers three units
Horsepower was never defined by a standards body first and adopted second. James Watt picked a figure that would let buyers compare a steam engine to the animals it replaced, and different industries in different countries then fixed that rough idea in their own terms. Nothing forced the results to agree, and they do not.
The British and American version was pinned to 550 foot pounds-force per second, which works out at 745.69987158227022 W once the foot, the pound and standard gravity are themselves exact. Continental Europe pinned its version to 75 kilogram-force metres per second instead, giving exactly 735.49875 W, and that is the PS on a German or Japanese spec sheet.
The electrical horsepower is the odd one. Motor manufacturers needed a round number for nameplates and ratings, so it was set outright at 746 W, which is 0.0402 percent above the mechanical horsepower and 1.43 percent above PS. NIST SP 811 also tabulates a boiler horsepower of 9,809.50 W and a water horsepower of 746.043 W, neither of which is a rating you will meet on a vehicle.
Why this matters when you read a spec sheet
The gap looks small until it decides an argument. At 1.39 percent, the difference between hp and PS is larger than the year-on-year power increase of most engine updates, so a car that appears to have gained power between two reviews may simply have been quoted in the other unit.
Worked through, a 300 PS engine is 295.9 hp and a 300 hp engine is 304.2 PS. Neither figure is a rounding of the other. If a specification lists both at the same round number, one of them has been converted badly or copied from a press release that did the same.
The safe habit is to convert through kilowatts. The kilowatt is the SI unit, it is what type approval documents in Europe actually record, and it has exactly one meaning. Once a rating is in kilowatts, any horsepower figure you produce from it is unambiguous as long as you say which horsepower you used.
Which one this site converts with
UnitGlide's power converter carries both of the units people search for: hp at 745.69987158227022 W and PS at 735.49875 W. Neither is rounded before the conversion runs, so a chain of conversions cannot accumulate error from the factor itself.
The electrical horsepower is deliberately not in the converter. It exists for motor nameplates rather than for general conversion, and offering three units all labelled horsepower in one dropdown would produce more wrong answers than it prevented. It is in this table because reading a nameplate is exactly when you need to know it exists.
If you are writing a specification rather than reading one, quote kilowatts first and put the horsepower figure in brackets with its definition named. That single habit removes the whole problem for whoever reads it next.
Frequently asked questions
Is 1 hp the same as 1 PS?
No. Mechanical horsepower is 745.69987158227022 W and metric horsepower is 735.49875 W, so hp is 1.39 percent larger. One PS is 0.98632 hp and one hp is 1.01387 PS.
How many kW is 300 hp?
300 mechanical horsepower is 223.71 kW. The same number of metric horsepower is 220.6 kW, and 300 electrical horsepower is 223.80 kW. The three answers differ by more than two kilowatts.
Why does the same car have different horsepower figures in different countries?
Usually because the rating was measured once, in kilowatts, and then quoted in the local horsepower. A 220.6 kW engine is 300 PS in Germany and 295.9 hp in the United Kingdom, and both figures are correct. Differences beyond that 1.39 percent come from different test procedures rather than from units.
What is electrical horsepower used for?
It is the horsepower on electric motor nameplates and in motor standards, defined as exactly 746 W so that ratings land on round numbers. It sits 0.0402 percent above mechanical horsepower, which is close enough that the two are often treated as interchangeable for motors and not close enough to be safe for anything precise.
Which horsepower does a dyno report?
Whichever the software is configured for, which is why a dyno sheet should always be read together with its units. The underlying measurement is torque and rotational speed, giving power in watts; every horsepower figure that comes out of it is a conversion, and choosing hp instead of PS changes the headline number without changing the engine.
Can I republish this table?
Yes, under Creative Commons Attribution 4.0 International (CC BY 4.0). Credit UnitGlide, https://unitglide.net and link back, and you may copy, adapt and republish it, commercial use included. The definitions themselves are public standards; the compilation and the calculated columns are what the licence covers.
Sources
- NIST Special Publication 811, Appendix B.8, for the watt value of horsepower (550 ft lbf/s), metric horsepower, electrical horsepower, boiler horsepower and water horsepower.
- NIST Special Publication 811 (full guide), for the SI usage rules and the exact foot, pound and standard gravity the mechanical horsepower is built from.
- ISO 80000-4, Quantities and units, Part 4: Mechanics, for power as an ISO quantity and the status of horsepower as a non-SI unit outside the standard.