What Does Energy Per 100g Mean? Your Comprehensive Guide

November 1, 2025 •

3 min read

According to the World Health Organization (WHO), maintaining energy intake and expenditure balance is crucial for a healthy lifestyle. The figure for what does energy per 100g mean on food labels is a critical metric for achieving this balance and making informed dietary decisions.

Quick Summary

Energy per 100g is a standardized food label metric that indicates a product's caloric value for a fixed weight, allowing for accurate comparison between items. This metric, displayed in both kilojoules (kJ) and kilocalories (kcal), helps consumers understand energy density and make informed dietary choices for better health and weight management.

Key Points

Standardized Metric: Energy per 100g is a standard measurement on food labels, offering a consistent way to compare the energy density of different products.
Two Units: Energy is displayed in both kilojoules (kJ) and kilocalories (kcal), often simply called 'calories', with 1 kcal roughly equivalent to 4.184 kJ.
Macronutrient Basis: The energy value is derived from the carbohydrates, protein, and fat in the food, with fat providing significantly more energy per gram than the other two.
Beyond Serving Size: Focusing on the 'per 100g' column helps avoid confusion caused by variable or small manufacturer-defined serving sizes, ensuring a true comparison.
Informed Choices: Use the energy per 100g value to gauge a food's energy density, which can guide choices towards healthier, less energy-dense options.
Healthier Eating: Combining the 'per 100g' energy data with information on fat, sugar, and fibre content helps in assessing overall product healthiness.
Weight Management: Lowering the average energy density of your diet by choosing more low-energy-dense foods can help manage overall energy intake.

The phrase "energy per 100g" is a foundational piece of information found on the nutrition labels of packaged foods. It represents the energy density of a product—that is, the number of calories and kilojoules contained within a standard, 100-gram mass of that food. Understanding this figure is key to effectively comparing different products and making healthier dietary choices. While serving sizes can vary between brands and product types, the 100g figure provides a consistent, apples-to-apples comparison.

Deciphering Energy Units: Kilojoules and Kilocalories

Food labels typically list energy in two different units: kilojoules (kJ) and kilocalories (kcal), often referred to simply as 'calories' by consumers.

Kilocalories (kcal): A kilocalorie is the amount of heat required to raise the temperature of one kilogram of water by one degree Celsius. For nutrition purposes, the term 'calorie' (with an uppercase 'C') is often used interchangeably with kilocalorie. One gram of fat contains about 9 kcal, while one gram of protein or carbohydrate contains about 4 kcal.
Kilojoules (kJ): The kilojoule is the official SI (International System of Units) unit for energy, and it is the standard measure in many countries. To convert between the two, you can remember that approximately 1 kcal is equal to 4.184 kJ. Food labels often show both values, making it easy to see the energy content regardless of your preferred unit.

The Role of Macronutrients

The energy provided by food comes from three main macronutrients: carbohydrates, protein, and fat. Each of these has a different energy density, which directly impacts the energy per 100g figure. A food's overall energy density is determined by the ratio of these macronutrients. For example, a food high in fat will have a much higher energy per 100g than a food rich in water and carbohydrates, because fat is the most energy-dense macronutrient.

How to Use the 'Per 100g' Figure for Comparison

The 'per 100g' column on a nutrition label is your most reliable tool for comparing similar products.

Standardized Comparison: Since serving sizes can be misleadingly small, comparing the energy per 100g removes this variable. For instance, a packet of cookies and a granola bar may have very different serving sizes, but comparing their 'per 100g' values gives a true indication of which is more energy-dense.
Product Choices: For those monitoring energy intake, using the 'per 100g' value helps identify products that offer more nutritional value for fewer calories. A lower energy per 100g often indicates a higher water or fibre content and can support weight management goals.

Practical Tips for Reading Labels

Always check the 'per 100g' column to compare products accurately, especially for similar food types.
Use the 5/20 rule to quickly assess fat and sugar content. Less than 5g per 100g is low, and more than 20g per 100g is high.
Recognize that food with high water content (like vegetables) will have a lower energy density, which can help promote satiety and reduce overall energy intake.

Energy Density Comparison Table

Here's a comparison to illustrate the difference in energy density across different food types. This table helps to visualize why a consistent 100g metric is so valuable.


Food Type	Example Food (Approx.)	Energy (kcal per 100g)	Energy (kJ per 100g)	Notes
Very Low Energy Density	Broccoli (raw)	34	142	High water and fibre content
Low Energy Density	Yogurt (plain, low-fat)	60	251	Water-rich with moderate protein
Medium Energy Density	Bread (whole wheat)	247	1034	Higher carbohydrate and less water
High Energy Density	Dark Chocolate (70-85% cocoa)	546	2284	High fat content
High Energy Density	Nuts (cashews)	553	2314	Very high in fats

Conclusion

Understanding what energy per 100g means empowers you to make informed decisions and better manage your dietary health. By consistently comparing the 'per 100g' values on food labels, you can look past misleading serving sizes and marketing claims to accurately assess the nutritional density of products. This knowledge is a fundamental tool for anyone looking to control their energy intake, manage their weight, or simply make more educated and healthier choices at the supermarket.

For more detailed guidance on reading food labels and understanding UK regulations, you can visit the Food Standards Agency (FSA) website.

Frequently Asked Questions

Calories (kcal) and kilojoules (kJ) are both units of energy. In nutrition, they measure the same thing, but kilojoules are the official metric (SI) unit. One kilocalorie is equal to approximately 4.184 kilojoules.

The 'per 100g' figure provides a standardized amount, making it the most accurate way to compare the energy and nutrient content of different products, since manufacturers often use varying and sometimes misleadingly small serving sizes.

The balance of macronutrients directly determines a food's energy per 100g. Fat is the most energy-dense, providing 9 kcal per gram, compared to carbohydrates and protein, which provide 4 kcal per gram. Therefore, fat-rich foods have a higher energy per 100g.

Yes, it can be a very effective tool. Foods with a lower energy per 100g (low energy density) often contain more water and fibre, which can help you feel full on fewer calories, supporting your weight management goals.

Not necessarily. While often a good indicator, it's important to look at the full nutrition label. A food could be low in energy but high in sugar or sodium. Always assess the overall nutrient profile.

Yes, alcohol provides energy and is included in the total energy count. Per gram, alcohol provides about 7 kcal, which is more than carbohydrates or protein.

Water has zero energy (0 kcal/g), so foods with a high water content, like fruits and vegetables, will naturally have a lower energy density and a lower energy per 100g value.