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. Many products carry both. Overseas products, and most of the internet, work the other way around.

The conversion

One calorie is 4.184 kilojoules. You’ll often see 4.2 used instead, 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. Across 9,000 kJ, a rough daily intake, using 4.2 loses about 8 calories. Trivial on its own.

The trouble is compounding. A value gets converted to calories, rounded, published, then converted back to kilojoules by someone else and rounded again. After a few hops it can sit several percent from where it started.

Figures here are stored as the kilojoules the source published. Calories are derived on the way out, so the kilojoule value is always the original.

The 8,700 kJ reference

The percentage daily intake column on Australian packs is calculated against 8,700 kJ, or about 2,080 calories. It’s a labelling convention. It assumes an average adult with average activity, which describes almost nobody exactly.

Why kilojoules stuck here

Australia went metric 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 never finished metrication and kept calories, and because most of the internet is American, so did most nutrition websites.

The practical effect when you’re cooking here: a recipe site showing “320 calories” uses a unit that doesn’t appear on the pack in your hand, and usually portion sizes that don’t either.