Kilojoules and calories, and why Australian labels use kJ

18 September 2026

Every packaged food sold in Australia has to declare energy in kilojoules. Calories are optional, and many products carry both. Overseas products, and most of the internet, work the other way around.

The conversion

One calorie is 4.184 kilojoules. Not 4.2, though you will see 4.2 used constantly, including by people who should know better.

• kJ to Cal: divide by 4.184
• Cal to kJ: multiply by 4.184

At small numbers the difference is invisible. At 9,000 kJ — a rough daily intake — using 4.2 instead of 4.184 loses you about 8 calories. Trivial on its own. The problem is compounding: a value converted to calories, rounded, published, then converted back to kilojoules by someone else and rounded again can drift by several percent from where it started.

This is why the figures here are stored as the kilojoules the source published, and calories are derived from them on the way out. The kilojoule value is always the original.

The “average adult” is 8,700 kJ

The percentage daily intake column on Australian packs is calculated against a reference of 8,700 kJ, or about 2,080 calories. It is a labelling convention, not a recommendation for you specifically — it assumes an average adult with an average activity level, which describes almost nobody exactly.

Why kilojoules stuck here

Australia moved to metric units in the 1970s and applied it to food energy along with everything else. The joule is the SI unit; the calorie is a nineteenth-century unit defined by heating water. The US, which never completed metrication, kept calories, and because most of the internet is American, so did most nutrition websites.

The practical consequence for anyone cooking here: a recipe site showing “320 calories” is using a unit that does not appear on the pack in your hand, and frequently portion sizes that do not either.