Horsepower (horse, HP)

Horsepower was formerly a widely used physical unit of power. Nowadays, horsepower is more often encountered only in the context of the performance of automobile and motorcycle engines. Horsepower has now been replaced by the more exact SI unit Watt. The English inventor James Watt came up with the definition of horsepower at the end of the 18th century.

In Czech, the abbreviation k (sometimes incorrectly as ks) is used for horsepower. The English designation for horsepower is hp (horsepower), while in Germany, you will come across the abbreviation PS (Pferdestärke). This unit describes power, meaning the work done in a certain time. Due to historical connections, this unit was linked to the force or work of a horse. Nowadays, the “horse” unit is gradually being replaced by the SI unit Watt.


At the end of the 18th century, English inventor James Watt faced the challenge of equating the performance of his steam engines to the performance of commonly used working horses at that time. Based on experiences with ponies working in a mine, he estimated that a pony harnessed in a gin could lift 22,000 foot-pounds per minute. He estimated the horse’s performance to be 1.5 times greater, i.e., one horsepower was equal to a performance of 33,000 foot-pounds per minute. James Watt thus established the following definition:

One horsepower is equal to the performance consistently delivered by a working horse, which, harnessed in a gin, lifts a load of 180 pounds and travels 144 revolutions per hour with a radius of 12 feet.

James Watt rounded the value calculated from the weight of the load and the distance traveled (according to the number of revolutions of the gin) to 33,000 foot-pounds per minute. At that time, Watt could not convert his unit to the current SI unit system, as the unified SI unit system did not yet exist. For historical reasons, there are two basic variants of horsepower. The difference is caused by different definitions of pounds and feet, which changed with the location and time. Mechanical horsepower is used mainly in England and the USA, while metric horsepower is used in Europe, Asia, and South America. This difference (1.4%) causes variations in the publication of engine performances.

Mechanical Horsepower Metric Horsepower
1 hp = 745.69987 W 1 hp = 735.49875 W

Did you know:

How is the power of an engine measured? Dynamometers, also known as power brakes, are used to measure engine power characteristics. Dynamometers use various physical principles. Practically, power brakes dissipate the energy of the tested engine in a measurable way, usually by converting it into easily measurable electrical energy.

Can horsepower be compared to human strength?
Horsepower is approximately ten times greater: 1 horse = 9.863 people

How do I convert power from horsepower to kilowatts? Simply multiply the power in horsepower by the factor 0.735. As a good mnemonic, you can remember that an engine with a power of 100 horsepower has 74 kW.