What is a Mechanical Horsepower?

Horsepower (hp) equals exactly 745.6998715822702 watts (mechanical horsepower) and is the American and British unit for car engines, motorcycles, motorboats, and traditional mechanical power ratings. James Watt invented the unit in the 1780s to market his steam engines: he calculated that a brewery horse could continuously turn a mill wheel at 33,000 ft·lb per minute, which became 1 horsepower. American cars typically range from 150 hp (economy) to 700 hp (sports cars), pickup trucks 250–500 hp, and large diesel trucks 400–600 hp. Outside the US, the metric horsepower (PS or CV, equal to 735.5 W — about 1.4% smaller than mechanical hp) is sometimes used. Horsepower relates to the watt (1 hp ≈ 745.7 W), the kilowatt (1 hp ≈ 0.746 kW), and the metric horsepower (1 hp ≈ 1.014 PS). Despite SI's preference for the watt, horsepower remains entrenched in automotive marketing.

Visual reference for how the mechanical horsepower relates to other power units. Each row links to the full converter for that pair.

• US car and motorcycle engine ratings
• Lawn mower and small engine specs
• US pump and compressor ratings

• ✓1 hp = 745.7 W = 0.7457 kW.
• ✓US car ads quote hp; European ads quote kW or PS.
• ✓Metric horsepower (PS) differs by ~1.4% — be aware.

• ✗Assuming hp and PS are identical — 1.4% difference.
• ✗Using horsepower for electrical devices — watts more appropriate.
• ✗Confusing horsepower at engine flywheel vs wheel (transmission losses).