Horsepower Calculator

Horsepower

Horsepower is a unit of power, the rate at which work is done. The Scottish engineer James Watt introduced it in the late eighteenth century to market his improved steam engines to mine and mill owners who used horses to drive pumps and machinery. He estimated that a horse could do 33,000 foot-pounds of work per minute, lifting 33,000 pounds by one foot in a minute, or 550 pounds by one foot in a second. That value is still the definition of mechanical, or imperial, horsepower.

Work is force multiplied by the distance over which it acts, and power is work divided by time. Pushing with a force of 10 newtons over 3 meters does 30 joules of work; doing it in 2 seconds takes 15 watts, about 0.02 horsepower. A healthy person can produce over 1 hp for a few seconds when sprinting up stairs, but only around 0.1 to 0.3 hp for long periods.

P = W ÷ t = F × d ÷ t

P

power in watts

W

work in joules

F

force in newtons

d

distance in meters

t

time in seconds

1 hp (mechanical) = 550 ft·lbf/s ≈ 745.7 W.

Watt

The watt is the SI unit of power, named after James Watt, and equals one joule per second. In electrical terms it is the power delivered by one ampere of current flowing under a potential difference of one volt. Kilowatts (1,000 W) are used for engines and appliances and megawatts and gigawatts for power stations. Most countries outside the United States rate car engines in kilowatts; one mechanical horsepower is about 0.7457 kW.

BTU

The British thermal unit is the amount of heat needed to raise one pound of water by one degree Fahrenheit, about 1,055 joules. As a unit of power, BTU per hour is common for furnaces, boilers, air conditioners and grills in North America. One watt equals about 3.412 BTU/h, and one horsepower about 2,545 BTU/h.

Different Horsepowers

Several slightly different horsepowers developed for different purposes. Metric horsepower, known as PS in Germany, CV in France and Italy, and pk in the Netherlands, is defined as lifting 75 kilograms by one meter in one second, which is about 1.4% less than mechanical horsepower. Electrical horsepower, used for rating electric motors, is exactly 746 W. Boiler horsepower measures a steam boiler's capacity to evaporate water and is far larger, about 9,810 W.

Frequently asked questions