Why cars use the term “horsepower”

It’s called horsepower because 18th‑century engineer James Watt coined the unit to compare early steam engines to the working ability of real horses, a familiar benchmark at the time; the auto industry later adopted the term to communicate engine power to buyers. One mechanical horsepower equals about 745.7 watts, while the “metric horsepower” used in some markets equals about 735.5 watts. This historical analogy stuck, shaping how car performance is described to this day.

Origins: James Watt’s 18th‑century benchmark

In the late 1700s, horses powered mills, pumps, and transport. To sell steam engines to customers who were used to hiring draft animals, James Watt needed a relatable measure of output. Observing horses at work, he standardized a unit of power—the rate of doing work—calling it “horsepower.” He defined one horsepower as 33,000 foot‑pounds per minute (550 foot‑pounds per second), which converts to roughly 745.7 watts. Historians note Watt likely chose a generous figure to favor his engines in marketing, but the unit quickly took hold across industry.

From mills to motorcars

When automobiles emerged, they directly replaced horse‑drawn carriages, so the comparison to horses was intuitive. Early carmakers published horsepower ratings to explain performance in familiar terms, and standardized test procedures later refined how that number is measured. The word became part of automotive language, bridging the mechanical past with modern powertrains.

What “horsepower” actually measures

Horsepower is a unit of power: how fast work is done. In engines, power depends on both torque (twisting force) and rotational speed. The basic relationship is power equals torque times rotational speed (with appropriate units and conversion factors). That’s why two engines with similar torque can feel very different depending on their rev range.

Automakers and regulators use several closely related horsepower standards. The list below outlines the most common versions you’ll see in car specifications.

• Mechanical horsepower (hp): The imperial/US standard defined as 745.7 W (550 ft·lbf/s). Common in North American media and marketing.
• Metric horsepower (PS, CV, ch): Defined as 735.5 W. Still used in European and Asian marketing; often appears alongside kilowatts (kW).
• SAE net horsepower (SAE J1349): The modern North American engine rating measured with standard accessories and intake/exhaust restrictions installed; lower and more realistic than pre‑1972 “SAE gross.”
• DIN horsepower (DIN 70020) and ECE/UN ECE R85: European-style net ratings under defined conditions; today, regulations typically cite power in kilowatts.
• Brake horsepower (bhp) vs wheel horsepower (whp): “Brake” refers to power measured at the engine’s crankshaft on a dynamometer; wheel horsepower is measured at the tires and is lower due to driveline losses.

Taken together, these standards aim to make comparisons fair and transparent, though numbers can still vary by method. For quick conversions: 1 hp ≈ 0.746 kW, 1 kW ≈ 1.341 hp. In many regions, kilowatts are the official legal unit, but horsepower remains widely quoted for consumer familiarity.

Why the term endures

Despite the global shift toward SI units (kilowatts), “horsepower” persists because it’s culturally entrenched and instantly understood by car buyers. Even as electric vehicles dominate headlines, manufacturers and media still express peak power in horsepower to convey performance at a glance.

The following reasons explain why horsepower remains part of the automotive vocabulary.

1. Cultural inertia: The term has been used for over a century and resonates with how drivers talk about performance.
2. Marketing clarity: A single, familiar number helps signal acceleration and capability in advertising and reviews.
3. Performance storytelling: Peak horsepower provides a headline figure, even if the full power curve is more informative.
4. Cross‑technology bridge: It offers a common yardstick for comparing internal combustion, hybrid, and electric vehicles.

As a result, you’ll often see horsepower and kilowatts listed together—kW for regulatory and engineering precision, hp for public communication.

Common misconceptions

The following points address frequent misunderstandings about horsepower in cars.

• Horsepower is not torque: Torque is force; horsepower reflects how quickly that torque is applied (torque times rotational speed).
• Peak horsepower isn’t the whole story: Driveability depends on the shape of the power and torque curves across the rev range.
• Old vs new ratings aren’t directly comparable: Pre‑1972 “SAE gross” figures are typically higher than modern net ratings for the same engine.
• EVs aren’t an exception: Electric motors are also rated in kW and often stated in hp; regulators may specify continuous and peak power separately.

Keeping these distinctions in mind helps consumers interpret spec sheets more accurately and avoid apples‑to‑oranges comparisons.

Summary

Cars use the term “horsepower” because James Watt created it to equate early engine output to the work of actual horses, and the automotive world adopted the familiar metric to explain performance. Today, horsepower coexists with kilowatts across various test standards (SAE, DIN, ECE), and while kW is the legal and engineering unit in many regions, horsepower endures in showroom brochures and headlines because it communicates power in a way drivers instantly recognize.

Why do they call it horsepower in a car?

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.

Is 1 hp equal to a horse?

In reality most horses can only manage 50% of Watt’s 33,000 foot-pound rate, and one horsepower doesn’t equal the strength of one horse.

Is 300 hp equal to 300 horses?

If you have a 300 HP engine, you can almost imagine 300 horses pulling your car forward. That’s definitely a lot of horses for one small car! An engineer named James Watt invented horsepower to sell his brand new steam engines back during the times when everything was horse-drawn.

How does horsepower get its name?

The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other power-generating machinery such as piston engines, turbines, and electric motors.