How to find kJ per gram using food labels and macronutrients

December 25, 2025 •

3 min read

Did you know that fat provides over twice the kilojoules per gram compared to carbohydrates and protein? Learning how to find kJ per gram is a fundamental skill for understanding a food's energy density and making informed dietary decisions.

Quick Summary

A food's kilojoules per gram (kJ/g) indicates its energy density. This value can be calculated from a nutrition label using macronutrient values or by referring to the 'per 100g' column directly.

Key Points

kJ/g vs. kJ/serving: Use the 'per 100g' value to compare the energy density of different foods, as 'per serving' sizes can be inconsistent.
Macronutrient Factors: Remember that fat provides significantly more kilojoules per gram (37 kJ/g) than protein or carbohydrates (17 kJ/g).
Calculation Method: Multiply the grams of fat by 37, protein by 17, and carbohydrates by 17 (per 100g) and sum them to find the total kilojoules.
Direct Reading: For packaged goods, the simplest method is to read the kilojoule value listed in the 'per 100g' column of the nutrition information panel.
Healthier Choices: Focus on low-energy-dense foods like vegetables and lean proteins to feel full and satisfied while managing your overall kilojoule intake.
Conversion: For converting between units, remember that 1 kilocalorie (Calorie) equals 4.184 kilojoules.
Weight Management: Understanding energy density helps make better food swaps; for instance, a nutritionally dense avocado may have similar total kilojoules to a chocolate bar but offers more health benefits.

Understanding Kilojoules and Energy Density

Energy is required to fuel all of our body's functions, and this energy comes from the food and drinks we consume. The standard metric unit for measuring this energy is the kilojoule (kJ). However, simply looking at the total kilojoule count for a serving can be misleading, as manufacturers define serving sizes differently. A more accurate metric is the food's energy density, which measures the amount of energy per gram or milliliter (kJ/g). By understanding how to find kJ per gram, you can more accurately compare different foods and make healthier choices.

Method 1: Calculating kJ/g from Macronutrients

If a nutrition information panel only provides the grams of macronutrients (carbohydrates, protein, and fat), you can calculate the kJ per gram yourself. This is based on internationally recognized conversion factors known as the Atwater system.

To perform this calculation, follow these steps:

Locate the nutrition panel: Find the 'per 100g' column on the food's nutrition label.
Identify macronutrient values: Note the grams of fat, protein, and carbohydrates per 100g.
Apply conversion factors: Multiply the grams of each macronutrient by its specific energy value in kilojoules:
- Fat: grams of fat × 37 kJ/g
- Protein: grams of protein × 17 kJ/g
- Carbohydrates: grams of carbohydrates × 17 kJ/g
Sum the values: Add the kilojoule results from the previous step to get the total kilojoules per 100g.
Calculate kJ per gram: Divide the total kJ by 100 to find the kJ/g. For example, if a food has 1500 kJ per 100g, its energy density is 15 kJ/g.

Method 2: Using the 'Per 100g' Value Directly

For many packaged foods, the nutrition information panel directly lists the energy content in kJ, typically in a 'per 100g' column. This value is the easiest way to find the energy density. Simply locate the 'Energy' row and find the kilojoule number listed under 'per 100g'. This number is the food's energy density in kilojoules per gram, without any further calculation required. This method is the most straightforward for quick comparisons between different products.

Macronutrient Energy Factor Comparison

Understanding the relative energy density of each macronutrient is key to interpreting the overall energy content of foods.


Macronutrient	Approximate kJ per Gram	Approximate kcal per Gram
Fat	37 kJ/g	9 kcal/g
Protein	17 kJ/g	4 kcal/g
Carbohydrates	17 kJ/g	4 kcal/g
Alcohol	29 kJ/g	7 kcal/g
Fibre	8 kJ/g	2 kcal/g
Polyols	10 kJ/g	2.4 kcal/g

Practical Application for Better Health

Applying the concept of kJ per gram can significantly improve your dietary choices, especially for weight management. Energy-dense foods, like many processed snacks high in fat and sugar, pack a lot of kilojoules into a small serving, which can easily lead to overconsumption. In contrast, low-energy-dense foods, such as fruits, vegetables, and lean protein, provide fewer kilojoules for the same volume, helping you feel fuller on less energy.

Consider this comparison:

High-Energy Density Meal: A plate of fried chicken and chips. The high fat content from the frying oil and the processed nature of the food results in a very high kJ/g value, meaning a small plate carries a large energy load.
Low-Energy Density Meal: A large salad with grilled chicken breast and a light vinaigrette. The meal is primarily composed of water-rich vegetables and lean protein, resulting in a much lower kJ/g, which allows for a larger, more satisfying portion for similar kilojoules.

For detailed nutritional information beyond what's on the label, authoritative resources like national food composition databases can be invaluable. For instance, the Food Standards Australia New Zealand food composition database provides comprehensive data for many foods, including fresh produce.

Conclusion

Knowing how to find kJ per gram is a powerful tool for navigating food labels and understanding the true energy content of what you eat. By moving beyond misleading 'per serving' values and focusing on energy density, you can make more informed choices that align with your health and wellness goals. Whether calculating from macronutrients or checking the 'per 100g' value, this practice offers a clearer picture of your dietary intake.

Frequently Asked Questions

Both kilojoules (kJ) and kilocalories (kcal or Calories) are units of energy measurement in food. Kilojoules are the standard metric unit, while kilocalories are commonly used in the United States. The conversion factor is 1 kcal = 4.184 kJ.

Comparing foods using kJ per gram is more accurate because manufacturers can set different and often smaller serving sizes to make their products appear lower in energy. The 'per 100g' value provides a standardized reference point for comparison.

For unpackaged items like fresh fruit, you can look up their macronutrient composition in a national food composition database. Then, use the Atwater conversion factors (17 kJ/g for carbs, 37 kJ/g for fat) to calculate the energy density based on the food's typical nutritional profile.

Yes, both dietary fiber and alcohol contribute to the kilojoule count. Fiber provides approximately 8 kJ/g, while alcohol provides a significant 29 kJ/g.

A high kJ per gram value indicates that a food is 'energy-dense,' meaning it contains a large amount of energy in a small volume or weight. These foods often have a higher fat content and may be less satiating.

A low kJ per gram value suggests that a food is 'low-energy-dense,' meaning it contains less energy per unit of weight. These foods are typically high in water and fiber, like fruits and vegetables, which can help promote feelings of fullness.

Yes, to calculate the total kJ for a whole package, find the total net weight of the product in grams. Multiply this weight by the food's kJ per gram (the 'per 100g' value divided by 100) to get the total kilojoules in the entire container.

Yes, understanding kJ per gram is very helpful for weight loss. By choosing foods with a lower energy density, you can consume a larger volume of food to feel satisfied while managing your total energy intake, which is crucial for creating a kilojoule deficit.