What is Horsepower?

L. S. Wynn
L. S. Wynn

Horsepower is a basic unit of energy output that is commonly assigned to engines, trains, and other machines that exert force. As its name suggests, this type of measurement is based on the hauling capacities of an average horse. Years ago, horses were the most efficient way for people to move materials from place to place. They were particularly useful when it came to pulling carriages and wagons, as well as working in mines and other industrial settings. Back in these times, the public was usually very familiar with how much work a horse could do; assigning “horsepower” to the new machines of the day, like the steam train, enabled most onlookers to immediately realize how powerful and efficient the technology really was. The terminology is primarily used today when it comes to classifying engine capacity and other quantifiable work outputs of machines, particularly cars and trucks, though it doesn’t always have a precise definition or universal means of calculation.

The EMD GP40 Diesel-electric locomotive has an output of over 3,000 hp, however newer locomotives can put out over 6,000 hp.
The EMD GP40 Diesel-electric locomotive has an output of over 3,000 hp, however newer locomotives can put out over 6,000 hp.

Inception and Basic History

This unit of work was first used in the context of coal mining. Horses were commonly used in the late 18th and early 19th centuries to haul coal out of shafts, but the innovation of the steam engine promised to make those tasks faster and more efficient. The idea was popularized in the mid-1800s by James Watt, a Scotsman who worked marketing steam engines; he figured, and was mostly correct, that people would be more readily able to understand the engine’s power when compared to the draft horses that were common at the time. Watt wasn’t concerned with piston-based engines, which is where the unit of measure is most commonly used in modern times. This adaptation came later.

Large inline and V-12 to V-16 engines provided the race cars of the late 1930s with over 400 horsepower.
Large inline and V-12 to V-16 engines provided the race cars of the late 1930s with over 400 horsepower.

How Power is Calculated

Watt based his calculation on a series of involved equations that were mostly centered on the power it would take to turn a mill wheel. Mill wheels were big water wheels that were common throughout Europe in centuries past, and it was relatively easy to measure a horse’s cadence and speed as it trotted the perimeter pulling water. Watt determined that horses are able to haul with a force of about 180 lbs (about 82 kg). He used this figure to determine how much force horses typically pull in a one-minute interval, and his ultimate conclusion was 33,000 foot-pounds per minute — with a “foot-pound” being basically the distance that could be covered (1 foot or 0.3 m) per one pound (0.45 kg) of force.


Understanding how this measurement is actually used to calculate force is sometimes easier when looking at fixed equivalencies. In general, a horsepower of one is equal to any or all of the following: lifting 33,000 pounds (14,968.5 kg) 1 foot (0.3 m) in one minute; lifting 1 pound (0.45 kg) 33,000 feet (10,058.4 m) in one minute; lifting 1,000 pounds (453.5 kg) 33 feet (10 m) in one minute; lifting 1,000 pounds (453.5 kg) 330 feet (100.5 m) in ten minutes; and lifting 100 pounds (45.35 kg) 33 feet (10 m) in 6 seconds.

Modern Usage

The analogy to horses remains a common way of expressing the power harnessed by automobiles and other engine-driven machines such as tractors and garden equipment. Modern cars typically have 125 to 200 horsepower, but some high-performance cars have a force potential of 400 or more. The measurement is also commonly used in marketing, usually as a way for salespeople to distinguish between models and for brands to compete against each other.

Variations and Geographical Differences

There are usually a couple of different ways of expressing power output with the same terminology. Mechanical operations are often calculated slightly differently than hydraulics or electric supplies, for instance, and there can also be discrepancies between things like braking force and propelling power.

Different countries sometimes also have different definitions, particularly where sales and marketing are concerned. There isn’t usually any fixed definition of exactly how much force is in each individual unit, which has caused some regulators to question whether the term should be applied to cars, engines, or other machinery that is sold commercially. Some countries actually outlaw the formal use of this measurement, opting instead for more standardized and measurable forces.

Some high performance cars have 400 horsepower or more.
Some high performance cars have 400 horsepower or more.

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Discussion Comments


@GlassAxe- Nice explanation of brake horsepower. What is the difference in horsepower and torque? It seems like the more torque that a vehicle has, the more power it has to pull trailers, and accelerate. How are the two different and how are the two similar?


@cinder- I can help break it down for you. All three of the terms you are asking about refer to work output, but they refer to power at different points in a Diesel or Otto (four-stroke gasoline) cycle engine. Brake horsepower, or bHP, is the power output of an unrestricted engine. This would be the output of an engine before a water pump, alternator, gearbox, differential, or exhaust are added, and it will be the highest published horsepower for a vehicle.

The horsepower at the wheels is the amount of power that is directly transferred to the roadway. The brake horsepower in cars and trucks can be much higher than the wheel horsepower in a vehicle. For example I have a pick-up truck that has 315 bHP with only about 271 HP at the wheels. The engine loses 44 HP from the engine to the wheels.


What about Brake Horsepower? Is that the same thing or not?

Article doesn't mention it, so it makes me think it isn't the same but I've read in other places that horsepower, brake horsepower and even something called wheel horsepower are the same, then other websites say they are different! I'm a bit confused.

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