Horsepower (hp or HP) is the name of several non-metric units of power. In scientific discourse, the term "horsepower" is rarely used because of its various definitions and the already existent SI unit for power, the watt (W). However, use of the term "horsepower" persists as a legacy in many languages and industries, particularly as a unit of measurement of the maximum power output of internal-combustion engines of automobiles; and often of trucks, buses, and ships. The use of 'HP' is being slowly replaced by kW (kilowatt) and MW (megawatt).

There are two important factors to consider when evaluating the measurement of "horsepower":

• The inconsistent definitions of the "horsepower" unit itself
• The various standards]] used in measuring the value of "horsepower"

These factors can be combined in unexpected ways — the power output for an engine rated at "100 horsepower" might not be what a reader expects. For this reason, various groups have attempted to standardize not only the definition of "horsepower" but the measurement of "horsepower". In the interim, more confusion may surface.

History of the term "horsepower"

The development of the steam engine provided a reason to equate the output of horses with the engines that could replace them. In 1702, Thomas Savery wrote in The Miner's Friend: "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work..." The term "horsepower" was coined later by James Watt to help market his improved steam engine. He had previously agreed to take royalties of one third of the savings in coal from the older [[Newcomen steam engines. This royalty scheme did not work with customers who did not have existing steam engines but used horses instead. Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet in radius, therefore the horse travelled 2.4 × 2π × 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds (assuming that the measurements of mass were equivalent to measurements of force in pounds-force, which were not well-defined units at the time). So:

$power\; =\; \backslash frac\{work\}\{time\}\; =\; \backslash frac\{force\; \backslash times\; distance\}\{time\}\; =\; \backslash frac\{(180\; \backslash mbox\{\; lbf\})(2.4\; \backslash times\; 2\; \backslash pi\; \backslash times\; 12\; \backslash mbox\{\; ft\})\}\{1\backslash \; \backslash mbox\{min\}\}=32,572\; \backslash frac\{\backslash mbox\{ft\}\; \backslash cdot\; \backslash mbox\{lbf\}\}\{\backslash mbox\{min\}\}.$

This was rounded to an even 33,000 ft·lbf/min.

Others recount that Watt determined that a pony could lift an average 220 pounds 100 feet (30 m) per minute over a four-hour working shift. Watt then judged a horse was 50% more powerful than a pony and thus arrived at the 33,000 ft·lbf/min figure.