Calories to Kilojoules
An instant calories-to-kilojoules converter, with the formula and a reference table below.
| Calories | Kilojoules |
|---|---|
| 1 cal | 0.004184 kJ |
| 2 cal | 0.008368 kJ |
| 3 cal | 0.012552 kJ |
| 5 cal | 0.02092 kJ |
| 10 cal | 0.04184 kJ |
| 25 cal | 0.1046 kJ |
| 50 cal | 0.2092 kJ |
| 100 cal | 0.4184 kJ |
How to use
- Enter a value in the Calories field.
- The Kilojoules figure appears immediately — no button to press.
- Typing in the Kilojoules field converts back to Calories the same way.
- Use the reference table for the values that come up most often.
Calories to Kilojoules formula
The factor from calories to kilojoules is 0.004184.
kJ = cal × 0.004184
Why this factor: the calorie and the kilojoule are both defined against the joule, so dividing one definition by the other gives 0.004184 directly. Both definitions are set out in full below.
To go the other way, divide by 0.004184 — or use the Kilojoules to Calories converter, which is the same calculation reversed.
Worked example
Take 500 cal and multiply by the factor 0.004184: the result is 2.092 kJ.
500 cal × 0.004184 = 2.092 kJ
The converter above does this for any value you type, in either direction.
What is a calorie?
A calorie is the energy needed to warm one gram of water by one degree Celsius.
Definition: The thermochemical calorie is defined as exactly 4.184 joules.
Where it is used: Chemistry and physics literature, and older engineering references.
Worth knowing: The 'calories' on a nutrition label are actually kilocalories — 1,000 of these. A 500-Calorie meal is 500 kcal, or about 2.1 million joules.
What is a kilojoule?
A kilojoule is 1,000 joules, the metric unit of food energy.
Definition: Exactly 1,000 joules.
Where it is used: Nutrition labels in Europe and Australia, and heating and cooling loads.
Calories vs Kilojoules
The calorie is a specialized unit while the kilojoule is a metric unit. That is why the factor between them is not a round number: the two systems were defined independently and only later tied together by exact agreement.
Frequently asked questions
How many kilojoules are in a calorie?
One calorie equals 0.004184 kilojoules.
What is the formula to convert calories to kilojoules?
Multiply the number of calories by 0.004184. To reverse it, divide by 0.004184 instead.
How do I convert kilojoules back to calories?
Divide by 0.004184, which is the same as multiplying by 239.005736.
How many kilojoules are in 10 calories?
10 cal works out to 0.04184 kJ.
Is there anything to watch out for with calories?
The 'calories' on a nutrition label are actually kilocalories — 1,000 of these. A 500-Calorie meal is 500 kcal, or about 2.1 million joules.
How is the calorie defined?
The thermochemical calorie is defined as exactly 4.184 joules.