Energy Converter
Convert joules, kJ, Wh, kWh, calories, kilocalories and BTU — for physics, nutrition labels, batteries and energy bills.
- InstantResults update as you type
- 27 categoriesFrom length to data storage
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How to use this tool
Steps for using the Unit Converter.
- Type a number into From — kilojoules are preselected. Commas and scientific notation such as 2.5e3 are accepted.
- Pick the units to convert between; the result in kilocalories (or whichever unit you choose) updates as you type. Use the swap button to reverse the conversion.
- Set the decimal places, turn on Show formula to see the calculation, and copy or share the result.
- Below the converter, the table lists every energy unit with its exact definition, and the links open dedicated pages for the most common pairs.
Example
500 kilojoules to kilocalories
500 kJ119.502868 kcal
Formula: kcal = kJ ÷ 4.184Common use cases
- Food energy on nutrition labels (kJ and kcal).
- Electricity use and battery capacity in Wh and kWh.
- Heating and cooling loads in BTU.
Energy unit table
| Unit | Symbol | System | Definition | In joules |
|---|---|---|---|---|
| Joule | J | SI | 1 N·m = 1 W·s, the SI unit of energy | 1 |
| Kilojoule | kJ | Metric | 1,000 J | 1,000 |
| Watt-hour | Wh | Metric | 1 W for 1 hour = 3,600 J | 3,600 |
| Kilowatt-hour | kWh | Metric | 1,000 Wh = 3.6 MJ | 3,600,000 |
| Calorie | cal | Other | Thermochemical calorie, exactly 4.184 J | 4.184 |
| Kilocalorie | kcal | Other | 1,000 cal = 4,184 J (the food “Calorie”) | 4,184 |
| British thermal unit | BTU | US customary | International Table BTU ≈ 1,055.056 J | 1,055.05585262 |
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About energy units
The joule is the SI unit of energy. A watt-hour is one watt for an hour, exactly 3,600 J, and a kilowatt-hour — the unit on electricity bills — is 3.6 MJ.
The calorie here is the thermochemical calorie, exactly 4.184 J. The “Calories” on food labels are kilocalories (1 kcal = 4,184 J), which is why food energy is often given as both kcal and kJ. The BTU used for heating and air conditioning is the International Table BTU, about 1,055 J.
Limitations
- Results are rounded for display only; the calculation keeps full precision (15 significant digits).
Frequently asked questions
Is a food Calorie the same as a calorie?
No. A food Calorie (capital C) is a kilocalorie, 1,000 small calories or 4.184 kJ.
How many joules are in a kWh?
3,600,000 J (3.6 MJ).
Which calorie does this use?
The thermochemical calorie, exactly 4.184 J. The International Table calorie (4.1868 J) differs by about 0.07%.
Are these energy conversions exact?
Each unit is defined from its official definition (for example 1 inch = 2.54 cm exactly). Results are calculated in double precision and only rounded to the decimal places you choose when shown.