A calorie (abbreviated cal) is the amount of heat necessary to raise the temperature of one gram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units called "calorie":
Of these, the 15 °C calorie is what is most commonly meant by calorie in contemporary English text.
One 15 °C calorie is the amount of heat necessary to raise the temperature of 1 g of water from 14.5 °C to 15.5 °C. This is approximately equal to 4.1855 J or 3.968×10-3 Btu. The International Table calorie is approximately equal to 4.1868 J and the thermochemical calorie 4.184 J.
Nutritionists, when describing the energy content of food, typically refer to Calories (capitalized and abbreviated as C or kcal); one Calorie equals 1000 15 °C calories, or about 4,186 J. The energy content of fat is 9 kcal/g. of proteins and carbohydrates 4 kcal/g.