The energy of a food is calculated by adding the energy produced by the energy-producing nutrients contained in the food in kilocalories (kcal) to the total number of kcal. As there are many nutrients in food, when calculating food energy, the energy producing nutrients such as proteins, fats and carbohydrates contained in the food are calculated separately, and then the energy produced is calculated according to the corresponding nutrients, and finally added up to arrive at the total energy value of the food. For example, if the food contains 10g of protein, 10g of fat and 10g of carbohydrates, and 1g of protein produces 4kcal of energy, 1g of fat produces 9kcal of energy, and 1g of carbohydrates produces 4kcal of energy, the total number of kcal contained in this food is 170. In some cases, the energy content can be indicated on the finished food packet, sometimes in kilojoules (kJ), which may be converted to kcal by using 1 kJ = 0.239 kcal, or, if less precision is needed, can be divided by 4 to obtain an approximate number.