Calories and kilojoules (often shortened to kcal or kJs) are both units of energy that are used to represent the amount of energy in a food. Calories and kJs both represent energy in the same way that centimetres and feet both measure height. The kilojoule (kJ) is an International system of units (SI) measurement whereas Calorie is a measure of energy using the metric system.
To convert 1kcal is 4.2kJ. Therefore multiply the kcal by 4.2 to get from kcal to kJ or divide by 4.2 to get from kJ to kcal