Kilojoules (kJ) are just a unit for measuring energy, like how kilometres measure distance or litres measure volume. In nutrition, they tell you how much energy a food or drink gives your body, and how much energy your body uses when moving, breathing, or just existing.

What a kilojoule actually is

A kilojoule is part of the metric (SI) system and equals 1,000 joules. A joule is the basic scientific unit of energy, defined as the energy transferred when a force of one newton moves an object one metre. So a kilojoule is a more practical size for talking about food and activity, instead of tiny joules.

In everyday life, kilojoules are used on food labels (especially in countries like Australia and New Zealand) to show how much energy is in a serving of food or drink. If you eat more kilojoules than your body burns, the extra gets stored as fat; if you burn more than you eat, you lose weight.

Kilojoules vs calories

Kilojoules and calories are measuring the same thing – energy – but they’re different units, like miles vs kilometres.

  • 1 kilocalorie (what most people just call a “calorie” on food labels) ≈ 4.2 kilojoules
  • 1 kilojoule ≈ 0.24 kilocalories (or about 0.24 “calories”)

So, for a rough idea:

  • 100 kJ ≈ 24 calories
  • 400 kJ ≈ 96 calories
  • 1,000 kJ ≈ 239 calories

If you see a food label in kJ and want to convert to calories, divide the kJ number by 4.2 (or roughly by 4 for a quick estimate).

Where you’ll see kilojoules

  • Food labels – In many countries, the “Energy” line shows kilojoules (and sometimes calories too) per 100 g or per serving.
  • Exercise apps & trackers – They often show how many kJ you’ve burned during a workout.
  • Daily intake guides – Health sites give recommended daily kJ intakes based on age, sex, and activity level (e.g., ~8,000–10,000 kJ/day for an average adult).

Why energy matters

Your body runs on energy from food, and kilojoules are one way to keep track of that “fuel”. The total kJ you need each day depends on:

  • Your age, sex, and body size
  • How active you are (sedentary vs. very active)
  • Whether you’re pregnant, breastfeeding, or growing (kids/teens need more).

If you’re watching your weight, balancing the kJ in (from food/drink) with the kJ out (from metabolism and movement) is the basic idea behind energy balance.

TL;DR
Kilojoules (kJ) are a metric unit for energy, used on food labels and in nutrition to show how much energy food gives you and how much your body uses. 1 “calorie” (kcal) is about 4.2 kJ, so kJ and calories are just different ways of measuring the same thing.