Calories and kilojoules (often shortened to kcal or kJ) are both ways to measure energy in food. The kilojoule (kJ) is an International system of units (SI) measurement, while Calories are used in the metric system. In the same way that centimetres and feet measure height, kJ and Calories measure energy.
1 Calorie is equal to 4.2 kJ
To convert Calories to kJ multiply 4.2
To convert kJ to Calories divide by 4.2