Calorie, a unit for measuring the amount of heat released or absorbed by a substance. A calorie is most commonly defined as the amount of heat that will raise the temperature of one gram of pure water from 14.5 to 15.5 degrees Celsius, and is equivalent to 4.186 joules. (There are also other definitions of the calorie, yielding slightly different values.) The traditional English unit is the British thermal unit (Btu). One Btu is the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit; it is equivalent to 252 calories.

The calorie used to measure the heat energy provided by food is actually a kilocalorie, or large calorie—equal to 1,000 calories. Nevertheless, it is commonly called simply the calorie. Thus, one cup of whole milk is said to provide 150 calories—meaning that it provides 150 kilocalories of heat energy for bodily activities.