What is horsepower? If you’ve had a conversation about cars at all, you likely know of the term ‘horsepower’. But what is horsepower, exactly?

Well, this guide explains all about this measurement of power including what it means, how it’s calculated and what relation it actually has to horses.

What is horsepower?

A horsepower is a unit of measurement used to denote power or the “rate at which work is done” by an engine or motor. Horsepower also refers to the maximum power available to a vehicle.

The higher the horsepower of a car, the more power is sent to the wheels and the faster it will go. This is the reason why some sports cars have astonishingly high horsepower figures.

However, the amount of acceleration you get depends on many factors, including the weight of the car and its torque. The vehicle with the highest horsepower may not be the fastest.

Horsepower figures can be calculated in different ways. Usually, the horsepower or “hp” figure quoted is the maximum power available to that vehicle. Most often, the figure gets calculated at different engine revs, which gives us brake horsepower. For example, the mightily powerful 2019 Dodge Challenger Hellcat produces 717 horsepower from its supercharged V8, that is 717 brake horsepower at 6000 rpm.

How do you calculate horsepower?

The answer is simple: horsepower is calculated by multiplying the amount of force (in pounds) by the speed (in feet per second).

So, let’s say your car’s engine can generate 200 pounds of force and move at a speed of 2 feet per second. The engine horsepower would be 400 (200 x 2).

Horsepower is often used to describe the power of cars, trucks and other vehicles. But it’s also used to express the power that a machine, motor or engine produces.

The term horsepower is also sometimes used informally to describe a person’s physical strength. For example, you might say that someone is “as strong as a horse” if they are very muscular.

The history behind horsepower

The term ‘horsepower’ was coined by Scottish inventor James Watt, who is often wrongly credited with inventing the steam engine, though he did improve the technology significantly.

How he arrived at the term ‘horsepower’ requires a bit of math, a bit of observation and a wee bit of obfuscation.

When Watt reimagined existing steam engine designs in 1776 to dramatically improve performance and improve fuel economy (they ran on coal, primarily), he needed a way to sell his new engine’s capabilities to a market still driven — figuratively and literally — by horses. So, he figured, what better way than to show how much better his engines were when compared with horse-driven machines, such as grain mills.

It is somewhat fitting that the same device that ushered in the industrial revolution also gave rise to a term we still use today.

How does horsepower measure engine power?

The various calculations are generally of interest only to mathematicians, but some numbers are unavoidable. Watt chose a relationship between how much weight a horse can lift — while pulling a rope running through a pulley to a weight on the ground — one foot high in one second.

Today, we say that a horsepower is equal to a horse lifting 550 pounds of weight a foot high in one second. That number is what Watt arrived at when he decided he needed to settle on a standard, given that horses vary in strength.

That may seem like a lot, but there are garage-door openers with more. The average homeowner’s garage door opener is 1/2 hp, but there are 1 hp and 2 hp models available.

Horsepower is a measurement of the rate at which work is done, and differs from torque, which is a measurement of the amount of force applied to do that work. In your Toyota’s engine, you can think of higher horsepower being the specification that will sustain a higher rate of work once moving, while torque is the figure that gets a car moving quickly.

In other words, engines high in horsepower but low in torque will feel less powerful from a stop than engines lower in horsepower but higher in torque. The higher horsepower engine will accelerate faster once at speed, however.

How horsepower works in vehicles?

How does this translate into the figures you see in vehicle specifications? Well, first you have to understand the figures quoted refer to peak horsepower. For example, the 2020 Corolla SE with the six-speed manual delivers 169 horsepower at 6,600 rpm, or revolutions per minute of the engine.

In other words, as the engine speed rises to 6,600 rpm, the horsepower it delivers rises to a peak of 169, then falls off slightly as engine speed rises further.

Horsepower is an important specification for buyers to consider, as it speaks directly to performance. For a given vehicle, an engine with more horsepower and more torque will accelerate more quickly — important for drivers who frequently use freeway on-ramps — and offer greater towing reliability. For the same vehicle, a lower-horsepower engine will offer greater fuel economy in exchange for more leisurely acceleration.

As for the reference to beer, legend states a brewer, one of Watt’s first customers for his steam engine, challenged the inventor to produce a machine as strong as a horse. The brewer chose the strongest horse he had and rode it as hard as he could. Watt’s resulting engine was even more powerful than the horse and it was the output of that engine that set the math in place for the horsepower.

Of course, in Watt’s day, a 5 hp steam engine would fill an entire room. Today, engines fifty times as powerful fit under your hood.

How is horsepower measured?

You may have heard theories that one horsepower is the equivalent to one horse, and that’s kind of true — just not in the way you may expect.

Horsepower is actually a completely made-up measurement. Back in the late 1760s, steam engine pioneer James Watt wanted to compare the effectiveness of his engines against the horse that previously did their work.

Watt defined one horsepower as the equivalent of the energy expended by a single horse raising 33,000 pounds of water one foot in the air from the bottom of a 1,000 foot deep well in 60 seconds.

Granted, that’s not an easy thing to imagine, but it did allow Watt a way to easily market his steam engines to potential customers.

Before you get ideas, manufacturers don’t plant cars in front of wells and pull water to work out how much performance they have nowadays. Rather, a device called a dynamometer (more commonly known as a dyno) is used to calculate its force.

How does horsepower impact a car’s performance?

As a simple rule, the more horsepower a car has — the faster it can go in a straight line.

An easy way to imagine the impact of more power is to imagine you’re pushing a car yourself. Now, if a friend came to help you, there’s more force behind it and thus you can push it faster, easier and further.

It’s not quite as simple as that, granted. For example, upgrading a 100hp car to have 200hp won’t make it twice as fast. There are other factors to consider — with key ones being weight, grip levels and aerodynamics.

Horsepower has less of an effect the higher your speed is, with aerodynamic efficiency playing an ever-more key part as you get faster.

What applications does horsepower have?

Horsepower is often used as a measure of engine power and other mechanical devices. For example, a small car might have a brake horsepower (bhp) of 100, while a large truck could have a brake horsepower of 500.

Horsepower is also used to describe the amount of work done by machines such as generators and pumps. For example, a small generator might have a horsepower of 5, while a large diesel engine generator could have a horsepower of 500.

Horsepower can also describe the amount of power that an electric motor can generate. For example, a small electric motor might have a horsepower of 1, while a large one could have a horsepower of 10. It can also be used to measure the power output of electric cars.

What is the difference between horsepower and torque?

Torque is force multiplied by distance or simply a measure of turning force. With cars, torque is “the rotational equivalent of linear force.” In other terms, torque is the amount of force applied to an object with a twisting motion, which consequently rotates the car tires due to the force applied on the crankshaft.

Horsepower and torque are two sides of the same coin as one goes with the other. Torque is the force while horsepower is the rate at which that force works. The difference in torque is doing the work, while horsepower is how fast that work gets done.

The horsepower and torque, together with where each falls on a vehicle’s engine rotation per minute (rpm) range and gearing, determine a car’s speed and acceleration.

In layman’s terms, torque allows you to feel the sensation of acceleration when you put your foot down, while horsepower is your overall speed and power.

This theory is understandable because horsepower gets calculated from torque.

Consider how you can calculate horsepower with this simple equation: Horsepower = Torque x RPM / 5,252 (where 5,252 is the conversion between lb-ft and horsepower). Horsepower and torque are closely linked. The higher the horsepower, the higher torque.

Before deciding to purchase a car, consider the torque and horsepower numbers. Horsepower is a good indicator of overall power but the torque can give a clear hint about what the vehicle is capable of doing. Torque is vital to every engine’s operation, but horsepower distinguishes a great engine from a good one.

Note that a higher number of horsepower or torque does not always mean more towing capacity, acceleration, or speed. The weight of the vehicle plays a huge part in its abilities. Take these into consideration when deciding to buy a used car.

Glance over the horsepower and torque figures when paying attention to the purpose of the vehicle. A car with good low-end torque is what you need in a hilly or mountainous area and a truck with a high-end torque is excellent for towing.

Are there other units for power?

Referring to vehicle power in kilowatts is becoming more common globally. In the U.S., kilowatts are often used when referring to electric power, so their use is becoming more common with the rise of electric cars. This unit may be used to describe both the charging power (indicating how quickly an EV recharges) and motor power (hinting at how quickly a vehicle accelerates). To convert from kilowatts to horsepower, multiply by 1.34102.

Metric horsepower is often used in European countries with the same “hp” abbreviation, but the two units are not equal. Metric horsepower is derived in a similar way to horsepower, but instead of the math being based on pounds and feet, it’s rooted in kilograms and meters. Naturally, one star-spangled American horsepower is stronger than the European version.

The French translation for metric horsepower is chevaux-vapeur (cv) and the German translation is Pfedestärke (ps). All these power units represent the same thing, and although the number for a given car will be very close to the horsepower rating we talk about in America, they won’t be the same.

If the automaker is being precise with its measurements, it will list different figures here and abroad. In the U.K., the Audi R8 is advertised with 570 ps. In America, it is advertised with 562 hp. To convert ps, cv, or metric horsepower to imperial (American) horsepower, multiply by 0.98632.

Should you look at horsepower figures when comparing vehicles?

Yes, but as a rough guide. If there are two vehicles within 20hp of each other, drive them both and see if you even notice a difference.

There are ways that HP figures can be skewed. Larry Webster wrote in a Car and Driver article that, “Since minute differences in the dimensions of engine components that naturally occur during series production—the compression ratio could be a smidge higher in one engine versus another—can result in two identical-looking engines making slightly different horsepower…”.

There are also different ways of quoting figures, such as BHP (brake horsepower), IHP (indicated horsepower), SHP (shaft horsepower), WHP (wheel horsepower). On top of that, other countries in the world have different systems. Germany uses Pferdestärke, which is quoted as PS. For example, a German vehicle might be rated at 200PS. 1 PS is not quite equivalent to 1 HP (PS x 0.9863 = 1HP).

There doesn’t seem to be any regulations as far which of these numbers should be quoted by manufacturers, although from my understanding, they do have the option to have the number SAE certified.

If you were a manufacturer, you’re probably going to choose the highest number, which would be the BHP measurement at the crank. “Brake horsepower (bhp) is the power measured at the crankshaft just outside the engine, before the losses of power caused by the gearbox and drive train”.

Why you should consider horsepower when buying a car

Because it will fundamentally affect how your car performs on the road. A car with a not-very-powerful engine (75hp, for example) will not be able to accelerate very hard, whereas a car with lots of power (500hp+) will have strong acceleration. The car’s weight will have a large impact on acceleration too, as the heavier the car, the more power is required to move it. That’s why small city cars tend not to have powerful engines, while large SUVs do.

If you travel a lot on the motorway, you are likely to want a more powerful engine, as you will want to be able to accelerate quickly along slip roads while joining the road, and accelerating at speed (IE when overtaking) is also much easier if you have a powerful engine.

If you have a less powerful engine, on a motorway you will typically need to accelerate hard to make progress, typically needing to change down a gear or two while using high engine revs that will be noisy.

People who regularly tow caravans, trailers and the like will also want powerful engines, as the extra weight will require more power to get things moving – although actually torque is a tow car’s best friend, hence why many caravanners have diesel cars.

FAQs

Does one horsepower equal one horse?

Horsepower was originally created based on a single horse lifting 33,000 pounds of water one foot in the air from the bottom of a 1,000 foot deep well. This was used by James Watt to provide context to the performance of his steam engines. So yes, it does equal one horse — but not quite in the way you may think.

What is considered good horsepower in a car?

‘Good’ horsepower is very subjective depending on the car. Other factors such as weight, mechanical grip, aerodynamics and even just the purpose of the car need to be taken into account.

What horsepower does my car have?

A quick search on the internet should be able to uncover a manufacturer’s stated figure for your car. If you’d like to find out the exact performance of your car, you can hire a dyno session to have it measured.

What car has the most horsepower?

The Rimac Nevera electric hypercar produces 1,914hp, making it the most powerful car in the world. The 1,600hp petrol-powered Bugatti Chiron SS follows the Rimac.

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