CalorieKilocalorie · kcal

A calorie is a unit of energy. The 'calories' printed on food labels and counted in apps are strictly kilocalories: one kilocalorie is the energy needed to raise a kilogram of water by one degree Celsius. The convention is universal enough that the distinction rarely causes confusion outside a chemistry lab.

It matters because energy balance — calories taken in against calories expended — determines whether body mass rises or falls. That is a thermodynamic constraint, not a diet philosophy: every approach that produces fat loss does so by creating a deficit, whatever mechanism it uses to get there.

In practice, the numbers are approximate at both ends. Food labels carry a permitted margin of error, the energy your body actually extracts varies with food processing and fibre content, and expenditure estimates are rougher still. Calorie counting works as a consistent yardstick to steer by, not as an exact measurement.