Why food energy has four units
The “Calorie” on a US nutrition label is really a kilocalorie — 1,000 of the small calories used in chemistry. Australia, New Zealand and much of the EU label in kilojoules instead, the SI unit, which is why an Australian label reading 8,700 kJ describes the same energy as a US label reading roughly 2,080 kcal. The joule is the base SI unit underneath it all.
The conversion factors
1 kcal = 4.184 kJ = 1,000 cal = 4,184 J. (Some sources use 4.1868, the international-table calorie; nutrition labelling uses the thermochemical 4.184, and the difference is under 0.1% — far below the tolerance of any food label.) Handy anchors: a 2,000 kcal day is ~8,370 kJ; a 100 kcal snack is ~418 kJ.
Where you'll need this
Comparing imported food labels, reading Australian or European nutrition research, or converting a TDEE result for an app that wants kilojoules. Every calculator on this site has a kcal/kJ toggle in the corner of the form, so you can also just flip the whole result.
Frequently asked questions
Is a food Calorie the same as a calorie?
No — and the capital letter is the giveaway. A food Calorie (kcal) is 1,000 chemistry calories. When a label says 250 Calories, that's 250 kcal, or 250,000 small calories. In practice nutrition always means kcal, whatever the capitalisation.
How do I convert calories to kilojoules in my head?
Multiply by 4.2 (or just 4 for a rough check). 500 kcal ≈ 2,100 kJ. Going the other way, divide kJ by 4.2 — an 8,700 kJ label is about 2,070 kcal.
Why do Australian labels use kilojoules?
Australia and New Zealand adopted SI units for food labelling, and the kilojoule is the SI energy unit. The energy is identical — only the unit changes.
Which factor is correct, 4.184 or 4.1868?
Both exist: 4.184 is the thermochemical calorie used in nutrition; 4.1868 is the international-table calorie used in some engineering contexts. For food, use 4.184 — and the 0.07% difference is far smaller than the legal tolerance on any nutrition label.
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Sources
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025. [link]