The joule (J) is the SI base unit of energy, defined as the work done when a force of one newton moves an object one meter. It is the universal unit in physics for measuring kinetic energy, heat, and electrical energy, appearing in everything from textbook problems to nutritional science. One joule equals the energy released by dropping a small apple one meter, making it tangible despite its small magnitude.

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.