Mechanical Horsepower to Watts Conversion

This calculator converts mechanical horsepower (hp) to watts (W). You enter the horsepower value, and it returns the equivalent power in watts, using the standard conversion factor of 1 mechanical horsepower = 745.69987158227022 watts. The result is shown with two decimal places, and you can also see the conversion in kilowatts for convenience.

Formula

The conversion is straightforward: Watts = Horsepower × 745.69987158227022. In symbols: W = hp × 745.7 (rounded). The constant 745.7 represents the number of watts in one mechanical horsepower. The unit “watt� (symbol W) is the SI unit of power, defined as one joule per second. Mechanical horsepower is a traditional unit of power, originally defined by James Watt to compare the output of steam engines with the power of draft horses.

Worked Example

Using the default value of 1 hp:

1. Start with the formula: W = hp × 745.69987158227022
2. Plug in hp = 1: W = 1 × 745.69987158227022
3. Multiply: W = 745.69987158227022
4. Round to two decimals: W = 745.70

So 1 mechanical horsepower equals 745.70 watts, which matches the calculator’s output.

Why Are There Different Horsepower Units?

Horsepower is not a single, universal unit. The most common types are:

• Mechanical horsepower (hp) – used in the US and UK for engines, motors, and machinery. It is defined as exactly 745.69987158227022 watts.
• Metric horsepower (PS or cv) – used in Europe and many other countries. It is defined as 735.49875 watts (the power needed to lift 75 kg at 1 m/s).
• Electrical horsepower – used for electric motors, defined as exactly 746 watts.
• Boiler horsepower – used for steam boilers, equal to about 9,809.5 watts.

When someone says “horsepower� without qualification, they usually mean mechanical horsepower, but it is always worth confirming which unit is being used, especially when comparing specifications from different countries.

How to Read the Result

The calculator gives you the power in watts and kilowatts. A watt is a small unit – a typical incandescent light bulb uses 60 W, so 1 hp is roughly the power of 12 such bulbs. In the automotive world, a car engine might produce 150 hp, which is about 111,855 W or 111.9 kW. For electric motors, power is often rated in kilowatts directly, so converting hp to kW helps compare motor sizes across different systems.

Common Mistakes and Misconceptions

• Using the wrong horsepower type – mixing up mechanical (745.7 W) and metric (735.5 W) gives a 1.4% error. For large values, this can be significant.
• Forgetting to convert between kW and W – if you need kilowatts, remember 1 kW = 1000 W. The calculator shows both, but when doing manual calculations, keep the units straight.
• Assuming horsepower is a measure of speed – horsepower is power, which is the rate of doing work. Speed depends on gearing, aerodynamics, and other factors. A high-horsepower car is not necessarily faster if it is very heavy.
• Rounding too early – if you round 745.7 to 746, the error is tiny, but for precise engineering work, use the full constant.

When to Use This Conversion

Use this calculator whenever you need to compare power ratings expressed in different units. For example:

• You are looking at a car engine rated in horsepower and want to know its power in kilowatts for a registration document or insurance form.
• You have an electric motor rated in watts and want to compare it with a gasoline engine rated in horsepower.
• You are doing physics homework and need to convert a horsepower value to SI units.

For most practical purposes, the conversion is exact and does not depend on temperature, pressure, or other conditions.

Limitations of the Conversion

The conversion factor is a definition, so it is exact. However, the horsepower value you start with may be measured under different conditions. For engines, “rated horsepower� is often measured at the flywheel (brake horsepower) or at the wheels (wheel horsepower). The difference is due to drivetrain losses. This calculator converts the number you give, but it does not tell you how that number was measured. Also, for electric motors, the input power (electrical) and output power (mechanical) differ by the motor’s efficiency. This calculator gives the mechanical equivalent of the horsepower value, not the electrical input power.

Frequently Asked Questions

Is 1 horsepower really equal to the power of one horse?

Not exactly. James Watt defined the unit based on the average power of a draft horse working continuously, but he actually overestimated it. A healthy horse can sustain about 0.5 to 0.7 horsepower over a full day. The unit is a historical standard, not a biological measurement.

What is the difference between brake horsepower and 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) can mean the same thing, but sometimes it refers to power at the wheels. The difference is typically 10-20% for cars. This calculator converts the number you enter, regardless of how it was measured.

How many watts is 5 horsepower?

Using the mechanical conversion: 5 hp × 745.7 W/hp = 3,728.5 W, or about 3.73 kW. The calculator will give the exact value: 3,728.50 W.

Why is electrical horsepower 746 watts instead of 745.7?

Electrical horsepower is a separate unit defined as exactly 746 watts, because it was historically used for electric motors and was rounded to a convenient whole number. It is slightly different from mechanical horsepower. Most modern electric motors are rated in watts or kilowatts, so the distinction is less common.

Can I convert watts to horsepower with this calculator?

No, this calculator only converts horsepower to watts. To go the other way, you would divide watts by 745.7. For example, 1000 W ÷ 745.7 = 1.34 hp.

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in hp | Horsepower (mechanical) | hp | 1 | 0
out watts | Watts | W | hp * 745.69987158227022 | 2
out kw | Kilowatts | kW | hp * 0.7456998715822702 | 2