Convert Calories to Kilojoules

About converting Calories to Kilojoules

Calories (cal) and Kilojoules (kJ) are both units of energy, so converting between them is a single multiplication: multiply your Calories figure by 0.004184. Ten Calories comes to 0.04184 Kilojoules, and a hundred works out at 0.4184 Kilojoules.

Rule of thumb: going from Calories to Kilojoules divides, so the figure always shrinks — it takes about 239.0057 Calories to make a single Kilojoule. If your answer came out bigger, you have the conversion the wrong way round.

What is a Calory?

The calorie is the unit most likely to be misread, because there are two of them. The small calorie — the one this converter uses — is approximately the heat needed to warm a gram of water by one degree Celsius, and it is a tiny amount of energy. The "calories" printed on food labels are not these: a food Calorie, traditionally written with a capital C, is a thousand small calories, properly a kilocalorie. So a 2,000-calorie daily diet is really about 8.4 million small calories. Several slightly different calories were defined historically; the thermochemical calorie used here is fixed at exactly 4.184 joules.

What is a Kilojoule?

A kilojoule is a thousand joules, the size at which the joule becomes practical for everyday quantities. In Australia and New Zealand it is the primary unit on nutrition labels — food energy is quoted in kilojoules first, with calories relegated to a secondary figure or dropped entirely — so shoppers there think about daily intake in thousands of kilojoules rather than thousands of calories. It is also the working unit of chemistry, where reaction and bond energies are tabulated per mole. European labels print both kJ and kcal, which is why the same packet carries two different numbers for the same food.

To put both in perspective: an apple holds roughly 95,000 small calories, while a slice of bread carries roughly 300 kilojoules.

The converter above updates as you type, and the table lists common values for quick reference. Need the opposite direction? Use the Kilojoules to Calories converter, or browse all Energy converters.