What Is a Calorie?

The short answer

A calorie is a unit of energy, not a substance. Food does not contain calories the way a bottle contains liquid; the figure on the packet is the energy the body can extract when it oxidises what you ate. The same gram of food can be worth slightly different amounts depending on how much of it you absorb and what it costs to process.

In nutrition the unit is the kilocalorie, written kcal, and in ordinary speech the kilo is dropped. This site follows that convention throughout: calorie means kcal, and 2,000 calories means 2,000 kcal.

Why we say calorie when we mean kilocalorie

Strictly, a calorie is tiny: the energy to warm a single gram of water by one degree. Nutrition needed something about a thousand times larger, so the kilocalorie became the working unit and the prefix fell off in speech. A 500 calorie lunch is a 500 kcal lunch, and a label that prints Calories with a capital C is printing the same thing.

This is not pedantry. Mixing the two units is the classic way an article ends up claiming a session burns 300 calories when it has measured 300 kcal. Here the two words are interchangeable, and both mean kcal.

Calories per gram of protein, carbohydrate, fat and alcohol

Converting grams of food into energy uses the Atwater factors: protein and available carbohydrate about 4 kcal per gram, fat 9, alcohol 7. They are averages measured on mixed diets, which is why the numbers are round rather than exact.

The table is the arithmetic behind almost every macro calculation on this site. It also explains why changing the fat in a diet moves the calorie total so much more than changing the carbohydrate by the same weight.

Where the 4, 9 and 7 come from

Atwater measured two different things. A bomb calorimeter gives the gross energy in a dried sample - roughly 5.65 kcal per gram of protein, about 4.1 for carbohydrate and 9.4 for fat. He then measured what people actually lost in urine and faeces, because the body does not extract everything a furnace can.

Protein is the interesting case. Burning it in the body leaves urea, which still holds energy, and the correction costs roughly 1.2 kcal per gram, which is how 5.65 becomes 4. Fat and carbohydrate are oxidised almost completely, so their factors barely move.

Fibre is the other exception. It reaches the colon largely intact and is fermented by bacteria, and the short-chain fatty acids that come back are worth far less than 4 kcal per gram. The result is a set of factors that describe average mixed diets: fit for planning, unfit for arguing about decimal places.

What 100 kcal looks like in real food

Energy density is the practical face of the 4 and 9 factors. Fat carries more than twice the energy of carbohydrate per gram, so foods built on fat pack the same calories into a fraction of the weight and volume.

Calories and body fat: where 7,700 comes from

Body fat is not pure fat. Adipose tissue is roughly 80 percent lipid with the rest water and protein, and a kilogram of it holds about 7,700 kcal. That is the figure used across this site for kilogram estimates, and 3,500 kcal per pound is the same number in imperial units.

It is a conversion, not a law. Losing a kilogram on the scale in a week rarely means losing a kilogram of fat, because glycogen, water and gut contents move faster and in both directions. The figure is most useful for setting a sensible deficit - 500 kcal a day is about 0.45 kg of fat a week - and least useful for predicting tomorrow morning.

Why a label is an estimate

Nutrition labels come from databases of average values rather than from testing your actual meal, and the rules accept a tolerance: US labels may be off by up to 20 percent for most nutrients, with similar latitude for energy in the EU. Cooking adds another layer, because oil absorbed in frying and water lost in roasting both change the energy per gram.

The body side of the equation adds more. Absorption varies with the food matrix and with the person, and processing a meal costs energy, so two foods with identical label values do not cost identical amounts to metabolise. Treat the number as a good estimate of what you ate, never as a measurement.

Do all calories behave the same in the body?

For energy balance, 500 kcal from any source arrives as 500 kcal. Beyond that the sources diverge. Protein costs roughly 20 to 30 percent of its own energy to digest and store, carbohydrate about 5 to 10 percent and fat almost nothing, so a higher protein diet burns a little more of what it takes in.

Protein and fibre also change how much you eat, because both raise fullness per calorie. In a year-long randomised trial comparing low-fat with low-carbohydrate diets, the difference in weight loss between the two groups was small next to the effect of actually staying on either one. The composition you can sustain beats the composition that looks best on paper.

How to use this page

If you want the daily energy that matches your size, the maintenance calorie calculator and the TDEE calculator both start from the Mifflin-St Jeor equation and multiply by an activity factor. If you want the gap that produces loss, the calorie deficit calculator takes the same baseline and subtracts from it.

The unit itself never changes. Once a calorie is 4.184 joules and fat carries 9 kcal per gram, the remaining decisions are about food you are willing to eat for years, and those are not arithmetic.

Frequently asked questions