What is a unit of food energy measurement called?

January 10, 2026 •

3 min read

Food energy is a measure of the energy content found in food and is most commonly quantified using the unit of a calorie or a kilojoule. Every day, the average adult human body requires a specific amount of this energy to function, with some dietary guidelines estimating this to be around 2,000 kilocalories. This energy powers all biological processes, from basic functions like breathing to more intensive activities like exercise.

Quick Summary

The most common units of food energy are the kilocalorie (often referred to as a "calorie") and the kilojoule, with terminology varying by region and context. The kilocalorie is equivalent to 1,000 small calories, while the kilojoule is the standard scientific (SI) unit, related to the joule.

Key Points

Calorie vs. Kilocalorie: In nutrition, the term "calorie" is commonly used to refer to a kilocalorie (1,000 small, scientific calories).
Kilojoule is the Standard: The kilojoule (kJ) is the official SI unit for measuring food energy, especially in countries like Australia and parts of Europe.
Conversion is Simple: The conversion is a fixed ratio, with 1 kilocalorie equaling 4.184 kilojoules.
Energy from Macronutrients: Fats provide the most energy per gram (9 kcal), followed by alcohol (7 kcal), and then protein and carbohydrates (4 kcal each).
Atwater System for Labels: Food energy values on most labels are estimated using the Atwater system, which applies average energy values to each macronutrient.

Understanding the Calorie vs. Kilocalorie Confusion

To fully answer the question of what a unit of food energy measurement is called, it is necessary to clear up the widespread confusion between the terms 'calorie' and 'kilocalorie'. In casual conversation, the word "calorie" is almost always used to refer to the larger unit of energy, the kilocalorie (kcal). For example, when a nutrition label lists a serving of food as containing 250 "calories," it is actually referring to 250 kilocalories. The smaller, scientific-grade "calorie" (with a lowercase 'c' and symbol cal) is the energy required to raise the temperature of one gram of water by one degree Celsius. Since this is a very small unit, the larger kilocalorie (with a capital 'C' or symbol Cal in some contexts) was adopted for nutritional purposes, requiring 1,000 of the smaller calories.

The Kilojoule: The Scientific Standard

While the kilocalorie is common in countries like the United States and the United Kingdom, the rest of the world and the broader scientific community tend to use kilojoules (kJ). The joule (J) is the standard (SI) unit of energy, and a kilojoule is simply 1,000 joules, making it a more universal measure. Many food labels in the European Union and Australia, for example, list energy in both kilojoules and kilocalories to avoid confusion. The conversion is straightforward: one kilocalorie is equivalent to 4.184 kilojoules.

How Food Energy is Measured

Measuring the energy content of food is a science that dates back to the 19th century and has evolved into two primary methods: direct and indirect calorimetry.

Direct Calorimetry: This method involves a "bomb calorimeter," a device that burns a food sample completely. The heat released during this process is used to raise the temperature of a surrounding volume of water. The change in water temperature directly indicates the food's energy content. While highly accurate, this method is too expensive and impractical for widespread use in food labeling.
Indirect Calorimetry (The Atwater System): This is the more common method for determining the energy content on nutrition labels. Developed by Wilbur Olin Atwater, this system assigns a standard energy value to each of the three main macronutrients: carbohydrates, protein, and fat. By analyzing the weight of each macronutrient in a food and applying these conversion factors, the total energy content can be calculated without burning the product. The rounded values are typically 4 kcal per gram for protein and carbohydrates, and 9 kcal per gram for fat.

Comparison of Food Energy Units


Feature	Calorie (kcal)	Kilojoule (kJ)
Equivalence (to other units)	1 kcal = 1,000 small calories	1 kJ = 1,000 joules
Conversion (to each other)	1 kcal = 4.184 kJ	1 kJ = 0.239 kcal
Geographic Usage	Primarily US, UK, and Canada	Primarily Europe, Australia, and New Zealand
Origin	Derived from the heating of water	SI unit based on work done
Nutritional Labels	Often labeled as "Calories"	Often labeled as "kJ"

Macronutrients and Energy

Not all sources of energy are created equal. The total energy in food is derived from the breakdown of macronutrients.

Fats: Provide 9 kcal (37 kJ) per gram. They are the most energy-dense macronutrient.
Alcohol: Contains 7 kcal (29 kJ) per gram.
Carbohydrates and Protein: Both supply 4 kcal (17 kJ) per gram.
Fiber: A type of carbohydrate that is not easily digested and provides less energy, around 2 kcal (8 kJ) per gram.

It is important to remember that these values are averages, and the actual energy a person absorbs can vary based on individual differences in digestion and metabolism.

Conclusion: Navigating Food Energy Terminology

In summary, the most common unit of food energy measurement is the kilocalorie (kcal), but it is colloquially referred to as a "calorie". The other major unit is the kilojoule (kJ), which is the official scientific unit used in many countries. Both measure the same thing: the energy potential of food. While the terminology can be confusing, the key takeaway is that when discussing nutrition in daily life, "calorie" refers to the larger kilocalorie. For scientific contexts or in regions that use the metric system more broadly, kilojoules are the standard. Understanding this distinction empowers consumers to accurately interpret food labels and make informed decisions about their dietary intake.

Learn more about calculating your personal energy requirements at the official Eat For Health website.

Frequently Asked Questions

A calorie (lowercase c) is a scientific unit equal to the energy needed to raise the temperature of 1 gram of water by 1°C. A kilocalorie (kcal), which is the unit typically listed on food labels and referred to as a "calorie" in common parlance, is 1,000 times larger, equal to the energy needed to raise the temperature of 1 kilogram of water by 1°C.

Globally, usage varies. In the United States and Canada, the kilocalorie (informally called a "calorie") is the most common. In Australia, New Zealand, and most of Europe, kilojoules (kJ) are the standard.

Some countries and regions use kilojoules (kJ) because it is the official unit of energy in the International System of Units (SI), providing a more globally standardized measurement system.

To convert kilojoules (kJ) to kilocalories (kcal), you can divide the kilojoule value by 4.184. For example, a product with 837 kJ of energy contains approximately 200 kcal.

Calorie counts on food labels are most often determined using the Atwater indirect system. This method calculates total energy by adding up the standardized calorie values for the protein, fat, and carbohydrates listed on the label.

No. The energy values on nutrition labels are estimates based on standard conversion factors. The actual amount of energy your body extracts can vary based on individual differences in digestion, absorption, food preparation, and other factors.

Fat provides the most energy per gram, at 9 kcal (37 kJ). Protein and carbohydrates both provide 4 kcal (17 kJ) per gram.