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Hagerty Employee

What is horsepower and why do the ratings differ by country?

There are numerous comparison points between various makes and models of cars, but there is one that both enthusiasts and the common driver latch onto with unapologetic fervor—horsepower. It’s a number manufacturers have always put front and center in advertisements, and it’s also a tidy number for enthusiasts to discuss and determine a hierarchy.


But what is horsepower? Many enthusiasts know a dynamometer can tell you how many horses you have, but what is that dyno doing in calculating that number? Does it actually relate to the power of a horse? Jason Fenske of Engineering Explained returned to his old form and dug into exactly what horsepower is and how it differs based on where you are in the world. 


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If my memory serves me correctly, it was stated to me one time that torque could be measured, and horsepower was simply "calculated". I may be misremembering, or have been told wrong. It's been a long time, and I had a lot of fun in the 80s so please don't quote me on this one.


That is true. We can measure the twisting force produced by an engine, but we have to apply math (multiplying by time) to calculate the horsepower.

This is your opportunity for a salient explanation of torque.  Good luck!

Pit Crew

Ha, now do the pound and find out how messed up that measure is!


Since testing engines is my day job, I'll chime in. The history on horsepower is correct. But regardless of that history the industry is very careful with how they do the measurement before they put that number into print. As another commenter pointed out what we actually measure is the torque from the engine. That is a measure of force times distance. Think of a 1-foot long wrench on a stuck bolt. If you push down with 10 pounds of force, you're applying 10 foot-pounds of torque. Once the bold starts turning, say you keep applying that 10 pounds of force while you turn the bolt at a certain RPM. Multiply the RPM by the torque (and do some unit conversion cleanup) and you get power. In the test lab we follow careful procedures to calibrate how we measure that force, use high accuracy devices to measure the rotating speed, and the computer gives us the power and torque. These days we all talk kilowatts and Newton-meters in the lab, but I still think in horsepower and pound-feet when I go to the garage. There is a Society of Automotive Engineers standard for how to measure power on an engine (what hardware is on it, details on test methods, and so on). For some engines you'll see an SAE Certified power - that means they've been as careful and consistent as they can be to give you those numbers. 


And another reason the power figures are different in other countries is that the engines are different. Europe has higher octane fuel than we do, so they often have higher compression ratio or at least a different calibration to take advantage of the octane. So sometimes the power is different because of metric vs. US horsepower, and sometimes it's really different.

Community Manager

This is fantastic insight, thank you for sharing! 

Pit Crew

Given 1 horsepower = 550 foot-pounds/second , implies a horse can move 550 lbs, 1 foot in one second. I have never seen reference to  180 lbs associated with HP as noted in this article... very interesting however. 


I've long known that horsepower was a weird measurement, but I thought it at least had some basis in reality--that some kind of testing had been done to determine the power a horse could produce. Who would have thought it was nothing more than a guess. Then, to make metric horsepower an inaccurate number based on the inaccurate American horsepower seems even dumber.


None of it really makes any sense in reality, anyway. Would it really require 5 horses to mow my lawn? If I could tie 400 horses to my auto chassis, I still could never get them to pull it to 150 miles per hour.

Pit Crew

If you coupled the horses to suitable pulleys and gearing, you could probably hit 150 mph no problem, as long as you didn't run out of rope first.
Pit Crew

Watt's first steam engine was to move water out of mines, which previously had been done with draft horses turning pumps. So it was logical to relate the work his steam engine could do to the work a horse could do. The horse plodded around a circular path, pulling at right angle on a 12 foot lever projecting from a capstan at the center of the circle. The capstan, in turn, was geared to operate the pump.


Watt estimated that the horse pulled with a force of 180 pounds. The circle it followed had a circumference of 2 times π times a radius of 12 feet or 75.4 feet. The horse could make 144 trips around the circle an hour; and 144 divided by 60 results in 2.4 trips per minute, for a speed of 2.4 times 75.4 feet or about 181 feet per minute.


To convert the demonstration of the horse’s ability into measurable leverage, or what is known as torque, Watt multiplied 180 pounds times 181 feet per minute obtaining 32,580 pounds-feet per minute. He rounded that figure to 33,000 pounds-feet per minute or 33,000 divided by 60 which is 550 pounds-feet per second, which became the norm for one (1.0) horsepower. Many of his contemporaries doubted his comparison, not so much because of the math, but because they believed he was being too generous to the ability of the horse! His engine would do this work all day, while a horse under this kind of effort would last an hour or two at the most.


Watt’s draft horse generated force around a circumference of a circle, due to the lever on the capstan. An engine does the opposite; it delivers force at the output end of the crankshaft. Imagine it turns a one-foot lever attached at right angles to the crankshaft. As the crank rotates, the free end of the lever will follow a circle with a one-foot radius. In order to convert the rotational force into horsepower, you need to know the distance the free end of the lever moves in one revolution. The answer of course is 2 times π times a radius of 1 feet or 6.28 feet.


Therefore, the total distance the free end of the lever will go in one minute is 6.28 feet times the Revolutions per Minute or RPM of the engine. Multiplying the total torque (pounds-feet or foot-pounds) output of the engine by distance traveled by the free end of the lever in one minute, and divided by Watt’s pounds-feet per minute per horsepower number will yield horsepower:


Horsepower = (6.28 x RPM x Torque) / 33,000


If I connect a driveshaft to my 400 watt amplifier, how fast could it do 0-60? 😄 

Pit Crew

Your question is not as silly as you think it is. Your 400W amplifier will produce just over 0.5 HP. If you have the skills, you could connect the amp's output to a suitable electric motor attached to your driveshaft, and apply a suitable signal to the amp's input. You've just created an EV! To get to 60 mph, though, you'll need a much bigger amplifier and motor. At least 8KW.

Pit Crew

As a fellow countryman of Mr Watt, I found the article and comments intriguing, to put it diplomatically. The 180 lbs is mentioned in Wikipedia, for what it is worth. Wiki also states that a sustainable rate of 1HP is reasonable for a horse doing agricultural work, from measurements done at the 1926 Iowa State fair. Regardless, the equivalence I use is 1HP = about 750 Watts. As an EE that keeps it in perspective for me, and I was surprised that no one mentioned this conversion factor.


I was taught that horsepower is defined by how much weight a horse can lift to a certain height in a certain time. And also taught that the horse that was used for the original test was an exceptionally strong draft horse and that in reality the average horse can produce about 3/4 HP. 

Intermediate Driver

Ok, doing the math, a 3,000lb VW GTi with 200hp  could go vertical at 25mph in 1st gear. At that rate it would climb to the top of Mt. Everest in just over 13 min.  Just so you know.