Converting Horsepower to Watts: hp to W Formula

This calculator converts horsepower to watts. Enter a value in horsepower (mechanical, metric, or electrical) and you get the equivalent power in watts instantly. The conversion is a simple multiplication by a fixed factor, but the factor depends on which horsepower definition you use.

Formula

The core formula is:

Watts = Horsepower × 745.6998715822702

Where:

• Horsepower (hp) – the power input, in mechanical horsepower (the most common type in the US).
• Watts (W) – the power output, the SI unit of power.

This factor (745.7) is exact by definition: 1 mechanical horsepower = 745.6998715822702 watts. It comes from the original definition: 1 hp = 550 foot-pounds per second. One foot-pound per second equals 1.3558179483314004 watts, so 550 × 1.3558179483314004 = 745.6998715822702 W.

Worked Example

Using the default value of 1 hp:

1. Start with the value: 1 hp.
2. Multiply by the conversion factor: 1 × 745.6998715822702 = 745.6998715822702 W.
3. Round to two decimal places: 745.70 W.

So 1 mechanical horsepower equals 745.70 watts. This matches the calculator output.

Why Are There Different Horsepower Definitions?

Horsepower was coined by James Watt in the 18th century to compare steam engines with draft horses. He defined it as the power needed to lift 550 pounds one foot in one second. That became the mechanical (or imperial) horsepower. However, other regions and industries developed their own variants:

• Mechanical horsepower (hp(I)) – 745.699872 W, used for cars, engines, and machinery in the US and UK.
• Metric horsepower (hp(M)) – 735.49875 W, used in Europe and Asia for car engines. It is defined as 75 kgf·m/s (the force needed to lift 75 kilograms 1 meter in 1 second).
• Electrical horsepower (hp(E)) – exactly 746 W, used for electric motors and generators.
• Boiler horsepower – 9,809.5 W, used for steam boilers, based on the heat output of a boiler.

When someone says “horsepower� without qualification, they almost always mean mechanical horsepower. But if you are converting a European car’s power rating, it is metric horsepower. Always check the context.

How to Convert Horsepower to Watts Manually

You can do the conversion in two steps:

1. Identify which horsepower type you have (mechanical, metric, electrical).
2. Multiply the horsepower value by the corresponding factor: 745.7 for mechanical, 735.5 for metric, 746 for electrical.

For example, a 150 hp car engine (mechanical) produces 150 × 745.7 = 111,855 W (about 111.9 kW). A 150 hp metric engine gives 150 × 735.5 = 110,325 W (about 110.3 kW). The difference is about 1.4%, which is small but not negligible in engineering calculations.

Common Mistakes and Misconceptions

Three errors appear frequently:

• Using the wrong factor – mixing up mechanical and metric horsepower. Always verify which definition the source uses.
• Confusing watts with watt-hours – watts measure power (rate of energy use), watt-hours measure energy (total amount). A 100 W light bulb uses 100 watts of power; over 1 hour it consumes 100 watt-hours of energy.
• Rounding too early – if you round 745.7 to 746, the error is small for single values but compounds in large calculations. Keep full precision until the final step.

Another misconception: “horsepower� is not a measure of speed or torque. It is a measure of the rate at which work is done. A car’s top speed depends on many factors, not just horsepower.

Real-World Context: What Does a Watt Look Like?

To make the numbers tangible:

• 1 hp ≈ 745.7 W – about the power of a small microwave oven (most are 600–1200 W).
• 10 hp ≈ 7,457 W – roughly the power of a small room air conditioner (which draws about 1,000–1,500 W).
• 100 hp ≈ 74,570 W – enough to power about 750 typical LED light bulbs (10 W each).
• A typical household in the US uses about 1,200 W on average, so 1 hp is about 60% of a home’s average draw.

When converting to kilowatts (kW), divide watts by 1,000. So 1 hp = 0.7457 kW. This is the unit used on European car specs and in electrical engineering.

Why Convert Horsepower to Watts?

You might need to convert for several reasons:

• Comparing engines – European car specs list power in kW, while US specs use hp. Converting lets you compare directly.
• Electrical system sizing – if you are connecting a motor to a generator or inverter, you need the power in watts to size the equipment.
• Energy cost calculations – electricity is billed in kilowatt-hours. To estimate running cost, you need power in kW and time in hours.
• Engineering standards – most technical documents use SI units (watts) rather than imperial (hp).

For example, if you have a 2 hp water pump running for 3 hours, the energy consumption is 2 × 0.7457 kW = 1.4914 kW, multiplied by 3 hours = 4.474 kWh. That tells you how much electricity it uses.

Limits of the Conversion

The conversion is exact for mechanical horsepower, but real-world measurements are never perfect. Engine output varies with temperature, altitude, fuel quality, and wear. The rated horsepower is usually measured under specific conditions (often SAE J1349 in the US). Also, the conversion only works for power – it does not tell you torque, efficiency, or fuel consumption. Those require additional data.

Frequently Asked Questions

How many watts are in 1 horsepower?

Exactly 745.6998715822702 watts for mechanical horsepower. For metric horsepower it is 735.49875 W, and for electrical horsepower it is exactly 746 W. The most common answer, 745.7 W, refers to mechanical horsepower.

Why is a horsepower 746 watts and not 1000?

Because it was defined before the metric system was standardized. James Watt based it on the work a draft horse could do, which he estimated as 550 foot-pounds per second. When the watt was later defined as 1 joule per second, the conversion factor turned out to be 745.7, not a round number.

What is the difference between hp and kW?

Horsepower is an imperial unit, kilowatt is the SI unit. 1 kW = 1000 W. To convert hp to kW, multiply by 0.7457 (for mechanical hp). To convert kW to hp, divide by 0.7457. For example, 100 kW ≈ 134 hp.

Does the conversion change for electric motors?

Yes. Electric motors are often rated in electrical horsepower, which is exactly 746 W. However, the motor’s output power (mechanical) is slightly less than the electrical input due to losses. The nameplate rating usually indicates the mechanical output, so use the mechanical factor (745.7) for that.

Can I convert horsepower to watts for a car engine?

Yes, but be careful: car engines are rated in mechanical horsepower (US) or metric horsepower (Europe). Check the owner’s manual or the manufacturer’s spec. Use 745.7 for US cars, 735.5 for European cars. The difference is about 1.4%.

Is 1 hp equal to 1 watt?

No. 1 hp is about 746 times larger than 1 watt. A watt is a small unit – a typical incandescent light bulb uses 60 W. One horsepower is roughly the power of a small microwave oven.

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in hp | Horsepower (mechanical) | hp | 1 | 0
out watts | Watts | W | hp * 745.6998715822702 | 2