A calorie is a unit of energy. In nutrition, it almost always means the kilocalorie (kcal): the "calories" on food labels, in calculators and in nutrition apps are kilocalories. Labels in many countries also list kilojoules (kJ), which express the same energy in a different unit.
How much energy nutrients provide
According to the Atwater system, these are the standard values per gram:
- Protein: 4 kcal
- Carbohydrates: 4 kcal
- Fat: 9 kcal
- Alcohol: 7 kcal
- Fiber: 2 kcal
Example: A meal with 40 g of protein, 60 g of carbohydrates and 20 g of fat provides 160 + 240 + 180 = 580 kcal. These numbers are a worked example.
Calories and body weight
To convert between energy and body weight, the usual approximation is about 7,700 kcal per kilogram. A deficit of 7,700 kcal therefore corresponds to roughly one kilogram less, a surplus of the same size to one kilogram more. The number is not a fixed constant, because a change in weight consists of different shares of fat, muscle and water (Hall 2008). For steering a bulk or cut, it is good enough.
A handy shortcut is 7,700 ÷ 7 = 1,100: each kilogram per week corresponds to about 1,100 kcal per day, each tenth of a kilogram to about 110 kcal.
Kilocalories or kilojoules
Studies often report energy in kilojoules. Slater and colleagues (2019), for example, suggest a surplus of 1,500 to 2,000 kJ per day, which is about 360 to 480 kcal. For your own calculations, stick to one unit, usually kcal.