What Are Kilojoules and Kilocalories?
Kilojoules (kJ) and kilocalories (kcal) are both units of energy commonly used to measure the energy content of food and drinks. A kilocalorie — often simply called a "calorie" on food labels — equals exactly 4.184 kilojoules. While most countries use kilojoules as the standard unit on nutrition labels, kilocalories remain the dominant unit in the United States, Canada, and several other countries. This converter lets you switch between the two instantly, so you can read any nutrition label no matter which unit it uses.
Understanding the relationship between kJ and kcal is especially useful when traveling, comparing products from different countries, or following a diet plan that uses the other unit. Since both measure the same thing — energy — converting between them is a simple multiplication or division.
Formula
The conversion between kilojoules and kilocalories uses a single constant:
1 kcal = 4.184 kJ
To convert kilojoules to kilocalories:
kcal = kJ ÷ 4.184
To convert kilocalories to kilojoules:
kJ = kcal × 4.184
The factor 4.184 is exact by definition. It comes from the thermochemical definition of the calorie, which is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius at standard atmospheric pressure.
Example
Suppose a food label from Australia lists an item as containing 840 kJ per serving and you want to know the equivalent in kilocalories.
- Divide by 4.184: 840 ÷ 4.184 = 200.8 kcal
So the serving contains roughly 201 kilocalories — about the same as a medium banana and a tablespoon of peanut butter.
Going the other way, if a US label says a snack has 250 kcal:
- Multiply by 4.184: 250 × 4.184 = 1,046 kJ
FAQ
Why do some countries use kJ and others use kcal? The International System of Units (SI) designates the joule as the standard unit of energy, so countries that follow SI — including Australia, New Zealand, and most of the EU — use kilojoules on food labels. The United States and Canada historically adopted the calorie (kilocalorie) and continue to use it. Many labels now show both units side by side.
Is a "calorie" the same as a kilocalorie? In everyday language, yes. When a food label says "200 calories," it means 200 kilocalories (200 kcal). The scientific calorie (lowercase c) is 1,000 times smaller and is rarely used outside chemistry and physics textbooks.
How many kJ should I eat per day? General guidelines suggest about 8,700 kJ (2,080 kcal) per day for an average adult, but individual needs vary widely based on age, sex, weight, height, and activity level. Consult a healthcare professional for personalised advice.
Does the 4.184 conversion factor ever change? No. The factor 4.184 kJ per kcal is a fixed definition, not an approximation. It will not change regardless of the food or context.