Understanding kilojoules

Before you can lose weight, gain weight or even understand how weight works, you need to know about kilojoules. Kilojoules are one of the greatest determining factors of your weight, and understanding their role in nutrition is the first – and biggest – step towards achieving your weight loss or specific diet goals.

What are kilojoules?

Living beings need to consume a certain amount of energy each day in order for their bodies to function. This energy provides the body with the fuel it needs to perform vital bodily tasks, such as sustaining the organs, repairing damage and creating new cells. Bodies also require energy to perform non-vital functions, such as daily physical activities and exercise.

The energy we require is obtained from the food and beverages we consume, and is known as kilojoules (kJ). All food is made up of kilojoules, which in turn are made up of proteins, carbohydrates and fats. By consuming food, kilojoules enter our bodies, and our organs metabolise (burn) these kilojoules to provide the energy we require.

But I thought food was made up of calories!

Yes, it is. Many people prefer to think of kilojoules in terms of calories (technically called kilocalories, but commonly referred to as calories), and 1 calorie = 4.18kJ.

Kilojoules (or calories) give an indication of the amount of energy a specific food or drink contains. Thus, if you know how many kilojoules (or calories) you should consume each day, you can easily ensure that you take in the right amount, by knowing the nutritional value of the foods you’re eating. In packaged items, these values are usually displayed on the wrapping. Otherwise, you can find the nutritional value of foods and beverages on this site.

To work how many calories are in food, bear in mind that:

1g of protein = 4 calories

1g of carbohydrates = 4 calories

1g of fat = 9 calories

Thus, if you know how many grams of protein, carbohydrates and fat are in the food you’re eating, you can work out how many calories it has. Use this kilojoule to calorie calculator to convert the number of kilojoules in food into calories:

How many kilojoules do I need to consume each day?

The amount of kilojoules you need to consume each day depends on whether you are eating for weight loss, weight gain, or weight maintenance. The average healthy adult requires 8 700kJ per day, but this differs between men and women (men generally require more energy, due to having a higher percentage of muscle), and from person to person. It’s also influenced by how much physical activity you do, which is why exercise is such an important part of weight loss.

Also, people require greater or lesser amounts of energy at different stages of their lives. For example, children and teenagers need greater amounts of energy in order to fuel their growth and development, while women who are pregnant or breastfeeding also need more energy. Meanwhile, older people who are less active, and people who lead sedentary lives, require less energy each day.

Consider this rough guideline:

  • To maintain her current weight, a woman aged 35 years, 1.62m tall, weighing 60kg, requires 7 600kJ per day if she is sedentary, and 9 800kJ if she is active.
  • To maintain his current weight, a man aged 35 years, 1.88m tall and weighing 80kg, requires 10 400kJ per day if he is sedentary, and 13 400kJ per day if he is active.

As you can see, the amount of daily kilojoules required is based on several factors, including age, height, current weight and level of activity. It’s important to consult a qualified dietician or nutritionist to determine the precise number of kilojoules you require for your weight or fat loss goals.

How do kilojoules contribute to weight gain or weight loss?

When we consume more kilojoules than our body requires, they are stored as fat, and we begin to gain weight. When we consume fewer kilojoules than our bodies require, our organs begin to burn existing fat reserves, and we lose weight.

A single kilogram of fat contains 37 000kJ, or 8 850 calories. You would need to burn this additional 37 000kJ in order to lose each extra kilogram of fat.

How can I tell how many kilojoules are in the food I eat?

By law, all packaged products must list their ingredients and nutritional content. The nutritional content shows the amount of protein, carbohydrates and fat in the product, and usually the kilojoule content as well. However, even if it doesn’t show the kilojoule content, you can work it out using the calorie conversions discussed earlier. Then simply multiply the calorie content by 4.18 to find out how many kilojoules it contains.

Better yet, put your calculator away and simply make use of the kilojoules and calorie counter charts on this site to work out the energy value of the food and beverages you consume. Alternatively, use our calorie calculators to convert kilojoules to calories (and vise versa).

But before you start counting kilojoules, be sure that you do in fact need to lose weight. Enter your height and weight into our BMI calculator, and instantly find out whether you need to lose weight, gain weight, or stay just as you are.