Watt Converter
Enter a power value and choose the input and output units to convert instantly between watts, kilowatts, megawatts, gigawatts, mechanical and metric horsepower, BTU per hour, BTU per minute, BTU per second, calories per hour and per second, kilocalories per hour, foot-pounds per second, and decibel-milliwatts. All conversions update as you type, with the formula shown step by step.
What is the watt?
The watt (symbol W) is the International System of Units (SI) derived unit of power, defined as one joule of energy transferred per second. It is named after Scottish engineer James Watt in recognition of his work on the steam engine. Because electricity, heat, and mechanical force all involve energy transfer, the watt serves as the universal bridge between electrical, thermal, and mechanical power measurement. One watt equals one volt times one ampere in an electrical circuit, 1 newton-metre per second in mechanics, or 1 joule per second in thermodynamics.
How to convert watts to other units
Every conversion passes through the watt as the SI base. To convert from any unit to any other unit, first multiply by the factor that turns your source unit into watts, then divide by the factor that turns watts into your target unit. For example, to convert 2 horsepower (mechanical) to kilowatts: 2 hp × 745.7 W/hp = 1,491.4 W, then 1,491.4 W ÷ 1,000 = 1.491 kW. Thermal units like BTU/h work the same way: 1 W = 3.41214 BTU/h, so 1,000 W = 3,412 BTU/h. The dBm unit is different because it uses a logarithmic scale: dBm = 10 × log10(watts × 1000), so 1 W = 30 dBm and 0.001 W (1 mW) = 0 dBm.
Horsepower: mechanical, electrical, and metric
Three different horsepower definitions exist and they give slightly different watt equivalents. Mechanical (or imperial) horsepower is the most common in North America: exactly 550 foot-pounds per second, which works out to 745.69987 W. Electrical horsepower, used for rating electric motors in North America, is defined as exactly 746 W. Metric horsepower (PS, from the German Pferdestärke) is used in Europe for vehicle power ratings: it equals 75 kilopond-metres per second, which is 735.49875 W. When comparing engine or motor specifications across regions, always check which definition is being used, because the difference is small but meaningful for precise engineering work.
Thermal power: BTU, calories, and tons of refrigeration
Heating and cooling professionals use thermal power units alongside or instead of watts. The BTU (British Thermal Unit) is the energy needed to raise one pound of water by one degree Fahrenheit; divided by one hour (BTU/h) it becomes a rate, or power. One watt equals 3.41214 BTU/h, so a 12,000 BTU/h air conditioner draws about 3,517 W of cooling capacity, which is one ton of refrigeration. Calories and kilocalories work similarly: 1 W = 0.8598 kcal/h. Nutritionists and fitness devices use kilocalories per hour or kilocalories per minute, while physicists prefer joules per second (watts). Knowing the watt equivalent lets you compare thermal ratings from different systems directly.
Common power unit conversion factors
| Unit | Symbol | Watts per unit | Category |
|---|---|---|---|
| Milliwatt | mW | 0.001 | SI prefix |
| Watt | W | 1 (base unit) | SI base |
| Kilowatt | kW | 1,000 | SI prefix |
| Megawatt | MW | 1,000,000 | SI prefix |
| Gigawatt | GW | 1,000,000,000 | SI prefix |
| Horsepower (mechanical) | hp | 745.7 | Mechanical |
| Horsepower (electrical) | hp(E) | 746 | Electrical |
| Horsepower (metric) | PS | 735.5 | Mechanical |
| BTU per hour | BTU/h | 0.2931 | Thermal |
| BTU per minute | BTU/min | 17.584 | Thermal |
| BTU per second | BTU/s | 1,055.06 | Thermal |
| Calorie per hour | cal/h | 1.163 | Thermal |
| Calorie per second | cal/s | 4,186.8 | Thermal |
| Kilocalorie per hour | kcal/h | 1,163 | Thermal |
| Foot-pound per second | ft-lb/s | 1.3558 | Mechanical |
| Decibel-milliwatt | dBm | 10^((dBm-30)/10) | Logarithmic |
Each row shows how many of that unit equal 1 watt. Multiply your watt value by the factor to convert.
Frequently asked questions
How many watts are in a kilowatt?
One kilowatt (kW) equals exactly 1,000 watts. To convert kilowatts to watts, multiply by 1,000. To convert watts to kilowatts, divide by 1,000. For example, a 3.5 kW electric kettle draws 3,500 W.
How do I convert watts to horsepower?
Divide the watt value by 745.69987 to get mechanical horsepower (the most common type in North America), or by 746 for electrical horsepower, or by 735.49875 for metric horsepower (PS). For example, a 150,000 W (150 kW) car engine produces about 150,000 / 745.7 = 201 mechanical horsepower.
How do I convert watts to BTU per hour?
Multiply the watt value by 3.41214 to get BTU per hour. For example, a 1,000 W space heater produces 1,000 × 3.41214 = 3,412 BTU/h of heat output. To go the other direction (BTU/h to watts), divide by 3.41214.
What is a decibel-milliwatt (dBm) and how is it related to watts?
The dBm is a logarithmic power unit referenced to 1 milliwatt. The formula is: dBm = 10 × log10(power in milliwatts). So 1 mW = 0 dBm, 1 W = 30 dBm, and 1 kW = 60 dBm. Because dBm uses a logarithmic scale, every increase of 10 dBm means the power is 10 times higher. This unit is common in telecommunications and radio engineering where power levels span many orders of magnitude.
How many watts does 1 ton of air conditioning equal?
One ton of refrigeration equals exactly 12,000 BTU/h, which is 12,000 / 3.41214 = 3,516.85 W, or roughly 3.517 kW. This is the cooling power needed to melt one short ton (2,000 lb) of ice in 24 hours. A typical residential 2-ton AC unit therefore has a cooling capacity of about 7,034 W.
Why are there three different definitions of horsepower?
Each definition arose in a different engineering tradition. Mechanical (imperial) horsepower was defined by James Watt himself as 550 foot-pounds per second and is used in the United States for engines and pumps. Electrical horsepower (746 W exactly) was standardized for electric motors to make nameplate ratings consistent. Metric horsepower (PS) was adopted in Germany and much of Europe; it equals 75 kgf-m/s or 735.499 W. The differences are small (under 1.5 percent), but they matter when comparing manufacturers specifications across continents.
How do I convert kilowatts to BTU per hour for HVAC sizing?
Multiply kilowatts by 3,412.14 to get BTU/h (because 1 kW = 1,000 W and 1 W = 3.41214 BTU/h). For example, a 5 kW heat pump delivers 5 × 3,412 = 17,060 BTU/h, or about 1.42 tons of heating. Going the other direction, divide BTU/h by 3,412.14 to get kilowatts.