The unit of measurement “horsepower” is well known to all motorists—and not just them. But what does it actually mean? Who came up with the idea to measure engine power in horses and why? And most importantly, how does this measure relate to the abilities of an actual horse? You would think that one horse equals one horsepower. But in reality, it’s more complicated.
The term “horsepower” as a unit of measurement was first introduced by Scottish engineer and mechanic James Watt, who is famous for developing the universal steam engine. A popular misconception is that Watt invented the steam engine itself, but in fact, he improved Thomas Newcomen’s design. Nevertheless, his invention sparked the Industrial Revolution in England.
Newcomen’s steam engine consumed too much fuel and wasn’t very efficient. To persuade industrialists to adopt his model, Watt proposed the following payment scheme: he would maintain the steam engine he installed and, in return, receive regular payments equal to one-third of the customer’s savings in coal costs. This “subscription” model was profitable, and the engine started to sell.
But when Watt was invited to assemble a steam engine for a mill owned by London brewers Godwin and Whitbread, the scheme failed. They were using horses to turn their millstones, so the argument of saving on coal didn’t work. Watt was at a loss: how could he demonstrate that his steam engine was more efficient than the horses?
It wasn’t feasible to directly compare the feed consumption of a horse and the machine, as the boiler wasn’t designed to run on hay.
In order to convince fans of the tried-and-true technology of draft horses, Watt developed the concept of “horsepower.” He observed the mill’s mechanism and determined that the average horse could turn a wheel with a radius of 12 feet (3.7 m) 144 times an hour, or 2.4 times a minute.
With this information, Watt calculated that the animal could exert a pulling force of 180 pounds-force, or about 800 newtons. Power is the work done per unit of time, and Watt determined that this work amounted to 32,572 foot-pounds per minute. He then rounded this number to 33,000 foot-pounds per minute (44,742 joules). So, one horsepower is about 745.7 watts.
Watt then calculated the power of his machine by using the area of its piston in square inches, the average excess steam pressure in pounds per square inch, and the speed of the piston in feet per minute. He concluded that his machine produced 10 horsepower.
This allowed the engineer to prove to his clients that his machine was superior to horses. It was not only more powerful but also didn’t need rest, didn’t produce manure, and couldn’t kick with its hooves.
With the Industrial Revolution, Watt’s horsepower became the primary unit of power measurement. For a long time, it was used worldwide. But since horses vary from one another, different mechanics came up with varying figures. Engineers and physicists like John Smeaton, John Desaguliers, and Thomas Tredgold all tried to calculate the most standardized horsepower, but their results varied.
It’s not surprising that at the Second Congress of the British Association for the Advancement of Science in 1882, 63 years after Watt’s death, the modern unit of power—the watt—was introduced, named in his honor to replace horsepower.
However, horsepower wasn’t abandoned—in fact, many different types of it were developed. There are metric horsepower units, used in most European countries today, which equal 75 kgf·m/s, or 735.49875 watts—slightly less than Watt’s original horsepower. There are also hydraulic, mechanical, electrical, and even taxable horsepower, the latter being used to determine vehicle taxes. In the U.S., there is also so-called boiler horsepower, used to measure a boiler’s ability to deliver steam to an engine. In general, the horsepower unit is no longer directly linked to the capabilities of real horses.
But does Watt’s definition of horsepower correspond to the actual power output of a horse? Not exactly. Watt based his calculation on the speed at which a horse could continuously turn a wheel all day, which wasn’t the animal’s maximum capacity. If the horse were forced to run at full speed around the wheel, it would generate much more power for a short time before collapsing from exhaustion.
Biologists Robert Stevenson from the University of Massachusetts Boston and Richard Wassersug from Dalhousie University in Canada calculated the maximum power a horse could physically produce. By evaluating the muscle mass of a large horse and considering that about 30% of its skeletal muscles could be used for mechanical work, they estimated that a horse could generate a peak power of 18,000 watts, or about 24 horsepower.
So, theoretically, a horse could generate as much power as 24 of its peers working at a slower pace.
Of course, such an overworked horse wouldn’t live long—it would suffer muscle failure from the strain. Therefore, 24 horsepower is the upper limit of a horse’s power output, and most horses will perform well below this. For example, at a 1925 tug-of-war competition in Iowa, horses demonstrated much more modest results, briefly generating 14.9 horsepower.
And in 2023, a group of engineers, out of curiosity, modified a car dynamometer to measure a horse’s power, and their draft horse produced only 5.7 horsepower (4.3 kW).
By the way, if you’re curious, a healthy average human can sustain a power output of about 0.1 horsepower for a long time and, during a brief maximum effort, about 1.2 horsepower. Trained athletes can momentarily generate up to 2.5 horsepower and maintain 0.35 horsepower for several hours. Jamaican sprinter Usain Bolt demonstrated a peak power of 3.5 horsepower—though only for 0.89 seconds.
To sum it up, a single horse can produce a maximum of 24 horsepower—or the equivalent of 6.85 Usain Bolts.
