CalorieKilocalorie · kcal

A calorie is a unit of energy. The one used in nutrition is technically a kilocalorie, the energy needed to raise the temperature of one kilogram of water by one degree Celsius, which is why food labels use kcal and why calorie and kilocalorie are used interchangeably in everyday speech.

Food labels also give kilojoules, the SI unit, where one kcal is about 4.18 kJ. The energy values themselves come from the Atwater system, which assigns roughly 4 kcal per gram to protein and carbohydrate, 9 to fat, 7 to alcohol and 2 to fibre.

Those numbers are averages with real error. Label figures are permitted to deviate by a substantial margin, and the Atwater factors overestimate the energy available from some whole foods, notably nuts, where a meaningful share passes through undigested.

How the food is prepared matters too. Cooking increases the energy you extract from many foods, and heavily processed foods generally cost less to digest, which is part of why calorie counting is more approximate than the decimal places on an app imply.

None of that makes the concept wrong. Energy balance still determines weight change, and controlled feeding studies in metabolic wards demonstrate it reliably. What the imprecision means is that counting should be treated as a consistent estimate to steer by rather than as a measurement.

The practical consequence is that your own weight trend over weeks corrects the errors. If the numbers say maintenance and the trend says otherwise, the trend is right.

In practice, use our TDEE and calorie deficit calculators to set a target, then adjust from three weeks of weekly averages.