How Horsepower Is Defined

Horsepower is a unit of power. In its most common form—mechanical horsepower—it is defined as exactly 550 foot-pounds of work per second, which equals 745.699871582 watts. A widely used alternative, metric horsepower (PS/CV), is defined as 75 kilogram-force meters per second, equal to 735.49875 watts. In electrical contexts, horsepower is often taken as exactly 746 watts. Beyond these headline figures, “horsepower” can refer to different testing methods and contexts, which affects how numbers are reported in spec sheets.

Origins and Core Meaning

James Watt popularized horsepower in the late 18th century to compare steam engines with draft horses. He characterized one horsepower as 33,000 foot-pounds per minute—equivalently 550 foot-pounds per second—capturing the rate of doing work. Today, horsepower remains a non-SI unit; the SI unit of power is the watt (W), and modern regulations often require kW to be shown alongside or instead of hp.

Standard Definitions of Horsepower

Several standard “horsepower” units are in use, each precisely defined and convertible to watts. The list below summarizes the most common variants and their exact or standard conversion factors.

• Mechanical horsepower (hp, imperial/US): Exactly 550 ft·lbf/s = 745.699871582 W.
• Metric horsepower (PS, CV, hk, ch): Exactly 75 kgf·m/s = 735.49875 W.
• Electrical horsepower (hpE): Exactly 746 W (commonly used for electric motors in North America).
• Boiler horsepower (hpboiler): Defined as the energy to evaporate 34.5 lb of water per hour at 212°F; ≈ 9.809 kW.

While the magnitude of each unit is fixed, industries and regions prefer different variants. In U.S. automotive contexts, “hp” generally means mechanical horsepower; in parts of Europe, “PS” or “CV” is common; and manufacturers increasingly publish kW alongside hp for clarity.

How Horsepower Relates to Torque and Speed

Horsepower ties directly to torque (twisting force) and rotational speed. These formulas show how engine or motor torque and RPM translate to power ratings, with constants reflecting the unit definitions.

1. hp (mechanical) = T(lb·ft) × RPM / 5252
2. kW = T(N·m) × RPM / 9550 (approximate, widely used constant)
3. hp (mechanical) ≈ T(N·m) × RPM / 7127
4. PS ≈ T(N·m) × RPM / 7023

These relationships highlight that horsepower is the rate of doing work: more torque at a given speed—or the same torque at higher speed—yields more power. The well-known 5252 constant arises from unit conversions and 2π radians per revolution.

Measurement Protocols and What Numbers Mean

Published horsepower figures depend on standardized test setups—what accessories are attached, how atmospheric conditions are corrected, and where power is measured. The unit’s size doesn’t change, but the test method can change the reported value.

• SAE J1349 “net” power: Common in North America; measures engine output with standard accessories and applies weather corrections.
• UN/ECE R85: Governs net power measurement of electric and hybrid vehicle drive units, widely used in Europe and elsewhere.
• DIN 70020 (historical): Older German method; many manufacturers now align with UN/ECE or ISO methods.
• ISO engine power codes (e.g., ISO 15550, related standards): International frameworks for net/gross power definitions and corrections.

Because these protocols differ, two engines can have identical true capability yet show different catalog horsepower depending on accessories, correction factors, and whether the number is “gross,” “net,” or measured at the wheels.

Related Terms You May See

Beyond unit definitions, several common terms describe where and how power is measured along the drivetrain.

• Brake horsepower (bhp): Power measured at the engine’s output shaft on a dynamometer.
• Shaft horsepower (shp): Power delivered to a shaft (e.g., marine, aerospace), typically synonymous with bhp in that context.
• Wheel horsepower (whp): Power measured at the driven wheels; lower than bhp due to drivetrain losses.
• Indicated horsepower (ihp): Power calculated from in-cylinder pressure, not accounting for mechanical losses.

These terms help pinpoint whether a figure reflects raw engine output, delivered shaft power, or usable power at the wheels—crucial distinctions when comparing specifications.

Conversions at a Glance

These conversions help translate horsepower among systems and to SI units for apples-to-apples comparisons.

• 1 hp (mechanical) = 745.699871582 W
• 1 kW = 1.34102209 hp (mechanical)
• 1 PS (metric hp) = 735.49875 W
• 1 PS = 0.98632007 hp (mechanical)
• 1 hpE (electrical) = 746 W
• 1 boiler hp ≈ 9.809 kW

When in doubt, convert to watts or kilowatts; SI units are universal and avoid ambiguity between hp variants and test protocols.

Summary

Horsepower quantifies power—the rate of doing work. By definition, mechanical horsepower is exactly 550 ft·lbf/s (745.699871582 W), metric horsepower is exactly 75 kgf·m/s (735.49875 W), and electrical horsepower is exactly 746 W. Differences you see on spec sheets mostly stem from testing standards and where power is measured, not from the unit itself. For clarity across regions and industries, converting to kilowatts provides the cleanest comparison.

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.

How was horsepower originally defined?

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.

Is 100 hp equal to 100 horses?

Not quite. It’s a common misconception that one horsepower is equal to the peak power production of a horse, which is capable of a maximum of around 14.9 horsepower. By comparison, a human being is capable of approximately five horsepower at peak power production.

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.