A calorie is a unit of heat measurement in the metric system. It is defined as the amount of heat needed to raise one gram of pure water by 1°C under standard conditions. The term standard conditions refers to atmospheric pressure of one atmosphere and a temperature change from 15.5 to 16.5°C.
A second unit of measurement is the Calorie. The Calorie (with a capital C) is 1,000 times the size of the calorie. The difference between the two is sometimes made clear by calling the calorie the gram-calorie and the Calorie the kilocalorie. The abbreviation for the two units are cal for the gram-calorie and Cal for the kilocalorie. When you read about the number of calories contained in food or the number of calories to include in your diet each day, the term intended is the kilocalorie. It is this unit that is used by nutritionists in talking about the food value of what we eat.
Many people other than health scientists are interested in the caloric content of substances. For example, engineers need to know the heat content (in calories) of various types of fuels. Ecologists are interested in the energy content (expressed in calories) of various organisms in the environment and in how that energy content changes over time.
One calorie in the metric system is equivalent to 3.968 British thermal units, the unit of measurement for heat energy in the British system. A calorie is also equivalent to 4.187 joules, the fundamental unit of heat energy in the Système International , or International System of Units, the measurement system used by scientists throughout the world.