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Watts, Kilowatts & Horsepower Converter

Convert power between watts, metric prefixes, mechanical horsepower, BTU per hour and other listed power units.

  • Formula & worked example
  • Private in your browser
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

1,000 watts equals 1 kilowatt or 1,000,000 milliwatts. One mechanical horsepower equals 745.69987158227022 watts.

Output power = input power × input-unit factor ÷ output-unit factor

Worked example

Enter these known values and leave the other values blank.

Watts
1000 W
Milliwatts
1000000 mW
Kilowatts
1 kW
Mechanical horsepower
1.341 hp(l)
Other power units
3412 BTU (IT)/h

Assumptions and limitations

  • All fields represent power, a rate of energy transfer. They do not represent energy consumed over time.
  • Mechanical horsepower, metric horsepower, electrical horsepower and boiler horsepower have different definitions. Select the intended unit.
  • BTU per hour uses the International Table BTU convention. Signed power values may describe a directional power balance.

Common questions

Can I convert watts straight to kilowatt-hours?

No. Kilowatt-hours measure energy. Multiply power in kilowatts by duration in hours when power is constant.

Are all horsepower units interchangeable?

No. Mechanical horsepower is about 745.7 W; metric horsepower is 735.49875 W. The selector keeps these definitions separate.

References

The boiler-horsepower selector uses the NIST tabulated 9809.5 watts per unit instead of the captured 9812.5-watt value. Mechanical horsepower uses exactly 550 foot-pounds-force per second instead of a rounded factor. The power unit uses the International Table BTU per hour consistently with the reviewed energy convention.

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