What is the difference between kilojoules and calories?

Have you ever looked at the nutritional label on a food package only to face a long list of numbers… and no idea what they mean? We get it – all that data on the back of the packet can be confusing at first. Are calories and kilojoules the same thing? And exactly how is food energy measured?

We spoke to accredited practising dietitian and allied health coach Georgie Britton, from Honeysuckle Health, to find out the difference between kilojoules (kJ) and calories (cal), how to convert them and just how many kJ per day we truly need to stay fit, healthy and strong.

Calories and kilojoules are simply a measurement of the amount of energy people get from food and drink. “When we talk about calories and kilojoules, we are talking about the same thing – a measure of how much energy is in the foods and drinks we consume,” explains Georgie. “Just like when we compare a mile to a kilometre, calories are the imperial system, whereas kilojoules are the metric system.”

Not all food labels offer both the kilojoule and calorie count of food, so if you prefer to use one over the other, you’ll need to know how to convert them. “Nutrition information panels for Australian products will always show the kilojoule content,” says Georgie. “However, international brands may show one or the other, depending on whether they use the metric or imperial system as a unit of measure.”

Which leads us to the question: how many kJ in a cal? Well, one calorie is around four kilojoules – so for a rough conversion when you’re standing in the supermarket aisle, simply divide the number of kilojoules by four to get the number of calories (or multiply calories by four to get kJ).

Want to get really specific when you convert kJ to cal (or vice versa)? “Consider that 1 calories = 4.18 kilojoules,” says Georgie. Whip out the calculator on your smartphone or use Google to find a kJ to calorie converter tool online to get the exact number.

As a general rule of thumb, food and drink that is high in fat, added sugar and alcohol is the most energy-dense – meaning it’s high in kilojoules. Protein and carbohydrates provide a moderate amount of kJ, fruits, and legumes are lower in kilojoules, while water has zero kilojoules.

However, while looking at the energy content of what we consume can be a helpful learning tool, it isn’t necessarily the best way to confirm how 'healthy' something is, explains Georgie. “This is because not all food is created equal. Just because a chocolate bar might have a lower kilojoule content than a chicken and salad roll doesn’t mean that they are equally ‘healthy’. What makes the chicken and salad roll the healthier option is that it is high in fibre, nutrient-dense, low in saturated fat and provides better satiety.”

The amount of energy we require (i.e. the number of kilojoules/calories) each day to fuel our bodies and keep us healthy relies on a lot of different factors and varies from person to person. While the average adult needs about 8700kJ (or around 2080 calories) each day to maintain a healthy weight, things like your activity levels, muscle density, age, gender, height and weight can all play into how many kJ per day a person needs. There are many kilojoule calculators online that can help you work out how much energy your body needs each day to stay healthy.

Keen for more expert tips, information on daily intake and dietitian-approved recipes? Check out the dedicated food section of The Check Up.