What this guide covers
The number printed beside "calories" on a food label is a measurement of energy, in exactly the sense used by the rest of this site. This guide explains that unit as physics: what it measures, how it is defined, how it converts to joules and kilowatt-hours, and what happens to food energy in terms of the conservation law.
Physics, not diet advice
This page does not say how much anyone should eat, does not compare diets and does not give health advice. Questions about food and health belong with a doctor, a registered dietitian or another qualified professional.
Food as stored chemical energy
The forms of energy guide described chemical energy as energy stored in the bonds of atoms and molecules. Food is one of the most familiar stores of it. The EIA uses food as its first example of energy changing form: the food you eat contains chemical energy, and your body stores that energy until you use it as kinetic energy during work or play.
A university teaching page from Penn State's College of Earth and Mineral Sciences names the parts of food that act as energy sources: carbohydrates, which include starches and sugars, fat and protein. All three can release energy when they are metabolised to carbon dioxide and water in the process of respiration, which takes place in the body's cells.
Seen as physics, that is a conversion of stored chemical energy into other forms, the same kind of change the EIA describes when wood burns in a fireplace and its chemical energy becomes thermal energy. The difference is that the body releases the energy gradually, inside its cells, rather than as a flame.
What a calorie is
The calorie is an old unit of heat that is still widely used for food. Its original idea is simple and is tied to water. The Penn State page explains that the "calories" used for food are in reality kilocalories (kcal) as defined in chemistry, and that one kilocalorie will heat one litre of water by one degree Celsius.
So there are two sizes of calorie in circulation:
- The calorie (cal) of chemistry, roughly the heat needed to warm one gram of water by one degree Celsius.
- The kilocalorie (kcal), 1,000 times larger, which is the unit meant by "calories" on food labels and in nutrition writing.
The naming is a historical accident, and it causes real confusion. When a food label gives a number of calories, the energy meant is that number of kilocalories.
Calories in joules
Defining a unit by the warming of water is not precise enough for science: NIST's table lists separate calories defined at particular water temperatures, such as the 20 °C calorie, because the heat needed changes slightly with temperature. Modern definitions tie the calorie to the joule, the SI unit of energy, work and amount of heat. NIST's table of conversion factors lists several versions; the two in common use are:
| Unit | In joules |
|---|---|
| Thermochemical calorie (calth) | 4.184 J |
| International Table calorie (calIT) | 4.1868 J |
| Thermochemical kilocalorie (kcalth) | 4,184 J |
| International Table kilocalorie (kcalIT) | 4,186.8 J |
NIST's table also lists a "kilogram calorie" for nutrition, which is the same size as the kilocalorie, in both the thermochemical and the International Table versions. The two versions differ by less than 0.1%, which is far smaller than the uncertainty in the energy content of any real meal. This site uses the thermochemical value, 4,184 joules per kilocalorie, in its arithmetic.
The same table connects the calorie to the electricity bill. One kilowatt-hour is 3.6 million joules, so one kilowatt-hour is 3,600,000 ÷ 4,184, or about 860 kilocalories.
Energy per gram
The three energy sources in food do not hold the same amount of energy per gram. The Penn State page gives approximate values: about 9 kilocalories per gram for fat, and about 4 kilocalories per gram for carbohydrate and for protein. Gram for gram, fat holds more than twice the energy of either.
Converted with NIST's thermochemical factor:
- Fat: 9 × 4,184 J, about 37.7 kilojoules (kJ) per gram.
- Carbohydrate or protein: 4 × 4,184 J, about 16.7 kJ per gram.
These are rounded, average figures for whole classes of nutrients, not measurements of any particular food. The Penn State page presents them as approximate, and they are used here only to show the size of the numbers involved.
The body as a machine
The EIA uses the human body as its example of an inefficient energy conversion. It describes the body as a machine whose fuel is food, giving a person energy to move, breathe and think. But the body is not very efficient at turning food into useful work: the EIA says it is less than 5% efficient most of the time, and the remaining energy is converted to heat.
In terms of the conservation guide, food energy is never destroyed. The chemical energy in food becomes a small amount of mechanical work, such as lifting, walking or climbing stairs, and a large amount of thermal energy. That heat may or may not be welcome, the EIA adds, depending on how warm a person wants to be.
Example: 100 kilocalories in other units
Worked example
An arithmetic illustration of the units, not a statement about any food or any person. It takes 100 kilocalories of food energy and expresses it in other ways using the factors above.
- In joules: 100 × 4,184 = 418,400 J, or about 418 kJ.
- In kilowatt-hours: 418,400 ÷ 3,600,000, about 0.116 kWh. That is the energy a 50 watt laptop would draw in about 2.3 hours.
- As heat for water: since 1 kcal warms 1 litre of water by 1 degree Celsius, 100 kcal could in principle warm 10 litres by 10 degrees, if none of the heat escaped.
- As lifting: raising 1 kilogram by 1 metre takes m g h = 9.81 J. A perfectly efficient machine could use 418,400 J to lift 1 kilogram through about 42.7 kilometres. With the body's efficiency of less than 5%, the useful work would be less than a twentieth of that.
The comparison shows two things at once. Food holds a surprising amount of energy for its weight, which is the everyday magic in the title of this site. And very little of it becomes motion, because the body, like every machine, turns most of what it converts into heat.
That is the last guide. Keep the units table to hand.