How to use
- Enter a value, then pick the unit you are converting from and the unit you want to.
- The result appears instantly, and the table below shows the same amount of energy in every unit.
- When you pick calories or BTU, a short note explains which definition is used.
Conversion factors
The SI unit of energy is the joule. One joule is one watt for one second, so conversions between watt-hours and joules are exact.
| Unit | In joules | Notes |
|---|---|---|
| 1 kJ | 1,000 J | |
| 1 calorie (cal) | 4.184 J | thermochemical calorie |
| 1 kcal (food Calorie) | 4,184 J = 4.184 kJ | the “Calories” on a food label |
| 1 Wh | 3,600 J | 1 W for 3,600 seconds |
| 1 kWh | 3,600,000 J = 3.6 MJ | the unit on an electricity bill |
| 1 BTU | 1,055.05585262 J | International Table BTU |
| 1 eV | 1.602176634 × 10⁻¹⁹ J | exact since the 2019 SI redefinition |
Examples
- A 2,000 kcal daily diet is 8,368 kJ — about 2.32 kWh.
- A label showing 100 kJ means about 23.9 kcal.
- 1 kWh of electricity equals about 3,412 BTU or 860 kcal.
- A therm of natural gas is 100,000 BTU, about 29.3 kWh.
Things to keep in mind
- There are two calories. Food labeling uses the thermochemical calorie (4.184 J); some engineering tables use the International Table calorie (4.1868 J). The difference is about 0.07%.
- A capital-C Calorie on a food label is one kilocalorie.
- A watt is a rate of energy use (power), not an amount of energy. To convert kW, horsepower or BTU/h, use the power converter. For other unit types, try the unit converter.
FAQ
How many kJ are in a kcal?
4.184 kJ. Divide kilojoules by 4.184 to get kcal — handy for food labels from Australia or the EU that lead with kJ.
How many BTU are in a kWh?
About 3,412.14 BTU.
Why do electronvolt results look so small?
An electronvolt is the energy of a single electron moved through one volt, so everyday amounts are enormous in eV and tiny the other way around. Very large or small results switch to scientific notation.