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 kilojoule (kJ) equals 1,000 joules and is the standard unit for expressing food energy content and chemical reaction energies in countries using the metric system. Food labels in Australia, Europe, and many other regions display energy values in kilojoules alongside or instead of calories. One kilojoule equals approximately 238.8 calories or about 0.239 kilocalories.