Kcal vs cal: what's the difference?
A calorie (cal), sometimes called a small calorie, is a tiny unit of energy: about the energy needed to warm 1 gram of water by 1°C. A kilocalorie (kcal) is 1,000 of those. The energy in food is measured in kilocalories, but in everyday life, and on US nutrition labels, the "kilo" is dropped and a kilocalorie is simply called a Calorie (sometimes with a capital C).
So the 250 "calories" in a snack are really 250 kilocalories, or 250,000 small calories. When you see kcal on a label or in an app, it means exactly the same thing as Calories on a US label.
cal = kcal × 1,000 · kcal = cal ÷ 1,000
1 kcal = 4.184 kJ · kJ = kcal × 4.184 · kcal = kJ ÷ 4.184
Kilojoules
The joule is the SI unit of energy. The UN's Food and Agriculture Organization notes that kilojoules and kilocalories are used side by side in most food regulations, so you'll often see kJ on labels outside the US. Nutrition uses the thermochemical calorie, defined as exactly 4.184 joules, so 1 kcal = 4.184 kJ. There is also an "International Table" calorie of 4.1868 J, used mainly in engineering. The difference is under 0.1%.
Worked example
A granola bar with 250 kcal: 250 × 1,000 = 250,000 cal (small calories), or 250 × 4.184 = 1,046 kJ. Going the other way, a European label showing 2,000 kJ is 2,000 ÷ 4.184 = 478 kcal, which a US label would call 478 Calories.
Kcal to cal and kJ reference table
| kcal | cal (small calories) | Food Calories | kJ |
|---|---|---|---|
| 1 | 1,000 | 1 | 4.184 |
| 10 | 10,000 | 10 | 41.84 |
| 50 | 50,000 | 50 | 209.2 |
| 100 | 100,000 | 100 | 418.4 |
| 200 | 200,000 | 200 | 836.8 |
| 250 | 250,000 | 250 | 1,046 |
| 500 | 500,000 | 500 | 2,092 |
| 1,000 | 1,000,000 | 1,000 | 4,184 |
| 1,500 | 1,500,000 | 1,500 | 6,276 |
| 2,000 | 2,000,000 | 2,000 | 8,368 |
| 2,500 | 2,500,000 | 2,500 | 10,460 |
When this doesn't apply
- Some older physics and chemistry books use the 4.1868 J calorie. If a problem says "IT calorie", multiply by 4.1868 instead.
- Label values are rounded, so converting a label's kJ to kcal may not match the kcal printed next to it exactly.
Want to see where the calories in a food come from? The macronutrient calories calculator turns grams of protein, carbs and fat into calories.
Sources
- NIST Special Publication 811, Appendix B.8: conversion factors — thermochemical calorie = 4.184 J exactly; IT calorie = 4.1868 J.
- NIST Guide to the SI, footnotes — the "large calorie" is the kilocalorie used for the energy in foods.
- FAO: Food energy, methods of analysis and conversion factors (2003), chapter 3 — 1 kcal = 4.184 kJ, and the Atwater factors 4, 4, 9 and 7.