Calories (cal for short) are defined as the heat required to raise 1 gram of water 1 degree Celsius at 1 atmospheric pressure.

Because the unit of "calorie" is still too small compared with the daily intake of calories, another unit of "kilocalorie" (kilocalorie or C, also known as "calorie") evolved from "calorie" is widely used in nutrition. 1 kcal is equal to 1000 kcal, which is about 4 186 joules. The calorie of fat is about 9 kilocalories per gram; Sugar and protein have only 4 kilocalories per gram.

A person's daily intake is about 2000 calories.