Convert joules to kJ

One joule equals 0.001 kilojoules. To convert any number of joules to kilojoules, multiply by 0.001.

The converter above works in both directions and updates as you type. Below you will find the formula, a worked example and a reference table of common values.

Energy conversions sit between physics and the kitchen. The joule is the SI unit and the one every equation uses; the calorie survives on food labels, where it is actually a kilocalorie a thousand times larger than the physicist’s calorie; and the kilowatt-hour is what an electricity meter counts. All three measure the same quantity, which is why an appliance’s consumption and a meal’s energy content can, in principle, be compared directly.

joules to kJ formula

To go the other way, use joule = kilojoule ÷ 0.001.

About these units

The joule is the SI unit of energy: one newton acting over one metre, or one watt for one second. Every other energy unit here is defined against it.

The kilojoule is the energy unit on food labels outside the United States. One kilocalorie is 4.184 kilojoules, which is why the kJ figure on a packet is always the larger number.

Joule is also written joules. Kilojoule is also written kilojoules.

Sources

1. Bureau International des Poids et Mesures, The International System of Units (SI), 9th edition, 2019.