Horsepower | Definition, Unit, And Facts - Britannica

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External Websites

• University of Toronto - Department of Physics - About the Horsepower of an Engine
• Livescience - Why do we still measure things in horsepower?
• NM State - Cooperative Extension Service Publications - Origin of the Horsepower Unit (PDF)
• Workforce LibreTexts - Horsepower and Efficiency
• University of Georgia Extension - Force, Speed, and Horsepower (PDF)
• Energy Education - Horsepower

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• Horsepower - Student Encyclopedia (Ages 11 and up)

horsepower unit of measurement Ask Anything Homework Help Also known as: hp Written and fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors History Britannica AI Ask Anything Table of Contents Table of Contents Ask Anything

horsepower, the common unit of power; i.e., the rate at which work is done. In the British Imperial System, one horsepower equals 33,000 foot-pounds of work per minute—that is, the power necessary to lift a total mass of 33,000 pounds one foot in one minute. This value was adopted by the Scottish engineer James Watt in the late 18th century, after experiments with strong dray horses, and is actually about 50 percent more than the rate that an average horse can sustain for a working day. The electrical equivalent of one horsepower is 746 watts in the International System of Units (SI), and the heat equivalent is 2,545 BTU (British Thermal Units) per hour. Another unit of power is the metric horsepower, which equals 4,500 kilogram-metres per minute (32,549 foot-pounds per minute), or 0.9863 horsepower.

Related Topics: electric motor horsepower thrust horsepower brake horsepower friction horsepower indicated horsepower (Show more) See all related content

Horsepower at the output shaft of an engine, turbine, or motor is termed brake horsepower or shaft horsepower, depending on what kind of instrument is used to measure it. Horsepower of reciprocating engines, particularly in the larger sizes, is often expressed as indicated horsepower, which is determined from the pressure in the cylinders. Brake or shaft horsepower is less than indicated horsepower by the amount of power lost to friction within the engine itself, which may amount to 10 percent or more of the indicated horsepower. Electric motor horsepower can be determined from the electrical input in watts, allowing for heat and friction losses in the motor itself. Thrust horsepower of jet engines and rockets is equal to the thrust in pounds force times the speed of the vehicle in miles per hour divided by 375 (which is equal to one horsepower measured in mile-pounds per hour).

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn.