Horsepower (hp)

Definition

Mechanical (imperial) horsepower, about 745.70 watts.

History

Horsepower was coined by James Watt in the 1780s to compare his steam engines to the draught horses they replaced, based on the work a horse could do. Mechanical horsepower is about 745.7 watts.

Where it is used

Horsepower rates car and engine power in the United States and United Kingdom.

Typical uses include car and engine power in the US/UK.

Conversion pages

These pages convert horsepower to or from another unit in power.

• Kilowatt to horsepower

Other Power units

• Kilowatt (kW)
• Watt (W)
• Megawatt (MW)
• Metric horsepower (PS)
• BTU per hour (BTU/h)

Sources

• NIST SP 811 NIST SP 811, App. B (power). Mechanical hp = 550 ft·lbf/s ≈ 745.70 W; metric hp (PS) = 75 kgf·m/s = 735.49875 W · Dataset reviewed Nov 11, 2026