Horsepower and watts
Mechanical horsepower is exactly 550 foot-pounds-force per second: about 745.7 watts. Car, boat and lawnmower engines in the US and UK, and electric motors, are rated in hp.
The watt is the SI unit of power: one joule per second. Light bulbs, appliances, chargers and solar panels are rated in watts.
Worked example
Convert 0.5 hp: 0.5 × 745.6998716 = 372.8499358, so 0.5 horsepower is 372.8499358 watts. Likewise 0.1 hp = 74.56998716 W.
Estimating in your head
Multiply by 750 for a quick figure; it is off by about 0.6%. For 0.5 hp that gives 375 W, against 372.85 exactly.
Going the other way
To convert watts to horsepower, divide by 745.6998716 (or multiply by 0.0013410220896). The swap button between the boxes opens that page.
How this works
W = hp × 745.6998716
1 hp = ≈ 745.6998716 W and 1 W = 1 W, both by definition, so the factor is their ratio: 745.6998716 (rounded here; the page uses the full value). Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many watts are in a mechanical horsepower?
1 mechanical horsepower = 745.6998716 watts. The factor is exact by definition but has endless decimals, so it is shown rounded; the page uses the full value.
How do I convert horsepower to watts?
Multiply by 745.6998716. For example, 0.1 hp × 745.6998716 = 74.56998716 W.
What is 0.2 hp in watts?
0.2 hp = 149.1399743 W. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- NIST SP 811 Appendix B.9 — Factors for units listed by kind of quantity
- Wikipedia — Horsepower
Reviewed Oct 17, 2026 · test vectors fixtures/units/power/hp-to-watts/