Horsepower (hp or HP or Hp) is several non-SI units of power. It was originally defined to allow the output of steam engines to be measured and compared with the power output of draft horses. The horsepower was widely adopted to measure the output of piston engines, turbines, electric motors and other machinery. Different regions adopted different definitions of the unit. Most countries now use the SI unit watt for measurement of power.

The definition of a horsepower unit is different in different applications; application outside of the context of a particular definition will be inaccurate.

• One mechanical horsepower of 550 foot-pounds per second is equivalent to 745.7 watts
• A metric horsepower of 75 kgf-m per second is equivalent to 735.499 watts
• A boiler horsepower is used for rating steam boilers and is equivalent to 34.5 pounds of water evaporated per hour at 212 degrees Fahrenheit, or 9809.5 watts
• One horsepower for rating electric motors is equal to 746 watts
• A Pferdestärke is a name for a group of similar power measurements used in Germany around the end of the 19th century, all of about one metric horsepower in size.

An ALAM or SAE horsepower is not a unit of power but instead is a measure of internal combustion engine displacement, and an RAC horsepower or tax horsepower is a differently defined unit of displacement.

Where units of horsepower are used for marketing consumer products, often measurement methods are designed by advertisers to maximize the size of the number produced for any product, even if this may not reflect realistic capacity of the product to do work when used in normal conditions.

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: