What does horsepower mean?
Horsepower is a unit of power based on the work a horse was traditionally believed capable of performing, especially when pulling or lifting. One mechanical horsepower equals approximately 746 watts, although the precise value can vary by context. In technical discussion, horsepower is often used to compare engines, vehicles, and machinery, while in everyday speech it can simply mean how powerful a machine feels or performs. The term is especially common in automotive and motorsport contexts, where manufacturers and enthusiasts compare engine output alongside torque and other specifications. Although the metric is not part of the International System of Units, it remains widely understood and is frequently used in advertising, engineering, and historical descriptions of steam power. The word reflects an early industrial practice of judging machinery by the work it could perform relative to that of working horses.
nounA unit of power equal to the amount of mechanical power produced by a typical horse, traditionally estimated at 550 foot-pounds per second or about 746 watts. The term is also used informally to describe a vehicle's or engine's power.
- A unit of power traditionally equivalent to about 746 watts.
- Informally, the power or performance of an engine, vehicle, or other machine.
"The vintage motorcycle engine produced nearly 50 horsepower."
"The tractor has enough horsepower to pull the heaviest loads."
"Although the car is light, it delivers surprisingly strong horsepower."
The plural is normally unchanged when the unit is counted.
"The two cars have 150 horsepower each."
The word horsepower was created by comparing steam engines with the work of horses, turning a familiar animal into a lasting unit of power.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does horsepower come from?
The compound was developed in the late 18th century as engineers sought a practical way to describe the power of steam engines. James Watt is credited with popularizing the term through comparisons between his engines and the work of horses. The first element comes from the longstanding English word for the animal, while power comes through Old French from Latin potestas, meaning ability or force. The expression became a lasting unit of mechanical power and later acquired a broader informal meaning for engine performance.
Related word forms
How horsepower is actually used
In technical contexts, horsepower is distinguished from the SI unit watt; in general use, it commonly serves as an informal measure of an engine's power. The spelling is generally used as an attributive modifier, as in "horsepower output."