Horsepower (hp)
The mechanical horsepower (hp) is 550 foot-pounds per second, about 745.7 W. It is the horsepower used for US engines and motors.
Symbol: hp
Quantity: power (energy per unit time)
SI relation: 1 hp = 745.6998715823 W (exact)
Derivation: 1 hp = 550 ft·lbf/s = 745.69987158227 W
Exact: yes, by definition
Reference conditions. Mechanical (imperial) horsepower, 550 ft·lbf/s. Metric horsepower (PS) is about 1.4 % smaller.
History
James Watt defined it around 1782 to compare steam engines with draught horses.
Typical uses
- Car and motorcycle engine power in the US and UK
- Electric motor ratings in North America
- Pumps, compressors and boats
Convert horsepower to other units
| Conversion | 1 hp equals | Reverse |
|---|---|---|
| horsepower to watt | 745.6998716 W | watt to horsepower |
| horsepower to kilowatt | 0.7456998716 kW | kilowatt to horsepower |
| horsepower to BTU per hour | 2544.433578 BTU/h | BTU per hour to horsepower |
| horsepower to refrigeration ton | 0.2120361315 TR | refrigeration ton to horsepower |
| horsepower to megawatt | 0.0007456998716 MW | megawatt to horsepower |
| horsepower to metric horsepower | 1.013869665 PS | metric horsepower to horsepower |
| horsepower to milliwatt | 745699.8716 mW | milliwatt to horsepower |
| horsepower to kilocalorie per hour | 641.186476 kcal/h | kilocalorie per hour to horsepower |
| horsepower to foot-pound per second | 550 ft·lbf/s | foot-pound per second to horsepower |
Open the Power Converter to convert any value between all units.
Sources
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "horsepower (550 ft·lbf/s)".
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
Cite this page
APA
Scientific Converters. (2026, October 15). Horsepower (hp). Retrieved October 17, 2026, from https://scientificconverters.com/units/horsepower/
IEEE
Scientific Converters, "Horsepower (hp)," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/units/horsepower/