How many kilojoules do you need to lose weight?

Experts explain what kilojoules are, where you should get yours from and how many you need to maintain or lose weight.

If you pick up a product in the supermarket, it’s easy to spot the kilojoule listing on the label for that particular food, or for a 100g serving. But what is a kilojoule? How does it differ from a calorie? And if you’re trying to lose a few kilos, how many kilojoules should you eat per day?

These are all great questions for anyone keen to manage their weight or adopt good nutrition habits in general, so let’s break it down.

What is a kilojoule?

Kilojoules are the way we measure the energy component of what we eat and drink, and the energy we use, each day. The original measure of this energy was the calorie, but in Australia we use the metric system of kilojoules (abbreviated to kJ).

You burn kilojoules simply by keeping your body going – breathing, thinking, physical activity and by your heart pumping blood around your muscles every time you move.

When it comes to weight loss or gain, kilojoules count because you’ll put on weight if you eat more kilojoules each day than you actually burn off. It sounds simple in theory, but it can be a bit of a learning curve figuring out how many kilojoules a day you should eat and the best way to ‘spend’ the kilojoules you do consume.

What about calories? Can we still use them?

Calories and kilojoules measure the same thing: how much energy is in the food we eat. And the calorie was the measurement used in Australia until 1988, when kilojoules became the standard measure. But it can be confusing as many food labels still include both measurements (kJ and cals) next to one another.

Which one should you use? In Australia, it’s probably easier to switch to kilojoules given that’s the metric used across the board, but it’s really up to you, as long as you know how to accurately convert the two whenever you need to.

Are all kilojoules created equal?

No. Rather than focusing purely on the kilojoules, consider the nutritional content of the food or drinks you choose.

“Take an avocado or nuts, they’re energy-dense [high kilojoule] foods but they’re full of healthy fats that keep the eyes, skin and brain healthy. An avocado has similar kilojoules to a small chocolate bar – but is far more nutritious,” says accredited practising dietitian Lisa Donaldson, spokesperson for Dietitians Australia.

To put it simply: one regular-sized avocado is 1210kJ. It contains at least 7% of the fibre, 25% of the folate and 30% of the potassium you need in a day – plus many other nutrients. It also contains around 12g of monounsaturated fat, which is shown to lower levels of cholesterol in the body.

A small, 50g bar of milk chocolate can contain 1120kJ (almost the same as an avocado), but also has 23% of the fat and 25% of the sugar that’s recommended in a day. Most of the fat is saturated, which raises cholesterol. Although chocolate can contain fibre, potassium and iron, it’s comparatively low in nutrients.

As a general rule when working out your average kilojoules per day, you should be focusing your food intake on wholefoods like fruits, vegetables, wholegrains, lean meats, oily fish, nuts and seeds if you want the whole package for health.

How many calories are in a kilojoule?

One calorie contains 4.184 kilojoules, whereas one kilojoule (kJ) is equal to 0.239 calories (cal). On many health websites you’ll find them rounded up or down when converted, to make it easier:

• 1kJ = 0.2cal
• 1cal = 4.2kJ

Some health professionals suggest we multiply the calories by four to get the kilojoules, but that’s not strictly correct.

For example, if you’re multiplying 1,000cals by four, you’ll only get 4,000kJ, instead of 4,184kJ – leaving you out by nearly 200kJ.

So if you need to convert calories to kilojoules, it’s more accurate to multiply calories by 4.2, or use a calorie/kilojoule convertor.

How many kilojoules should you eat per day for good health?

It’s important to consider the average kilojoules a day you should eat, and how to choose your kilojoules wisely.

For example, fat, protein and carbohydrates all contain a set number of kilojoules per gram. Fat contains 37.7kJ per gram, protein and carbohydrates each contain 16.7kJ per gram. This is one reason why cutting back on unhealthy fat automatically cuts down on the kilojoules you consume.

As well as limiting unhealthy fats – by avoiding the likes of deep-fried foods, processed foods and margarine – you need to lose foods known as ‘empty-calorie foods’.

“These are foods that supply energy to the body but few nutrients – lollies, chips, cake, soft drinks and alcohol all fall into this group,” says Lisa.

The labels on the back of all processed foods give you details of the kilojoule count. You can track kilojoule content of foods through apps but it’s a good idea to check their data against the Australian Dietary Guidelines.

A University of Sydney study found considerable inaccuracy among weight loss apps. Among the Aussie apps that made the researchers’ top 10 list was by CalorieKing.

How many kilojoules a day should you eat to lose weight?

If you’re trying to lose weight, which is better: limiting the number of kilojoules you eat or exercising? Both are important. Cutting kilojoules is far easier than burning them off. It takes just seconds to cut 420kJ (by, say, skipping a handful of hot chips with your dinner) but to burn off that food once you’ve eaten it could take around 10 minutes of jogging.

“But simply decreasing kilojoule intake to control weight means you miss out on the health benefits of exercise like improving cardiovascular fitness and building muscle,” explains exercise physiologist Dr Jarrod Meerkin.

If you want to lose weight, you need to burn more energy than you consume. To lose weight at the rate of around half a kilo a week, it’s recommended you burn around 2,100kJ per day more than you consume.

How many average kilojoules do you need in a day?

“You may have heard that 8,700 kilojoules is the average intake for a healthy Australian adult, but this is only a guide,” explains Lisa.

“A number of factors including your age, ethnicity, gender, body composition and level of physical activity all influence exactly how much you need.”

The best way to tell how many kilojoules you need in a day is to have a DEXA scan (dual energy X-ray absorptiometry), which measures your exact body composition and calculates your energy needs. These cost anywhere from $50 to $160, depending on how comprehensive they are. You can get one with an exercise physiologist or at a radiology clinic. You don’t need a doctor’s referral.

You can also get a good idea of your needs using an online kilojoule calculator.

Having an idea of the average kilojoules you can consume in a day can help you lose or maintain weight. But it’s important to remember that nutrients come first.

Weight management support

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Words by Helen Foster and Rachel Smith
Updated August 2022