The difference between calories and kilojoules is that one calorie equals 4,2 kilojoules and vice versa. So, if you read on a food label that a portion of the food contains 100 calories, you can multiply that value with 4,2 to work out how many kilojoules the food contains:
Example: 100 calories x 4,2 = 420 kJ
Conversely, you can calculate that a portion of food that contains 420 kJ (metric system) will contain 420 divided by 4,2 = 100 calories The same applies when you want to work out how many calories or kilojoules a diet contains. Multiply calories by 4,2 to obtain kilojoules and divide kilojoules by 4,2 to obtain calories.
Although we use larger numbers when working with metric kilojoules this does NOT mean that kilojoules contain more energy, just that we are using a different system to express energy content.