Kilojoules (kj) & calories (cal) are both used to measure the amount of energy in foods and drinks, the same as kilometres and miles are both used to measure distance. It can be confusing having two measurements for the same thing particularly when they're both commonly used. Let's unpack some of this confusion and discover an easy conversion factor to make working with both measurements easier...
In Australia kilojoules are the preferred measurement however US TV shows (such as the biggest loser), apps for mobile devices and cardio equipment (such as rowers, treadmills etc) use calories which makes it difficult to let go of calories completely and move on to kilojoules. Calories are also a smaller number which makes them easier to work with, understand and remember for most people. It's for these reasons I believe that calories are still very commonly used despite Australia's preference for kilojoules.
Having two different units in use means it's important to be able to quickly convert between them if necessary. 1cal = 4.2kj. You can do a rough calculation of 'cal x 4 = kj' or 'kj / 4 = cal' which will be close enough for most purposes.
My suggestion would be to pick one and stick with it to help you get familiar with amounts & reduce confusion as much as possible. Kilojoules is what you'll see most commonly on nutrition labels but you may prefer to work in calories for other reasons. It ultimately doesn't matter which one you prefer to use but it's important you can convert one way or the other if needed.
It's interesting to note that (very confusingly) kilocalorie (kcal) & cal are the same thing when used in the context of energy in foods and drinks - no conversions required, kcal = cal.
David Eggins | Personal Trainer at Drive Fitness in Brisbane | firstname.lastname@example.org