One Horsepower Equals Watts? The Exact Conversion

One horsepower equals 745.7 watts (exactly 745.69987158227022 W). This is the most common definition, called mechanical horsepower, and it’s what you’ll see on engines, motors, and pumps in the United States. This page explains the conversion, where the number comes from, and how it applies to real-world machines.

Formula

The conversion formula is simple:

P(W) = P(hp) × 745.7

Where:

• P(W) = power in watts
• P(hp) = power in mechanical horsepower
• 745.7 = the exact conversion factor (745.69987158227022 W per hp)

Worked Example

Using the default value of 1 horsepower:

1. Start with the formula: P(W) = P(hp) × 745.7
2. Plug in 1 hp: P(W) = 1 × 745.7
3. Multiply: P(W) = 745.7 W

So 1 horsepower equals 745.7 watts. The calculator above gives the same result (745.70 W) when you enter 1 hp.

Where Does the 745.7 Number Come From?

The unit was defined by James Watt in the late 18th century to compare the output of steam engines with the horses they replaced. He measured that a typical horse could lift 550 pounds one foot in one second. That rate of work—550 ft·lb/s—became the standard mechanical horsepower. When you convert that to the metric system, you get exactly 745.69987158227022 watts.

That’s why the number isn’t a round figure in watts: it’s a legacy of the imperial unit “foot-pound per second� that was chosen for marketing and practicality, not for metric neatness.

Different Types of Horsepower

Horsepower isn’t a single universal unit. Several variants exist, and using the wrong one can throw off calculations by 1–15%. The most common are:

• Mechanical horsepower (hp) — 745.7 W. Used for car engines, lawnmowers, and most US machinery.
• Metric horsepower (PS or cv) — 735.49875 W. Used in Europe and Asia; defined as 75 kgf·m/s.
• Electrical horsepower — exactly 746 W. Used for electric motors in some contexts.
• Boiler horsepower — about 9,809.5 W. Used for steam boilers; based on heat output, not mechanical work.

When someone says “horsepower� without qualifiers, they almost always mean mechanical horsepower. But if you’re comparing European and American specs, check which type is being used.

How to Convert Horsepower to Watts Yourself

You don’t need a calculator for most conversions—just multiply by 745.7. Here are common values:

• 0.5 hp = 372.85 W
• 1 hp = 745.7 W
• 2 hp = 1,491.4 W
• 5 hp = 3,728.5 W
• 10 hp = 7,457 W

For metric horsepower, multiply by 735.5 instead. If you’re working with electrical horsepower, use 746. To convert watts back to horsepower, divide by 745.7.

Why the Conversion Matters in Real Life

Knowing the watt equivalent helps you estimate electricity costs, size generators, or compare power sources. For example, a 1-hp electric motor running for one hour consumes about 0.746 kWh (if it’s 100% efficient, which it never is). At a typical electricity rate, that’s a few cents. But an engine rated 100 hp doesn’t use 74.57 kW of electrical power—that’s the mechanical output. The fuel or electrical input is always higher due to losses.

When sizing a generator for a 2-hp pump, you need at least 1,491 W of continuous capacity, but starting surge can be 2–3 times that. So the conversion is just the starting point; real-world factors like efficiency and startup current matter.

Common Mistakes and Misconceptions

• Using the wrong horsepower type — mixing metric and mechanical hp gives a 1.4% error, which can be significant in engineering.
• Confusing watts with watt-hours — watts measure power (rate), watt-hours measure energy (total). A 745.7 W motor running for 2 hours uses 1,491.4 Wh, not 1,491.4 W.
• Assuming 1 hp = 746 W exactly — that’s electrical horsepower, not mechanical. For most purposes 745.7 is correct.
• Ignoring efficiency — a motor’s input power is higher than its output. A 1-hp motor might draw 1,000 W from the wall because it’s only 75% efficient.

Frequently Asked Questions

Is 1 horsepower equal to 746 watts?

Almost—1 mechanical horsepower is exactly 745.69987158227022 W, which rounds to 745.7 W. The 746 W figure comes from electrical horsepower, a different unit used mainly for electric motors. For most everyday conversions, 745.7 is the right number.

How many watts is 5 horsepower?

5 mechanical horsepower equals 5 × 745.7 = 3,728.5 W. If you’re using metric horsepower, it’s 5 × 735.5 = 3,677.5 W. Always check which type you’re dealing with.

Why is horsepower still used instead of watts?

Tradition and marketing. Consumers are familiar with horsepower for cars and lawnmowers, and manufacturers have used it for over a century. Watts are the scientific standard, but horsepower persists in the automotive and small-engine industries, especially in the US.

How do I convert watts to horsepower?

Divide watts by 745.7. For example, 1,500 W ÷ 745.7 = 2.01 hp. For metric horsepower, divide by 735.5. This is useful when comparing the power of electric heaters or motors to engine power.

Does 1 horsepower really equal one horse?

No. James Watt’s estimate was based on a horse working continuously for a full day, which is about 50% more than what an average horse can sustain. The unit is a convenient benchmark, not a biological measurement. A healthy horse can briefly produce over 10 hp, but only for seconds.

What is the difference between horsepower and brake horsepower?

Brake horsepower (bhp) is the power measured at the engine’s output shaft, before losses from the transmission, alternator, and other accessories. Horsepower (hp) in the US often refers to the same thing, but in some contexts it may be measured at the wheels (whp) or with different standards. The conversion to watts is the same for both—you’re just measuring at different points.

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in hp | Horsepower | hp | 1 | 0
out watts | Equivalent power | W | hp * 745.69987158227022 | 2