Calories (or kilo-calories as they should be correctly called) are a non-metric unit of energy, while kilojoules (kJ) are units of energy used in the Metric System.
1 cal = 4.184 kJ (or approx. 4.2 kJ)
To calculate the kJ content of a diet or a food, you need to multiply the calories by 4.184.
For example: a 1 300 cal diet = 1 300 x 4.184 = 5 439 kJ.
To calculate the calorie content of a diet or food, you need to divide the kilojoules by 4.184.
For example: a 5 439 kJ diet = 5 439 ÷ 4.184 = 1 300 calories.
- (Birgit Ottermann, Health24, December 2010)