kJ vs calories: The simple guide to understanding energy units

Anyone who's ever begun a weight loss or fitness journey (so, let's be real — most of us) is likely to have heard a thing or two about calories.

"It's all about calories in versus calories burned," one person says.

"Calories are fuel. They give you the energy you need to get through the day and push yourself in your workouts; you can't build muscle mass without energy," another will add. And look, they're not wrong!

Calories and kilojoules are different measures of energy that you're consuming via food and drink each day, and weight loss is largely about creating a calorie deficit in your diet. Furthermore, a big part of creating a calorie deficit is about how much energy you burn compared to how much energy you're consuming each day.

But anyone who's ever been to a supermarket (again, that's most of us!) will know that one look at any nutrition label on the back of packaged foods will tell you how many kilojoules are in a serving — not how many calories it contains. For many, the question then becomes, "Wait, what's the difference?" followed by, "How many kilojoules are in one calorie?"

In this article, we'll break down everything you need to know about kilojoules (kJ) versus calories, as well as how to convert kilojoules into calories, and more.

What are calories?

A calorie is a unit of measurement that tells us how much energy the food and drink we consume provides [1]. However, you may have heard that there are small calories (cal) and big calories (kcal). A small calorie is the amount of energy needed to raise the temperature of a gram of water by one degree Celsius, at one atmospheric pressure. A large calorie (kcal) is equal to 1,000 small calories.

When it comes to diet and nutrition and the calories in food and drink, though, we're usually talking about kilocalories, or large calories. One large calorie represents the amount of energy required to raise the temperature of a kilogram of water by one degree.

Your body derives energy content from food and drinks via their macronutrients, that is, your carbohydrates, proteins, fats and alcohols. As you digest food, your body breaks these macros down and uses them for fuel. But while it's common to count calories, particularly when you're trying to lose weight, most countries list kilojoule content on food packaging nutrition labels.

What are kilojoules?

Kilojoules are another way to measure the energy value of food, and like 1,000 small calories is equal to one kilocalorie (kcal), one kilojoule is equal to 1,000 joules [2].  Kilojoules also represent the amount of energy we get from the food and drink we consume, as well as the energy our bodies require for basic functions and physical activity. While the terminology is different, the concept is the same: both are ways of describing food energy.

The kilojoule (kJ) is the standard unit of energy in the International System of Units (SI), also known as the metric system. That's why nutrition labels often show the kilojoule content of foods, rather than the caloric content.

Are calories and kilojoules the same thing?

Although calories and kilojoules are both measures of energy, they are not the same thing [2]. Essentially, it's like any other difference in measurement, for example, centimetres versus inches — they both measure distance, but 50 centimetres and 50 inches will be vastly different lengths. In the same way, 100 calories when compared to 100 kilojoules will give you vastly different amounts of food.

When looking at nutrition labels, you'll see that the kilojoule content will appear much higher than the calorie content. This is because one calorie (kcal) is equal to 4.184 kJ.

What is the difference between calories and kilojoules?

The difference lies in the measurement system used to measure the energy you get from various foods and drinks. Both describe the amount of energy in food, but the numbers look different. For example, a snack that has 100 kcal will appear as 418 kJ on a nutrition label. This can sometimes be confusing if you’re comparing guides from different countries, but don't worry, there aren't secretly more kilojoules hiding in the snack! The underlying energy content is the same.

A kilocalorie (large calorie) is an older unit, but it remains more common in the US and online diet tools, which could explain why we know so much about calories, despite it not being our standard unit of measurement here in the UK.

How to convert calories to kilojoules

If you're following a diet that counts either calories or kilojoules, it can be handy to know how to convert the measurements. And thankfully, it's quite easy to do!

To convert calories into kilojoules, you multiply the number of calories (kcal) by 4.184. For example, if you ate a meal that was 400 calories, you would multiply that by 4.184 to find that 400 calories is equal to 1673.6 kilojoules.

On the other hand, if you're tracking calories rather than kilojoules, you would take the kilojoules of your meal, and divide it by 4.184.

Whether you're tracking kilojoules or calories, this simple calculation will allow you to track your energy intake consistently, no matter which units appear on the nutrition information panel.

Is it easier to count calories or kilojoules?

Whether you'll find it easier to count calories or kilojoules often comes down to context and preference.

Some people find that it's easier to track kilojoules, because that's the unit shown on our nutrition information panels and on some restaurant or takeaway menus. Not having to do any additional work to convert the number into calories is a hassle-free approach to tracking energy intake.

On the other hand, because so much of the scientific research and online nutrition advice around energy content still uses calories, many people find that it's easier to track their caloric intake.

When it comes to tracking the amount of energy you're consuming through the day, either system can be helpful, whether your goal is to lose weight, maintain your current weight, fuel your workouts, or more. Like all things, when it comes to a well-balanced diet and exercise routine, consistency is key. In this case, that means picking the system that works best for your diet and sticking with it, because these small changes can have a big impact.

How accurate are the labels?

While nutrition labels and nutrition information panels are a useful guide, they’re not always 100 per cent accurate.

Food manufacturers must provide a reliable estimate [3] of the energy content and nutrients in their products, but natural variation in fresh foods (like fruit and vegetables) and preparation methods means the amount of energy can differ.

In fact, one small study from 2006 found "a significant discrepancy between actual and declared values was detected with an average variation in precision of -13 per cent to +61 per cent for individual nutritional components" [3].

However, labels also don’t account for how each individual's body digests and absorbs food, which can vary depending on factors like age, muscle mass, and metabolism. Still, packaged foods are labelled in the same way across the UK, which offers a consistent reference point when comparing options.

Understand nutrition with Juniper

However you prefer to count your daily intake, understanding these units can help you make more informed choices. But at Juniper, we believe that knowledge is only one part of the journey toward reaching a healthy weight and then sustaining it long-term.

The Juniper Programme combines weight loss medication with dietitian-led health coaching and tracking, so you can lose weight and reach your goals for good.

Kickstart your weight loss journey with Wegovy or Mounjaro, two groundbreaking weight loss medications that mimic the GLP-1 and GIP hormones, respectively, to help control cravings, delay stomach emptying, and keep you feeling fuller for longer. These clinically backed treatments help members experience up to 23% body weight loss.

Plus, we have a team of expert dietitians and health coaches you can consult with to help you work through any challenges you're experiencing during your weight management journey. This approach sets you up for sustained weight loss, rather than a quick-fix solution.

This means you can continue to enjoy the things you love in moderation without ever feeling deprived.

Ready to see if Juniper is right for you? You can check your eligibility here.

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