How Much Is 1 Horsepower?

One horsepower is about 746 watts in the mechanical (U.S.) definition and about 735.5 watts in the metric (PS/CV) definition. The exact value depends on which standard is used, and a few specialized variants exist for electrical motors and steam boilers. Here’s what that means in practical terms and how to convert between units.

What “horsepower” means and why it varies

Horsepower is a historical unit of power, coined to compare early steam engines to the work rate of draft horses. While the SI system uses watts, horsepower remains common in automotive marketing and some engineering contexts. Crucially, “hp” is not single-valued: different industries standardized different definitions, which are all close but not identical.

Common definitions and exact values

The following list outlines the most commonly used horsepower definitions and their exact or widely accepted values. Knowing which version applies will ensure your conversions are correct.

• Mechanical (imperial) horsepower: 1 hp = 550 ft·lbf/s = 745.6998716 W ≈ 0.7457 kW
• Electrical horsepower: 1 hp (electric) = exactly 746 W = 0.746 kW
• Metric horsepower (PS, CV, ch): 1 hp (metric) = 735.49875 W ≈ 0.7355 kW (defined as 75 kgf·m/s)
• Boiler horsepower (steam): 1 bhp (boiler) = 33,475 BTU/h ≈ 9.81 kW (used only for boiler/steam capacity)

For vehicles and general machinery, mechanical hp and metric hp are the norms; electrical hp appears in motor ratings, and boiler hp is a specialized legacy unit unrelated to engines.

Quick conversions

Between horsepower, kilowatts, and other units

These conversions cover the most common back-and-forth calculations you’ll see in spec sheets and discussions.

• 1 mechanical hp ≈ 0.7457 kW; 1 kW ≈ 1.341 mechanical hp
• 1 metric hp (PS/CV) ≈ 0.7355 kW; 1 kW ≈ 1.3596 PS
• 1 mechanical hp = 550 ft·lbf/s = 33,000 ft·lbf/min
• 1 mechanical hp ≈ 2,544.43 BTU/h (since 1 W = 3.412142 BTU/h)

When converting, match the hp type to the context: automotive marketing in Europe often uses PS (metric hp), while U.S. engineering specs typically mean mechanical hp unless otherwise noted.

How engineers compute horsepower from torque and RPM

Horsepower relates directly to rotational torque and speed. These formulas allow you to compute power given torque and revolutions per minute (RPM), with constants tailored to the unit system.

• Mechanical horsepower: hp = (Torque in ft·lbf × RPM) / 5252
• SI power: kW ≈ (Torque in N·m × RPM) / 9550 (more precisely, divide by 9549.3)
• Electric motors (single-phase): kW ≈ (V × A × power factor × efficiency) / 1000; for three-phase systems, multiply by √3

These relationships link the “feel” of torque to the rate of doing work. For example, the same torque at double the RPM produces roughly double the power.

Real-world context: what does 1 hp look like?

It helps to anchor the number to familiar devices and human performance, illustrating how much work 1 hp represents.

• Home equipment: small shop vacs and treadmills often use ~1–2 hp motors
• Lawn/garden: walk-behind mowers typically 3–5 hp; 1 hp is modest but useful
• Human output: sustained cycling power is ~0.1–0.3 hp for trained adults; brief sprints can approach ~1–2 hp
• Cars: modern passenger vehicles commonly range from 100 to 400+ hp; 1 hp is tiny by automotive standards

In everyday terms, a single horsepower is enough to drive small appliances or tools, but it’s only a fraction of what’s needed to move a car at highway speeds.

Regional and regulatory usage

In the United States, mechanical hp is widely used alongside kW. In the European Union and many other regions, kW is the official unit, though metric horsepower (PS/CV) still appears in marketing. Always check whether figures are quoted in hp, PS, or kW to avoid mismatches.

Summary

One horsepower is roughly three-quarters of a kilowatt—but the exact value depends on the standard. Mechanical hp is about 745.7 W, electrical hp is exactly 746 W, and metric hp is about 735.5 W, while boiler hp is a much larger, specialized unit (~9.81 kW). For most engine and motor contexts, assume mechanical hp (U.S.) or metric hp (Europe) unless clearly stated otherwise.