What unit is used to measure engine power in the United States?

What unit is used to measure engine power in the United States? Horsepower is the customary answer.

Horsepower measures power—the rate at which an engine can do work. Scottish engineer James Watt popularized the term in the late 18th century as a way to compare steam engines with the work output of draft horses. The name remained useful even after engines became far more powerful and precise measurement equipment became available.

In the United States, automotive power is commonly advertised in horsepower, often written as hp. Torque is different: it measures twisting force, while horsepower combines torque with rotational speed. Kilowatt is the SI unit of power, and it is increasingly shown alongside horsepower, but it is not the traditional U.S. automotive label. PS is metric horsepower, a closely related but slightly different standard.

One mechanical horsepower equals about 745.7 watts. Engine figures can also vary depending on whether they are measured as gross, net, brake, or crankshaft power, so two ratings with the same horsepower number may not have been tested under identical conditions.