Watts to Horsepower Converter: Formula & Examples

This calculator converts power in watts (W) to mechanical horsepower (hp) using the standard conversion factor 1 hp = 745.699872 W. Enter the power in watts, and you get the equivalent horsepower, plus the reverse conversion (hp to W) for convenience. The first result is the horsepower value you asked for.

Formula

The conversion is a simple division: hp = W / 745.699872

Where:

• hp = mechanical horsepower (the unit used for engines, motors, and most machinery in the US and UK)
• W = power in watts (the SI unit of power)
• 745.699872 = the exact number of watts in one mechanical horsepower

This constant comes from the definition of mechanical horsepower: 550 foot-pounds per second. Converted to SI units, 550 ft·lbf/s equals 745.69987158227022 W, usually rounded to 745.7 W.

Worked Example

Using the default value of 745.7 W (which is almost exactly 1 hp):

1. Start with the formula: hp = W / 745.699872
2. Plug in the value: hp = 745.7 / 745.699872
3. Calculate: hp ≈ 1.00000017
4. Rounded to 4 decimal places: 1.0000 hp

So 745.7 W is essentially 1 horsepower, as expected.

Why Are There Different Horsepower Values?

Horsepower is not a single universal unit. The most common is mechanical horsepower (the one used here), but you may also see:

• Metric horsepower (PS) – defined as 735.49875 W, used in Europe and Asia. To convert watts to PS, divide by 735.49875.
• Electrical horsepower – exactly 746 W, used for electric motors in some contexts.
• Boiler horsepower – about 9,809.5 W, used for steam boilers.

For most automotive and machinery applications, mechanical horsepower is the standard in the US and UK. If you are comparing specs from different countries, check which horsepower is being used – the difference between mechanical and metric is about 1.4%, which can matter for precise engineering.

Common Mistakes and Misconceptions

A frequent error is using 746 W instead of 745.7 W. While the difference is tiny (0.04%), it can accumulate in large power values. Another misconception is that watts and horsepower measure the same thing in the same way – both measure power (energy per time), but the conversion factor is fixed and does not depend on the type of machine.

Also, be careful with kilowatts (kW). Many electric motors are rated in kW, not W. To convert kW to hp, multiply by 1000 first, then divide by 745.7. For example, a 7.5 kW motor is 7500 W, which is about 10.06 hp.

When to Use This Converter

Use this calculator when you need to compare power ratings across different units. Common situations include:

• Converting an electric motor’s watt rating to horsepower for comparison with a gasoline engine.
• Understanding the power output of a generator or appliance in terms you are familiar with.
• Checking specifications of imported equipment that uses metric horsepower.

Remember that this converter gives the mechanical horsepower. If you are working with metric horsepower, use the factor 735.5 W per PS instead.

Limits of the Conversion

The conversion is purely mathematical and does not account for real-world efficiency. A motor rated at 1 hp does not deliver 745.7 W of useful work – it consumes that much electrical power, but some is lost as heat and friction. The actual output (shaft power) is lower. This is why you should not use this converter to estimate operating costs or performance; it only converts units, not actual power delivery.

Also, the conversion assumes you are using the same definition of horsepower throughout. Mixing mechanical and metric horsepower without adjusting will give incorrect results.

Frequently Asked Questions

How many watts are in one horsepower?

Exactly 745.699872 watts in one mechanical horsepower. For most practical purposes, you can round to 745.7 W or even 746 W, but for precise engineering use the exact value.

What is the difference between hp and PS?

PS (Pferdestärke) is the metric horsepower unit, defined as 735.49875 W. It is about 1.4% smaller than mechanical horsepower. A 100 hp engine is about 101.4 PS, and vice versa.

Can I convert kilowatts to horsepower with this calculator?

Yes, but you need to enter the value in watts. Since 1 kW = 1000 W, multiply the kilowatt value by 1000 before entering it. For example, 5 kW = 5000 W, which is about 6.7 hp.

Why do electric motors have different horsepower ratings than their wattage suggests?

Electric motor ratings often list input power (watts consumed) and output power (shaft power). The output is lower due to losses. Some motors are rated in “electrical horsepower� (746 W), which is different from mechanical horsepower. Always check which rating you are converting.

Is horsepower still a useful unit?

Yes, especially in the automotive and machinery industries where it is deeply ingrained. However, the SI unit watt (or kilowatt) is increasingly used worldwide, especially in electrical engineering and electric vehicles. The conversion remains important for comparing traditional and modern equipment.

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in watts | Power (W) | W | 745.7 | 0
out hp | Horsepower (mechanical) | hp | watts / 745.699872 | 4
out watts_back | Reverse: hp to W | W | hp * 745.699872 | 2