The Exact HP to kW Conversion
One mechanical horsepower is exactly 745.699872 watts — not approximately. Here is where that number comes from and why other horsepowers disagree.
Last updated · 6 min read
Most conversion tables give 1 HP = 0.7457 kW and leave it there. But that figure is not a measurement or an approximation — it is an exact defined value, and understanding why explains all the small discrepancies you find between sources.
Where the Number Comes From
James Watt needed a unit that would let him sell steam engines against the draught horses they replaced. He settled on the rate a strong horse could work continuously: raising 550 pounds by one foot every second, which is 33,000 foot-pounds per minute.
That definition is entirely imperial, so converting it to watts means converting each component:
- One pound-force is exactly 4.4482216152605 newtons.
- One foot is exactly 0.3048 metres.
- Multiply: 550 × 4.4482216152605 × 0.3048 = 745.699871582 W.
Why It Is Exact, Not Approximate
This surprises people. Surely converting an old imperial unit into SI involves some measurement error?
It does not, because since 1959 the international yard and pound have been defined in metric terms rather than measured against physical artefacts. The inch is exactly 25.4 mm by definition; the pound is exactly 0.45359237 kg by definition. Everything derived from them inherits that exactness.
So 745.699871582270222 W is not the result of an experiment. It is arithmetic on definitions, and it will never be revised. Any source quoting 745.7, 746, or 0.746 has simply rounded — sometimes appropriately, sometimes not.
The Other Horsepowers
Here is where genuine disagreement creeps in, because "horsepower" names at least four different units:
| Unit | Exact value | Definition | Used for |
|---|---|---|---|
| Mechanical HP | 745.699872 W | 550 ft·lbf/s | UK, US engines |
| Metric HP (PS, KM, KS) | 735.498750 W | 75 kgf·m/s | Continental Europe, Japan |
| Electrical HP | 746 W exactly | Defined by convention | US motor ratings |
| Boiler HP | 9,809.5 W | Steam raising rate | Boiler capacity |
The mechanical and metric versions differ by about 1.4%, which is why a car sold as 150 PS in Germany is quoted as roughly 148 hp in Britain. Nothing about the engine changes — only the yardstick. Our kW to PS converter handles the metric scale.
Electrical horsepower being exactly 746 W is a deliberate simplification adopted for motor ratings, not a different physical quantity. Boiler horsepower is a genuinely different thing altogether — it measures steam-raising capacity, is over thirteen times larger, and cannot be compared to the others at all. See our boiler horsepower calculator for that.
How Much Precision Do You Actually Need?
Almost never more than four figures. Dynamometers have measurement uncertainty of a percent or more, and atmospheric correction factors move results further than any rounding in the conversion constant.
| Using | 200 HP becomes | Error vs exact |
|---|---|---|
| 0.745699872 | 149.1400 kW | exact |
| 0.7457 | 149.1400 kW | 0.00002% |
| 0.746 | 149.2000 kW | 0.04% |
| 0.75 | 150.0000 kW | 0.58% |
Even the crude 0.75 shortcut is within 0.6% — comfortably good enough for mental arithmetic. Use 0.7457 when writing anything down, and the full value only when a specification demands it.
Getting the Direction Right
The reverse factor is 1.34102209, not 1.34 — and not the reciprocal you might guess from memory. A frequent error is using 1.36, which is the metric horsepower factor. If your kW to HP answer looks about 1.4% high, that is almost certainly what happened.
For quick conversions use our HP to kW calculator or kW to HP, and the HP to kW chart for a full lookup table from 1 to 1000 HP.
Frequently Asked Questions
The exact value is 0.745699871582 kW. Rounding to 0.7457 errs by under one part in a million — far below any dyno resolution.
Since 1959 the inch and pound have been defined in metric terms — exactly 25.4 mm and 0.45359237 kg. Everything derived is arithmetic on definitions, not measurement.
From 550 ft·lbf/s. Multiply 550 × 4.4482216152605 N/lbf × 0.3048 m/ft = 745.699872 W.
That is electrical horsepower, defined as exactly 746 W by convention for US motor ratings — a deliberate simplification, differing by about 0.04%.
Mechanical HP is 745.699872 W; metric PS is 735.49875 W. PS is about 1.4% smaller — a 150 PS car is roughly 148 hp.
Within about 0.6% — fine for mental arithmetic. 200 HP becomes 150 kW instead of the exact 149.14 kW.
Multiply kW by 1.34102209. A common error is using 1.36 — that is the metric factor and gives an answer 1.4% too high.