Kilojoule (kJ)

The kilojoule is a unit of energy equal to 1000 joules. Its symbol is kJ. It is the working unit of nutrition labelling in most of the world outside North America, and the natural scale for chemical reactions, mechanical impacts and small quantities of stored energy.

Food labelling is where most people meet it. European Union regulations require energy to be declared in kilojoules with kilocalories alongside, and Australia and New Zealand use kilojoules alone. A slice of bread holds roughly 300 kilojoules, a banana about 400, and daily requirements run from about 8000 to 10,500. The figures are larger than the kilocalorie equivalents by a factor of 4.184, which is why the two scales are rarely confused once seen together.

Chemistry adopted it wholesale. Enthalpies of reaction, bond dissociation energies, lattice energies and activation barriers are tabulated in kilojoules per mole, and the numbers land conveniently: a typical single covalent bond takes between 150 and 500 kilojoules per mole to break, and the combustion of methane releases 890.

Mechanical impacts sit at the same scale. A car at 50 kilometres per hour carries around 150 kilojoules of kinetic energy per tonne, a rifle bullet a few, and a boxer's punch about one. Protective equipment standards specify the energy a helmet or a barrier must absorb in kilojoules, which is what makes the ratings comparable across designs.

The unit describes stored energy in devices too small for kilowatt-hours. A camera flash capacitor holds a few kilojoules, a defibrillator discharge is around 0.2, and a compressed-air tool cycle uses a handful. Physiology uses it for the energy cost of movement, where a flight of stairs costs a person around 20 kilojoules.

Its main competitor for public use is the kilocalorie, which persists in the United States and in casual speech nearly everywhere. Where both appear, the kilojoule is the one that connects to the rest of physics without a conversion factor.

Materials testing uses the unit in a distinctive way. The Charpy test drops a weighted pendulum onto a notched bar and reports the energy absorbed in breaking it, typically a few tens of joules for steel at room temperature but falling sharply as the temperature drops. Plotting that energy against temperature reveals the ductile-to-brittle transition, the point at which a metal stops bending and starts shattering. The investigation of the Liberty ships that split in half in cold Atlantic water during the Second World War rested on exactly this measurement, and modern steel specifications still state a minimum absorbed energy at a stated temperature.

One kilojoule equals 1000 joules, about 239 calories, or about 0.278 watt-hours.

Kilojoule conversion table