Horsepower (hp)

US & ImperialCurrent unit

The mechanical horsepower: 550 foot-pounds per second, about 745.7 watts. James Watt coined it to compare steam engines with draft horses.

1 horsepower = 745.6998716 watts (approximately).

About the horsepower

Horsepower was a sales pitch before it was a unit. Late in the 18th century the Scottish engineer James Watt adopted the term to compare the output of steam engines with the draft horses they replaced. His 1782 experiments found that a brewery horse could produce 32,400 foot-pounds-force per minute, and in 1783 he and Matthew Boulton standardized the figure at 33,000 foot-pounds-force per minute. Customers could then be told how many horses an engine would replace.

That figure works out to 550 foot-pounds-force per second, or about 745.7 watts: the power needed to lift 550 pounds by one foot in one second. Several cousins exist, including metric horsepower and electrical horsepower, defined as exactly 746 W. Car ratings added more confusion. Early British taxable horsepower was a formula based on bore and cylinder count, not measured output, and American ratings before 1972 used SAE gross brake horsepower, measured without accessories. In the European Union, horsepower may be used only as a supplementary unit under Directive 80/181/EEC, in force from 1 January 2010.

Sources: Wikipedia: Horsepower; Wikipedia: Metric horsepower

Horsepower conversion chart

1 horsepower expressed in every other power unit:

More US & Imperial units

• electrical horsepower (power)
• BTU per hour (power)
• ton of refrigeration (power)
• foot-pound per second (power)
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• foot (length)
• yard (length)
• mile (length)
• thou (length)
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• rod (length)
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