Horsepower to Watts: Conversion & Formula

One mechanical horsepower equals exactly 745.69987158227022 watts (often rounded to 745.7 W). This calculator converts horsepower to watts using the standard mechanical definition, and you can also convert in reverse by entering a watt value.

Formula

The conversion is a simple multiplication: Watts = Horsepower × 745.69987158227022. The number 745.7 is the exact conversion factor defined by the international standard for mechanical horsepower (sometimes called imperial horsepower).

In symbols: P_W = P_hp × 745.7, where P_W is power in watts and P_hp is power in horsepower.

Worked Example

Using the default values from the calculator above (1 hp and 745.7 W):

1. Start with 1 horsepower.
2. Multiply by the conversion factor: 1 × 745.69987158227022 = 745.69987158227022 watts.
3. Rounded to two decimal places, that gives 745.70 W.

For the reverse direction, 745.7 W divided by 745.69987158227022 = 1.00 hp, confirming the round-trip.

Why Are There Different Horsepower Definitions?

Horsepower was coined by James Watt in the 18th century to compare the output of steam engines with that of draft horses. He defined one horsepower as the power needed to lift 33,000 pound-feet per minute (or 550 pound-feet per second). That works out to 745.7 watts under the modern SI definition.

However, several other horsepower units exist:

• Metric horsepower (PS or cv): equals 735.49875 W. Used in Europe and many other places for car engines.
• Electrical horsepower: exactly 746 W, used for electric motors in some contexts.
• Boiler horsepower: about 9,809.5 W, used for steam boilers.
• Hydraulic horsepower: calculated from flow and pressure, not a fixed unit.

When someone says “horsepower” without a qualifier, they usually mean mechanical horsepower (745.7 W). But always check the context—especially with car specs from different countries.

How to Convert Horsepower to Watts Manually

If you don’t have a calculator handy, the process is straightforward:

1. Identify which horsepower unit you have (mechanical, metric, electrical, etc.).
2. Multiply by the appropriate factor: 745.7 for mechanical, 735.5 for metric, 746 for electrical.
3. Round to the precision you need—two decimal places is usually enough for everyday use.

For example, a 150 hp car engine (mechanical) produces 150 × 745.7 = 111,855 W, or about 111.9 kW. A 150 PS engine (metric) gives 150 × 735.5 = 110,325 W, or 110.3 kW—a small but real difference.

Common Mistakes and Misconceptions

One frequent error is mixing up mechanical and metric horsepower. If you convert a European car’s PS rating using the mechanical factor, you’ll overstate the power by about 1.4%. Another mistake is forgetting that watts are a small unit—a typical car engine produces tens of thousands of watts, so kilowatts (1 kW = 1000 W) are more convenient for discussion.

Also, don’t confuse power (watts or horsepower) with torque (newton-meters or pound-feet). Power is the rate of doing work; torque is a rotational force. They are related but not interchangeable.

When to Use This Conversion

You’ll need this conversion when comparing equipment rated in different units. For example:

• An electric motor rated in horsepower but you need watts for a solar or battery system.
• A car engine rated in PS (metric) and you want to compare with a US-spec car rated in mechanical hp.
• Generators and inverters often list power in watts, while engines are rated in horsepower.

In most real-world situations, the difference between mechanical and electrical horsepower (745.7 vs 746 W) is negligible—less than 0.05%—so you can use 746 for quick mental math without meaningful error.

Limits of the Formula

The conversion factor is exact for mechanical horsepower, but the formula only converts units—it doesn’t tell you anything about efficiency, losses, or actual output. A motor’s nameplate horsepower is its rated output, but the electrical input will be higher due to heat and friction losses. Similarly, an engine’s horsepower is measured at the crankshaft, not at the wheels; drivetrain losses can reduce wheel power by 10–20%.

Also, the watt is an SI unit, so the conversion is straightforward. But be aware that some industries use “horsepower” loosely—for example, air conditioner capacity is sometimes quoted in “tons” or “BTU/h,” not horsepower, even though people might say “a 1-hp AC.” Always verify the actual unit.

Frequently Asked Questions

Is 1 horsepower really equal to 746 watts?

For mechanical horsepower, the exact value is 745.69987158227022 W, which rounds to 745.7 W. The number 746 is used for electrical horsepower, a slightly different unit. For most purposes, 746 is a fine approximation.

Why do European cars use PS instead of hp?

PS (Pferdestärke, German for “horse strength”) is the metric horsepower unit, defined as 735.49875 W. It originated in Germany and became the standard in continental Europe. The difference from mechanical hp is about 1.4%, so a 100 PS engine is roughly 98.6 mechanical hp.

How do I convert kilowatts to horsepower?

Divide the kilowatt value by 0.7457 (or multiply by 1.341) to get mechanical horsepower. For example, 100 kW ÷ 0.7457 = 134.1 hp. If you’re dealing with metric horsepower, divide by 0.7355.

Is horsepower a unit of force or energy?

Neither. Horsepower is a unit of power, which is the rate at which work is done or energy is transferred. One horsepower equals 745.7 joules per second (since 1 W = 1 J/s). It’s not a force (like pounds) or an energy quantity (like joules).

Why do we still use horsepower if watts are the SI unit?

Tradition and marketing. Horsepower has been used for over 200 years, and consumers associate it with engine performance. It’s also a convenient number—a typical car has 100–300 hp, while in watts that’s 75,000–225,000 W, which is less intuitive. However, many countries now list engine power in kilowatts alongside horsepower.

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in hp | Horsepower | hp | 1 | 0
in w | Watts | W | 745.7 | 0
out watts | Watts | W | hp * 745.69987158227022 | 2
out hp_out | Horsepower | hp | w / 745.69987158227022 | 2