The exact equivalent
A kilocalorie is exactly 4.184 kilojoules. The food Calorie with a capital C is a kilocalorie, which is why the numbers on a label are not what a physicist means by a calorie.
1 kcal = 4.184 kJ
1 kJ = 0.23900574 kcal
How to convert it by hand
Just multiply by the conversion factor. To go from kilocalories to kilojoules, multiply by 4.184; to come back, divide by that same number or multiply by 0.239006.
For example: 25 kcal × 4.184 = 104.6 kJ.
A tip that saves mistakes: check the result goes the way you expect. If kilojoules are a smaller unit than kilocalories, the number has to come out bigger. If it comes out smaller, you divided when you should have multiplied.
Conversion table
| kcal | kJ |
|---|---|
| 1 | 4.184 |
| 2 | 8.368 |
| 5 | 20.92 |
| 10 | 41.84 |
| 20 | 83.68 |
| 50 | 209.2 |
| 100 | 418.4 |
Reference points to picture it
| Reference | Equals |
|---|---|
| A daily intake | 2,000 kcal = 8,368 kJ |
| A chocolate bar | 250 kcal = 1,046 kJ |
| An apple | 80 kcal = 335 kJ |
| 1 kcal | 4.184 kJ |
The trick for doing it in your head
Multiply by 4 and add 5%. For 500 kcal: 2,000 plus 92 gives 2,092 kJ, which is exact.
It is fine for getting by without a phone, but not for anything that depends on precision: in a recipe, on a building site or in a dose, use the exact conversion.