A calorie can easily be calculated if you know the exact nutrition information of the food item you are referring to. You can easily rote the exact value of calorie each nutrient has in grams and calculate it to the total calorie. Before this it is important to know the relation between Kilo Joule (KJ, which is unit of energy) and calories.
Just remember one basic calculation: 4.2 Kilo Joules are equal to 1 calorie.
On the packets of food items actually 1 Kilo Calorie is written as calorie because we refer to KJ as calorie in general discussions.
Generally on the packets of food items exact amount of contents is given even in case when the calorie info has been avoided totally. For this you just need to know amount of energy present in different nutrients. For example:
Water has 0 KJ in each gram, so automatically calories are nil.
Fat has 37 KJ per gram, so according to the KJ value, you multiple 37 KJ with 1 (1 gram, the amount for which you are calculating calories) and divide it by 4.2 (value of 1 KJ in each calorie). The answer is 8.81 Kilo Calorie, which is actually referred to as calories. So you get 8.81 calorie which could be rounded off to 9 cal to make calculations easy and to get approx results.
Similarly, Protein has 17 KJ per gram, which when calculated similarly would give 4.05 Cal as result, which could be rounded off to 4
Dietery fibre has 13 KJ per gram, thus 3.1 calories
Alcohol has 29 KJ per gram, thus 6.90 cal
Carbohydrate 16 KJ gram, thus 3.81 cal.
You can round off the results to make calculations easier. However, if you wish to find accurate results don't round it off.
