Joules to Calories Converter
The small calorie and the food Calorie, a factor of a thousand apart, shown separately rather than merged.
Enter a number.
The largest routine unit error there is. The Calorie on a nutrition label is a kilocalorie, 1,000 of the calories used in physics. The capital C is meant to distinguish them and is applied inconsistently. Getting this wrong isn't a rounding difference, it is a factor of a thousand.
This page converts energy units. It gives no dietary guidance, and a converted figure says nothing about what anyone should eat.
The formulas
1 Cal (food) = 1 kcal = 4,184 J (exact)
1 watt-hour = 3,600 J (exact)
All exact by definition. The joule is the SI unit of energy, defined as one newton-meter of work.
Conversion table
| Joules | Calories (cal) | Food Calories (kcal) | Watt-hours |
|---|---|---|---|
| 1 J | 0.24 | 0.0002 | 0.0003 |
| 10 J | 2.39 | 0.0024 | 0.0028 |
| 100 J | 23.90 | 0.0239 | 0.0278 |
| 1,000 J | 239.01 | 0.2390 | 0.2778 |
| 4,184 J | 1,000.00 | 1.0000 | 1.1622 |
| 10,000 J | 2,390.06 | 2.3901 | 2.7778 |
| 100,000 J | 23,900.57 | 23.9006 | 27.7778 |
| 1,000,000 J | 239,005.74 | 239.0057 | 277.7778 |
Which calorie is which
Nutrition labels in the United States say Calories and mean kilocalories. European labels usually show kilojoules and kilocalories side by side, which removes the ambiguity. Scientific work uses joules and, where calories appear at all, the small one.
The safest habit is to treat any energy figure on food as kilocalories and any figure in a physics or chemistry context as small calories, then check the unit symbol rather than the capitalization, since the capital C convention isn't reliably followed.
Common questions
What is the difference between a calorie and a Calorie?
A capital C usually marks the food Calorie, which is a kilocalorie, or 1,000 thermochemical calories. Nutrition labels use it. Physics and chemistry use the small calorie. Confusing them is a factor of 1,000, the largest routine unit error there is.
How many joules in a calorie?
The thermochemical calorie is exactly 4.184 joules. A food Calorie is exactly 4,184 joules.
Why does my food label say kJ and kcal?
Because many countries require energy to be shown in kilojoules, the SI unit, alongside the more familiar kilocalorie. 100 kcal is 418.4 kJ.
Are there other calories?
Yes, which is part of the problem. Besides the thermochemical calorie there are the 15 degree calorie and the International Table calorie, differing in the fourth significant figure. This tool uses the thermochemical calorie, which is the one defined exactly.
Related converters and guides
Sources
- NIST Guide to the SI, Appendix B: Conversion Factors. National Institute of Standards and Technology. Source for every conversion factor on this page, and for which factors are exact rather than rounded.