The term calorie refers to a unit of energy. It can be further divided into two terms: small calorie and large calorie. A small calorie (or the gram calorie [cal]) is defined as a unit of energy that is equivalent to the amount of heat needed to increase the temperature of one gram (g) of water (H2O) by one degree of Celsius (C).

As a unit of food energy, the large calorie (or the kilogram calorie [kilocalorie or Cal]) is the amount of energy needed to increase the temperature of one kilogram (kg) of water by one degree of Celsius. French chemist and physicist Nicolas Clément (1779–1842) became the first known person to define and utilize the calorie as a unit of heat in 1824. The definition for kilocalorie as a unit of energy has since been replaced with the unit of joule (J) by the International System of Units (SI), but it is still used as a measure of food energy. When people in the United States refer to “calories” in a diet or with food, they are really talking about kilocalories.

Experiment to measure the calorific content of food by burning it. The food sample (lower center-right) is burned and used to heat a test tube of water.

Experiment to measure the calorific content of food by burning it. The food sample (lower center-right) is burned and used to heat a test tube of water. The rise in temperature of the water is then noted and used to calculate the amount of energy released.


Calories are needed to sustain life. Food provides energy in the form of calories—when a person eats food, the calories are either converted to physical energy and burned off by the body or are stored in the body as fat. All people have a base rate at which their bodies burn calories, called the basal metabolic rate. This is the number of calories burned by the body while at rest. Exercise or other movement burns more calories and increases the total number of calories burned in a day. Excess calories that are not expended by the body as energy are instead stored as fat within the body. A person's caloric intake may vary from day to day, but eating more food than the body can turn into energy for an extended period of time can result in weight gain.


Everyone consumes calories, but people eat different foods every day. Some foods, because of the various essential nutrients they contain, possess more calories than others (by mass). Macronutrients such as carbohydrates, fats, fiber, and proteins all contain calories and release energy to the body, but they do so in varying amounts. Fats release a large amount of energy, about 9 kilocalories per gram (38 kJ/g). Carbohydrates and proteins release a bit less, at about 4 kilocalories per gram (17 kJ/g). In addition, alcohol is a source of calories, with about 7 kilocalories per gram (30 kJ/g). Two foods may have the same mass, but if one food is high in fat and the other is high in protein, the high-fat food will have more calories.

Calories and weight maintenance

Calories are burned (expended) during exercise or physical activity. Even moving a finger to change channels on the television burns a tiny bit of calories within the body. The amount of calories expended depends on the length and type of exercise, though it varies from person to person. For instance, according to the National Heart, Lung, and Blood Institute, for a 175-pound (79 kg) man and a 140-pound (63.5 kg) woman performing light activity (such as daily housework, playing a round of golf, or working in an office), the number of calories expended in one hour is about 300 calories and 240 calories, respectively. For moderate activities such as bicycling, dancing, and walking briskly, men burn about 460 calories per hour, while women expend around 370 calories. Strenuous activities, such as playing football, swimming, or jogging, burn even more calories.

A scale and unit of measurement for temperature, in which the freezing point of water is 0°C and the boiling point of water is 100°C.
The ability of a physical system to do work on other physical systems.
A metric unit of mass.
The International System of Units (SI) unit of energy.
The International System of Units (SI) unit of mass, in which one such unit is equal to one thousand grams.

Regardless of where dietary calories come from, they are either converted to physical energy (as when people perform physical exercises) or are stored in the body as fat. If exercises and physical activities do not convert all of the calories consumed in one day, then the remaining calories (not consumed) are stored as fat in the body. These stored calories remain until they are used up another day with either additional physical activity or a caloric deficit. If more calories are consumed than are expended, a person gains weight. On the other hand, if fewer calories are consumed, a person loses weight. If the same number of calories are consumed as are expended in any particular day, the person remains at the same weight. For example, if 3,500 calories are burned in one day from playing basketball, walking, doing yard work, and all the other daily activities, and 3,000 are consumed as food in the form of calories, then 500 calories have been taken from fat and burned by the body for energy to the muscles and other life processes. If this happens for one week (seven days), then 3,500 calories of fat have been eliminated from the body and the person has lost about one pound. Conversely, if 3,500 calories are eaten and only 3,000 are burned, that person will gain one pound. When it comes to weight loss and maintenance, it all comes down to calories consumed (intake) versus calories expended (output).

Calorie tracking

People trying to alter their weight can track their calories by “counting them,” or measuring the number of calories in foods against a daily calorie balance. Many free online calorie counters are available on the Web. The U.S. Department of Agriculture's (USDA) Super-Tracker, released in December 2011, is a free calorie-tracking tool that also accounts for physical activity (calorie expenditure). The SuperTracker MyPlan program will also assign users daily calorie balances based on weight loss goals and help them make meal plans to meet these goals.


Eating too many calories on a daily basis increases the risk of becoming overweight or excessively overweight (obese). However, not eating enough calories puts a person at risk of nutrient deficiencies, which can lead to undernutrition and malnutrition. Consuming too much of a specific nutrient, by way of foods or supplements, can also cause problems.

Undernutrition and malnutrition are more common than overnutrition in developing countries. The opposite is true in developed countries, such as the United States and Australia, two countries with highest percentages of obese citizens. Medical conditions may also cause caloric complications, whether as the result of eating too much or too little.

Older people are more apt to eat less calories and nutrients than younger people. Some older people in the United States live on less than 1,000 calories each day. Such a small caloric intake of food is not adequate to maintain healthy nutrient levels.

Parental concerns

Infants, children, and adolescents are often at increased risk from undernutrition because of their need for extra calories and nutrients as they grow rapidly into adulthood.

See also Academy of Nutrition and Dietetics ; Adolescent nutrition ; Adult nutrition ; Alcohol consumption ; Calorie restriction ; Fats ; Malnutrition ; Obesity .



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Academy of Nutrition and Dietetics, 120 South Riverside Plz., Ste. 2000, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600,, .

U.S. Department of Agriculture, 1400 Independence Ave. SW, Washington, DC, 20250, (202) 720-2791, .

Weight-Control Information Network (WIN), 1 WIN Way, Bethesda, MD, 20892-3665, (202) 828-1025, (877) 946-4627, Fax: (202) 828-1028, win@ , .

William A. Atkins, BB, BS, MBA

  This information is not a tool for self-diagnosis or a substitute for professional care.