Horsepower to Watts Formula & Converter

This calculator converts horsepower to watts instantly. Enter the horsepower value (mechanical, metric, or electrical), and the calculator returns the equivalent power in watts. The conversion factor depends on which horsepower definition you use, so the calculator includes a selector for the three common types.

Formula

The basic formula for converting horsepower to watts is:

Watts = Horsepower × 745.7

Where:

• Watts (W) is the SI unit of power.
• Horsepower (hp) is the power value you start with.
• 745.7 is the conversion factor for mechanical horsepower (the most common type in the US).

However, there are actually three different horsepower definitions, each with its own conversion factor:

• Mechanical horsepower (hp(I)): 1 hp = 745.69987158227022 W (often rounded to 745.7 W). This is used for most engines and motors in the US.
• Metric horsepower (hp(M)): 1 hp = 735.49875 W. This is used in Europe and many other parts of the world for car engines.
• Electrical horsepower (hp(E)): 1 hp = 746 W. This is used for electric motors and generators.

The calculator above uses the mechanical value by default, but you can switch to the others with the dropdown.

Worked Example

Let’s convert 1 mechanical horsepower to watts using the default values.

Step 1: Identify the horsepower value and type. Here, hp = 1, type = Mechanical (factor = 1).

Step 2: Apply the formula:

Watts = 1 × 1 × 745.69987158227022 = 745.69987158227022 W

Step 3: Round to two decimal places: 745.70 W.

So, 1 mechanical horsepower equals 745.70 watts. If you had 2 hp, it would be 2 × 745.7 = 1491.4 W.

Why There Are Different Horsepower Definitions

Horsepower was first defined by James Watt in the 18th century to compare the output of steam engines with that of draft horses. He estimated that a horse could do 33,000 foot-pounds of work per minute, which became the mechanical horsepower. Later, different industries and countries adopted slightly different standards. The metric horsepower was defined as the power needed to lift 75 kilograms one meter in one second (735.49875 W). Electrical horsepower was set to exactly 746 W to match the electrical system standards. These differences matter when comparing specs across regions or when doing precise engineering calculations.

How to Use the Calculator

To get the most out of this converter:

• Enter the horsepower value you have. For example, if a car engine is rated at 150 hp, type 150.
• Select the correct horsepower type. If you’re not sure, mechanical (hp(I)) is the default and most common in the US. For European car specs, use metric (hp(M)). For electric motors, use electrical (hp(E)).
• Read the result in watts. The calculator shows the answer rounded to two decimal places.

For quick reference, here are some common conversions:

• 1 hp (mechanical) = 745.7 W
• 5 hp = 3,728.5 W
• 10 hp = 7,457 W
• 100 hp = 74,570 W

Common Mistakes and How to Avoid Them

One of the most frequent errors is using the wrong conversion factor. For example, using 746 W for mechanical horsepower instead of 745.7 W introduces a small error that can add up in large numbers. Another mistake is mixing up horsepower and kilowatts (kW). Remember that 1 kW = 1000 W, so to convert hp to kW, you divide the watts by 1000. For instance, 745.7 W = 0.7457 kW.

Also, be careful with units when doing manual calculations. Always write down the units and cancel them out. If you’re converting from horsepower to watts, the horsepower unit cancels, leaving watts.

When to Use Horsepower vs. Watts

In everyday life, horsepower is still used for car engines, lawn mowers, and some appliances in the US. Watts are the standard unit in science, engineering, and most of the world for all types of power, including electricity (e.g., light bulbs, heaters, solar panels). If you’re comparing power across different systems, converting to watts gives a common baseline. For example, a 2-hp motor is about 1491 W, which tells you how much electrical power it would consume if it were 100% efficient (though real motors are less efficient).

Limitations of the Formula

The conversion formula is exact for the definitions given, but the real-world power output of an engine or motor can vary due to factors like temperature, altitude, and mechanical losses. The horsepower rating is usually measured at the flywheel (for engines) or shaft (for motors), not at the wheels or output device. So, the watts you calculate represent the theoretical power produced, not necessarily the power delivered to the ground or to the load. For electric motors, the input electrical power is higher than the output mechanical power because of losses (efficiency).

Frequently Asked Questions

How many watts are in 1 horsepower?

One mechanical horsepower equals 745.7 watts. One metric horsepower equals 735.5 watts. One electrical horsepower equals 746 watts. The exact value depends on which definition you use.

What is the difference between mechanical and metric horsepower?

Mechanical horsepower is based on the imperial system (550 foot-pounds per second) and equals 745.7 watts. Metric horsepower is based on the metric system (75 kgf·m/s) and equals 735.5 watts. The difference is about 1.4%.

How do I convert horsepower to kilowatts?

First convert horsepower to watts, then divide by 1000. For example, 1 hp (mechanical) = 745.7 W = 0.7457 kW. So, multiply the horsepower by 0.7457 to get kilowatts.

Why is electrical horsepower exactly 746 watts?

Electrical horsepower was defined to match the electrical system standards of the late 19th century. It is exactly 746 watts, which is slightly higher than mechanical horsepower. This definition is used for electric motors and generators.

Does the conversion change if I use imperial vs. metric units?

No, the conversion factor is fixed for each horsepower definition. The units you use for the horsepower value (e.g., hp, kW) don’t change the conversion factor. Just make sure you’re using the correct horsepower type.

Can I use this calculator for metric horsepower?

Yes, the calculator has a dropdown to select Metric horsepower. When selected, the conversion factor changes to 735.49875, and the result will be in watts.

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in hp | Horsepower | hp | 1 | 0
sel type | Horsepower Type | Mechanical=1, Metric=0.98632, Electrical=1.00034
out watts | Watts | W | hp * type * 745.69987158227022 | 2