HP to Watts Calculator: Convert Horsepower Easily

This calculator converts horsepower (hp) to watts (W) using the standard mechanical horsepower definition, where 1 hp equals exactly 745.699872 W. Enter the horsepower value (decimal numbers are fine) and you immediately get the equivalent power in watts, plus the result in kilowatts for convenience.

Formula

The conversion is a simple multiplication:

Watts = Horsepower × 745.699872

Where:

• Horsepower (hp) – the power input value you enter, in mechanical horsepower.
• Watts (W) – the resulting power output, the SI unit of power.
• 745.699872 – the exact conversion factor for mechanical horsepower to watts.

For kilowatts, divide the watts by 1000, or multiply horsepower by 0.745699872 directly.

Worked Example

Using the default value of 10 horsepower:

1. Start with the formula: Watts = 10 × 745.699872
2. Multiply: 10 × 745.699872 = 7456.99872 W
3. Round to two decimals: 7457.00 W
4. Convert to kilowatts: 7456.99872 ÷ 1000 = 7.457 kW (rounded to three decimals: 7.457 kW)

The calculator shows 7457.00 W and 7.457 kW for 10 hp, matching the example.

Why Are There Different Horsepower Values?

Horsepower has several definitions depending on the context. This calculator uses mechanical horsepower (also called imperial horsepower), which is the most common in the United States and the UK for engines and motors. However, you may encounter:

• Metric horsepower (PS) – used in Europe, equals 735.49875 W.
• Electrical horsepower – used for electric motors, equals exactly 746 W.
• Boiler horsepower – used for steam boilers, equals 9,809.5 W.

If you are converting a value from a European car specification, it is likely metric horsepower. Multiply by 735.49875 instead. For electric motors, use 746. The differences are small (about 1.4% between mechanical and metric), but they matter in engineering calculations.

How to Use the HP to Watts Calculator

Using the calculator is straightforward:

1. Enter the horsepower value in the input field. You can use decimals, such as 5.5 or 0.25.
2. The calculator immediately shows the power in watts and kilowatts.
3. For metric horsepower, multiply the result by 0.98632 (since 735.49875/745.699872 ≈ 0.98632).

For example, a 150 hp car engine produces 150 × 745.699872 = 111,854.98 W, or about 111.85 kW. That is the mechanical power at the crankshaft, not the power that reaches the wheels (which is lower due to drivetrain losses).

Common Mistakes and Misconceptions

One frequent error is using the wrong horsepower definition. If you are converting a European car’s power rating, using mechanical horsepower will overestimate the watts by about 1.4%. Another mistake is confusing watts with watt-hours. Watts measure power (rate of energy use), while watt-hours measure energy. A 1000 W appliance running for 1 hour uses 1000 Wh (1 kWh), but the power is still 1000 W.

Rounding too early can also cause errors. For precise engineering work, keep at least four significant figures during intermediate steps. The conversion factor 745.699872 is exact for mechanical horsepower, so use it fully.

Practical Applications

Converting hp to watts is useful in many real-world situations:

• Comparing engines and motors – electric motors are rated in kW, while gasoline engines are often in hp. Converting allows a direct comparison.
• Sizing generators – a generator’s output is in watts or kW, so you need to know the total wattage of the equipment you plan to run.
• Calculating electricity costs – if you know an appliance’s power in watts, you can estimate its energy consumption and cost.
• Engineering design – pumps, compressors, and conveyors often have power ratings in hp, but electrical systems are designed in watts.

For example, a 5 hp water pump consumes about 5 × 745.7 = 3728.5 W. If it runs for 3 hours, it uses 3728.5 × 3 = 11,185.5 Wh, or about 11.19 kWh.

Limits of the Conversion

This conversion only changes the unit of power; it does not account for efficiency. An electric motor rated at 10 hp consumes more than 7457 W from the wall because of losses in the motor and drive. The input power (electrical) is higher than the output power (mechanical). Similarly, a car engine’s rated horsepower is the brake horsepower measured at the flywheel, not the power at the wheels. The conversion gives you the equivalent power in watts, not the actual energy consumed or delivered.

Also, horsepower is a unit of power, not force or torque. Do not confuse it with foot-pounds (torque) or miles per hour (speed). Power is the rate at which work is done.

Frequently Asked Questions

Is 1 hp exactly 746 watts?

Not always. 746 W is the electrical horsepower, used for electric motors. Mechanical horsepower is 745.699872 W, and metric horsepower is 735.49875 W. The difference is small, but for precise calculations, use the correct definition for your application.

How many watts is 1 hp?

One mechanical horsepower equals 745.699872 watts. For quick mental math, you can use 746 W, which is within 0.04% of the exact value.

How do I convert hp to kW?

Multiply the horsepower by 0.745699872. For example, 10 hp × 0.745699872 = 7.45699872 kW, which rounds to 7.46 kW.

Why do car engines use horsepower instead of watts?

Horsepower is a historical unit that became standard in the automotive industry. Watts are the SI unit and are used in electrical and scientific contexts. The conversion is simple, so both units coexist.

Does this calculator work for metric horsepower?

No, it uses mechanical horsepower. For metric horsepower (PS), multiply the result by 0.98632, or use the factor 735.49875 W per PS instead.

What is the difference between horsepower and brake horsepower?

Brake horsepower (bhp) is the power measured at the engine’s output shaft, before losses from the gearbox and drivetrain. Horsepower can refer to any of the definitions, but in automotive contexts, it often means the same as bhp. The conversion to watts is the same regardless of the measurement point.

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in hp | Horsepower | hp | 10 | 0
out watts | Power | W | hp * 745.699872 | 2
out kw | Power | kW | hp * 0.745699872 | 3