What is a Kilojoule?

The kilojoule equals 1,000 joules and is the standard unit on European nutrition labels and in mid-scale chemical and physical calculations. EU regulations require food packaging to display energy in kilojoules (typically with kilocalories also shown for consumer familiarity): a chocolate bar might show '2,100 kJ / 500 kcal' per 100 g. The kilojoule is also common in chemistry (bond energies), electrical engineering (capacitor storage), and small-mechanical-energy contexts. A car battery typically stores about 1,800 kJ; lifting a person 1 meter requires roughly 0.7 kJ. The kilojoule relates to the joule (1,000 J = 1 kJ), the kilocalorie (1 kcal = 4.184 kJ), the watt-hour (1 kWh = 3,600 kJ), and the megajoule (1,000 kJ = 1 MJ). The conversion factor 4.184 (kJ per kcal) is one of the most-used numbers in nutrition labeling.

Visual reference for how the kilojoule relates to other energy units. Each row links to the full converter for that pair.

• EU and UK nutrition labels
• Mechanical work in engineering
• Battery capacity in vehicles

• ✓1 kJ = 1000 J = 0.239 kcal.
• ✓EU nutrition labels must show both kJ and kcal.
• ✓For quick conversion: kcal × 4.2 = kJ.

• ✗Assuming European labels use kcal only — they use both kJ and kcal.
• ✗Confusing kJ with kW (power).
• ✗Using kcal when kJ is requested in scientific context.