What is a calorie?
Calorie is the word that comes up most often in conversations about food and is defined least often. It is neither a type of food nor an enemy; it is simply a unit of energy.
Short answer
A calorie is a unit of energy. The “calorie” used in nutrition is in fact a kilocalorie (kcal), the energy needed to raise the temperature of 1 kg of water by 1°C. The energy in food comes from its macronutrients: carbohydrate and protein provide about 4 kcal per gram, and fat about 9 kcal. Daily needs vary from person to person and from day to day.
A unit of measurement, nothing more
A metre measures length and a kilogram measures weight; a calorie measures energy. In its technical definition, 1 calorie is the energy needed to raise the temperature of 1 gram of water by 1°C. The unit we use in nutrition is a thousand times that, the kilocalorie (kcal), and this is what people mean when they say “calorie” in everyday speech. Some labels also give energy in kilojoules (kJ); 1 kcal is about 4.2 kJ. A 300-calorie meal, then, is a meal carrying a certain amount of energy the body can use, no more and no less.
Where does the energy in food come from?
Three macronutrients carry the energy in a food, and each has a known approximate value per gram:
- Carbohydrate: 4 kcal/g
- Protein: 4 kcal/g
- Fat: 9 kcal/g
Alcohol also provides about 7 kcal per gram. Fat carrying more than twice the energy per gram explains why a small amount of a fatty food is high in calories: a tablespoon of olive oil on its own passes 100 kcal. Water, fibre, vitamins and minerals provide no energy, which is why vegetables are bulky but low in calories.
What does the body use this energy for?
Most of the energy taken in goes not to exercise, as many assume, but to the body's basic workings: breathing, circulation, body temperature and cell renewal. This baseline cost is called basal metabolism and is described in the BMR guide. The rest is shared between everyday movement, planned exercise and digestion itself. The long-term relationship between the energy taken in and the energy spent is covered in the energy balance guide.
What does “empty calories” mean?
Two foods with the same amount of energy can be very different in what else they carry. A 200 kcal lentil dish brings protein, fibre, vitamins and minerals. A 200 kcal sweetened drink brings very little beyond the energy, and the phrase “empty calories” describes that situation. This is not about sorting calories into good and bad but about seeing that the same energy can arrive with very different nutritional value. To read the energy and nutrient information on a label, the food label guide will help.
How are calories measured and calculated?
The classic way of measuring the energy in a food directly is a laboratory device called a bomb calorimeter: the food is burned in a sealed chamber and the heat released is measured. The body does not work like a furnace, though; it cannot digest most fibre and cannot convert part of the protein fully into energy. Nutrition therefore uses corrected coefficients obtained by subtracting digestive losses from the burning values, and the figures of 4 kcal per gram for protein and carbohydrate and 9 for fat are the product of those corrections. Today the calorie figure on a label is almost never measured by burning: the manufacturer determines the macronutrient amounts in the product and calculates the energy by multiplying them by the standard coefficients. The number you see on a label is the result of a calculation rather than a measurement, which explains why the values are averages and why small deviations are unavoidable. The scale of that uncertainty is the subject of a separate guide: why calorie counts are never exact.
Daily needs: there is no single right number
Expressions such as “2000 calories a day” are used on labels as a reference, but that is an average rather than a personal target. Real needs vary from person to person, and even in the same person from day to day, according to age, body size, muscle mass and activity level. If you are curious about your own estimated range, the TDEE guide explains the logic of the calculation; remember that all of these calculations are starting points. In situations such as pregnancy, growth or chronic illness, energy needs should be assessed with a specialist.
Frequently asked questions
What is the difference between a calorie and a kilocalorie?
In the scientific definition, 1 kilocalorie is a thousand calories. The “calorie” used on nutrition labels and in everyday speech is in fact a kilocalorie, so 250 kcal on a label and 250 calories in conversation are the same thing.
Are zero-calorie drinks really zero?
Regulation allows very low values to be written as zero, so “zero” actually means “at a negligible level”. In practice their energy contribution is unimportant.
Which matters more, calories or nutritional value?
They answer different questions: calories tell you how much energy you have taken in, and nutritional value tells you what came with that energy. A balanced view keeps both in sight.
How is the calorie figure on a label calculated?
Usually by multiplying the amounts of carbohydrate, protein and fat in the product by standard coefficients. The result is therefore a calculation based on averages rather than a measurement.
This guide draws on the standard Atwater energy coefficients and on the general framework of the energy definitions in food labelling regulation.