The calorie (cal) is a small energy unit defined as the amount of heat needed to raise one gram of water by one degree Celsius, primarily used in physics and chemistry. In everyday language, 'calorie' usually refers to the kilocalorie (kcal), but the true calorie is 1/1,000th of that amount. One calorie equals exactly 4.184 joules, providing a direct bridge between thermal and mechanical energy.

The kilocalorie (kcal), commonly called the 'Calorie' on food labels, equals 1,000 calories or 4,184 joules, the standard unit for measuring dietary energy worldwide. When nutrition facts list '200 Calories,' they mean 200 kilocalories, representing the energy your body can extract from that food. One kilocalorie equals exactly 4.184 kilojoules, the figure used on food packaging in many countries.