What Exactly Is 1 Horsepower?

One horsepower is a unit of power most commonly defined as 745.7 watts in the “mechanical horsepower” used in the U.S. and U.K.; the similar “metric horsepower” used in parts of Europe equals 735.5 watts. Originating with James Watt’s 18th-century marketing for steam engines, horsepower measures the rate of doing work—how quickly energy is converted or delivered.

Origins: James Watt’s Benchmark for Power

In the late 1700s, Scottish engineer James Watt needed a relatable way to sell the capability of his improved steam engines. Observing horses working in coal mines, he estimated a strong horse could turn a mill wheel with the equivalent of 33,000 foot-pounds of work per minute. He set “1 horsepower” equal to that rate—33,000 ft·lbf/min—cementing a benchmark that bridged animal labor and industrial machinery.

Not All Horsepower Is the Same

Several definitions of horsepower are used in different industries and regions. The differences mostly reflect historical standards and application-specific conventions, but they can change the number you see on a spec sheet.

• Mechanical horsepower (hp, imperial/US): 1 hp = 550 ft·lbf/s = 33,000 ft·lbf/min = 745.699871582 W (≈ 745.7 W).
• Metric horsepower (PS, CV, hk, ch): 1 PS = 75 kgf·m/s = 735.49875 W (≈ 735.5 W); 1 PS ≈ 0.98632 mechanical hp.
• Electrical horsepower: exactly 746 W (often used for electric motors in North America).
• Boiler horsepower (steam generation): 1 boiler hp = 33,475 BTU/h = 9.8095 kW = evaporation of 34.5 lb/h of water “from and at” 212°F (100°C).
• Hydraulic horsepower: a fluid-power context where power is still power; 1 hydraulic hp = 745.7 W. Often computed as hp = (flow in GPM × pressure in psi) / 1714.

For engines and motors, mechanical horsepower and metric horsepower are the most relevant. Boiler horsepower is a specialized thermal rating, and electrical horsepower is a practical convenience for motor ratings.

How Horsepower Relates to Torque and RPM

Power is the rate at which work is done. For rotating machines, that depends on how hard the shaft is twisting (torque) and how fast it’s spinning (revolutions per minute).

Formulas engineers use

Imperial: horsepower = (torque in ft·lbf × rpm) / 5252

SI: power in kW = (torque in N·m × rpm) / 9549

Quick example

If an engine produces 200 ft·lbf at 3000 rpm: hp = (200 × 3000) / 5252 ≈ 114 hp. In SI, 200 N·m at 3000 rpm gives kW = (200 × 3000) / 9549 ≈ 62.8 kW, which is ≈ 84.2 hp (metric vs mechanical and rounding explain differences—use consistent units).

Conversions and Equivalences

These conversions help compare ratings across regions and datasheets. Be sure to match the type of horsepower referenced in the source.

• 1 mechanical hp = 745.699871582 W ≈ 0.7457 kW.
• 1 kW = 1.34102209 mechanical hp (approx.).
• 1 mechanical hp = 550 ft·lbf/s = 33,000 ft·lbf/min = 745.699871582 N·m/s.
• 1 metric hp (PS) = 735.49875 W ≈ 0.7355 kW ≈ 0.98632 mechanical hp.
• 1 electrical hp = exactly 746 W.
• 1 boiler hp = 9.8095 kW = 33,475 BTU/h = 34.5 lb/h steam (from and at 212°F).

When in doubt, convert to watts or kilowatts. Power in watts is unambiguous and SI-consistent.

How Horsepower Is Reported

Automakers and equipment makers use various testing and reporting conventions that can change the headline number without changing the underlying physics.

• Brake horsepower (bhp): Power measured at the engine’s crankshaft (or on a brake dynamometer), before drivetrain losses.
• Shaft horsepower (shp): Power available at the output shaft—common for turbines and marine/aviation applications.
• Wheel horsepower (whp): Power measured at the wheels on a chassis dynamometer; lower than bhp due to drivetrain losses.
• Indicated horsepower (ihp): Theoretical cylinder power derived from pressure measurements; higher than bhp because it ignores mechanical losses.
• Standards: Modern “net” ratings (e.g., SAE J1349, ISO 1585/ECE) include accessories and standardized conditions; older “gross” ratings (pre-1972 SAE) were higher because accessories and stock exhausts were often removed during testing.

If two engines appear to have different horsepower, check whether they’re rated as gross or net, and whether the figure is at the crank or at the wheels.

Where You’ll See Horsepower Today

Horsepower remains a popular headline figure, even as kilowatts are the legal unit in many markets and the default in electric power.

• Cars: North America often quotes hp; the EU requires kW, though hp is still widely referenced alongside.
• Electric vehicles: Motor power is typically specified in kW; many marketing materials also list hp for familiarity.
• Trucks and heavy equipment: Rated in hp or kW depending on region; torque curves matter as much as peak hp.
• Marine and aviation: Shaft horsepower is common; turbines may use shp with separate thrust specifications.
• Consumer products: Beware of “peak” or “developed” horsepower claims (e.g., vacuums, treadmills). Continuous/rated power in watts or kW is the more reliable indicator.

Ultimately, horsepower is part of a broader picture; torque, rpm, and how long a machine can sustain its power are equally important for real-world performance.

Summary

One horsepower is a measure of power—the rate of doing work. The most common definition, mechanical horsepower, equals about 745.7 watts; metric horsepower equals about 735.5 watts, and an electrical horsepower is exactly 746 watts. The unit traces to James Watt’s 18th-century benchmark of 33,000 ft·lbf per minute. In practice, horsepower connects directly to torque and rpm, and published ratings vary by test standard and where the power is measured (engine, shaft, or wheels). Converting to watts or kilowatts provides the clearest, standard comparison across machines and markets.

How do you explain 1 horsepower?

horsepower, the common unit of power; i.e., the rate at which work is done. In the British Imperial System, one horsepower equals 33,000 foot-pounds of work per minute—that is, the power necessary to lift a total mass of 33,000 pounds one foot in one minute.

What defines 1 hp?

Its name does indeed come from the animal: 1 mechanical horsepower, an imperial measurement, is the amount of energy required to lift a weight of 250kg by 1 foot every second.

Is 1 hp equal to a horse?

No, 1 horsepower (hp) does not equal the maximum power of a horse; rather, it is a unit of power, defined as 33,000 foot-pounds of work per minute, based on the sustained output of a draft horse over a workday. While a single horse can momentarily produce around 15 hp, the term was coined by James Watt to market steam engines by comparing their consistent output to the average, sustained power of a working horse.
 
The Origin of Horsepower

• James Watt’s Calculation: Opens in new tabIn the late 18th century, inventor James Watt needed a way to sell his new steam engines. He compared the power of his engines to the familiar power of horses, the standard at the time. 
• A “Working” Horse: Opens in new tabWatt calculated the average rate of work a draft horse could perform over a full day and defined this as one horsepower. This equates to 33,000 foot-pounds of work per minute. 

The Power of a Real Horse

• Peak vs. Sustained Output: A healthy, working horse’s peak power output can be much higher than 1 hp, with some sources stating it can reach up to 15 hp for short bursts. 
• Varying Performance: The actual power a horse can generate varies depending on its breed, physical fitness, and the specific task it is performing. 
• The “Average” Horse: The definition of 1 hp represents the power an average horse could sustain over a working shift, not its maximum potential. 

What is an example of 1 horsepower?

A single horsepower (hp) is defined as the continuous power needed to lift 550 pounds one foot in one second, and is equivalent to about 746 watts in the SI system. Items and concepts that can exhibit one horsepower of power include: 

• An average horse’s sustained output, though a horse can produce much more for short bursts. 
• A 16-amp electric motor at 115 volts, which produces approximately 1 hp. 
• An average human’s output for a short period, with a person capable of producing about 1 horsepower for a few seconds, but only around 0.15 to 0.4 horsepower for longer durations. 

Understanding the Definition of Horsepower

• Origin: The term “horsepower” was coined by James Watt in the 18th century to help people understand the power of his new steam engines by comparing them to the labor horses provided at the time. 
• Work vs. Power: A common misconception is that horsepower means one horse’s power. Instead, it’s a unit of power (the rate at which work is done), not just the ability to do work. 
• Work Example: To perform work equivalent to one horsepower, a 33-pound bucket could be lifted 1,000 feet in one minute. 

Modern Comparisons

• Electrical Outlets: A 16-amp electrical outlet can provide roughly 1 horsepower of power. 
• Appliances: While a typical light bulb consumes less, the overall horsepower output of engines in cars and motors in appliances can be easily assessed using this unit.