kilojoules

Kilojoule (symbol kJ) is a unit of energy in the International System of Units (SI). It equals 1000 joules. The joule, the base SI unit for energy, is defined as the energy transferred when a force of one newton acts over a distance of one meter.

In nutrition and everyday use, energy content of foods is commonly expressed in kilojoules or kilocalories (Calories). The conversion is 1 kilocalorie (kcal) = 4.184 kilojoules (kJ), and therefore 1 kilojoule ≈ 0.239 kilocalories. Food labeling may present energy values per serving or per 100 grams, depending on regional regulations.

In science, kilojoules appear in thermodynamics and chemistry, where energy changes are reported as enthalpy changes (ΔH) in kilojoules per mole (kJ/mol) or per reaction. Kilojoules are also used to quantify heat transfer and metabolic energy expenditure in physiology and engineering.

History and naming: the joule is named after James Prescott Joule, and the kilojoule is the SI unit scaled by a factor of 1000. The kilojoule is widely used in science, industry, nutrition, and energy accounting as a practical unit for moderate to large energies.