Outdoor group training
Outdoor group training

Kilojoules vs. Calories

Food energy is measured in kilojoules (kJ). The common term for this used to be ‘calorie’, but ‘kilojoule’ is the term now accepted internationally. This unit of measurement allows us to see how much energy a food contains and how much energy is burned up during exercise.

What is a kilojoule?

A kilojoule (kj) is a metric unit of which is used to measure energy.

The energy in food and the energy we expend during exercise is measured in units called kilojoules (kJ). The ‘joule’ (small unit of energy) is defined as “the amount of energy expended in applying a force of 1 Newton through a distance of 1 metre” – which to me is quite confusing!! In easier words – the kilojoule or kJ is a Big Joule and represents 1000 joules.

What is a ‘calorie’?

The word “calorie” is a non-metric unit which is used to measure how much energy we burn or consume.

Calories come in TWO sizes! The little calorie and the Big Calorie – which is actually called the kilocalorie (kcal). When we talk about food energy and energy burned from exercise – we refer to the Big Calories (kcal).

A calorie (with a small ‘c’) is defined as the amount of heat energy required to raise the temperature of 1 ml of water at 15 degrees Celcius by 1 degree. So you can see why we don’t talk about the “little” calories!

Each kilocalorie (kcal) equals 1,000 little calories. So it’s easier to just call 1,000 ‘little calories’ – 1 (Big) Calorie – or as you know now – 1kcal.

How many kcal (Big Calories) in one kilojoule (kJ)?

The basic rule is that 1 kJ = 4.185 (or 4.2) kcal.

So when we talk about how many kcals you can eat a day and want to convert them to kilojoules, you multiply the kcal by 4.2. On the other hand if you would like to convert kJ to kcal then you divide by 4.2

I thought I understood all of this but now I’m completely confused! Can you make it easier for me to understand??

YES! Basically the rule of thumb is as follows.

4.184 kilojoules = 4,184 joules = 1 Calorie = 1 kilocalorie = 1,000 calories
To make it easier we round it up…

4.2 kilojoules = 4,200 joules = 1 Calorie = 1 kilocalorie = 1,000 calories