Convert Kilojoule to Calorie

One thermochemical calorie equals exactly 4.184 joules, so one kilojoule equals 1000 ÷ 4.184 = 239.006 calories. Dividing a kilojoule value by 4.184 and multiplying by 1,000 (or simply multiplying by 239.006) gives small calories. A 500 kJ serving of food contains 119,503 small calories.

Here is the unavoidable confusion: the 'Calorie' printed on food labels in the United States, Canada and most English-speaking countries is actually a kilocalorie (kcal) — 1,000 small calories, or 4,184 joules. The food industry capitalised the C to signal the difference, but packaging rarely spells out kcal. A chocolate bar labelled '250 Calories' contains 250 kcal = 1,046 kJ. European nutrition labels instead show kilojoules alongside kilocalories, so the same bar might read '1046 kJ / 250 kcal.'

The practical implication: if you divide a European kJ figure by 4.184, you get small calories, not food Calories. To compare a kJ reading with the Calorie count on a US label, divide by 4.184 and then divide again by 1,000 — or just divide the kJ figure by 4,184. A 2,000 kcal daily target is 8,368 kJ; an 8,000 kJ target is 1,912 kcal.

Exercise trackers and sports nutrition complicate things further. A running app might report energy burned in kJ in one language setting and in kcal in another. Switching languages in the app settings can make a 600 kJ ride look like it became a 600 kcal ride — an apparent fourfold difference in effort that is purely a display artefact.