Why is food not measured in joules? A historical and practical look at calories

November 2, 2025 •

2 min read

A standard kilocalorie is equal to 4.184 kilojoules, yet most food labels, particularly in the US, use calories for nutrition labeling. So, why is food not measured in joules, the internationally recognized SI unit for energy, on many food labels?

Quick Summary

Food energy is measured in calories due to historical reasons stemming from early measurement techniques involving heat. The term became entrenched in public consciousness and regulation, persisting despite the joule's status as the standard SI energy unit.

Key Points

Historical Context: Calories were adopted because early nutritional science measured food energy by heating water, a concept easily translated into the calorie unit.
SI Standard vs. Tradition: The joule is the internationally recognized SI unit for energy, but the calorie's long-standing use in nutrition, particularly in the US, persists due to consumer familiarity and historical momentum.
Practicality and Perception: Calories often result in smaller numbers on food labels, which many consumers find easier to manage and understand than the larger numerical values of kilojoules.
Indirect Measurement: Modern food labels are typically calculated using the Atwater system, applying standard energy values to macronutrients, rather than direct calorimetry.
Global Compromise: Many countries use dual labeling, displaying both calories (kcal) and kilojoules (kJ) to inform consumers using both systems and to align with international standards.
Regulatory Inertia: Adherence to the calorie system in some regions is reinforced by regulatory policies and historical labeling practices, making a transition to joules a complex process.
Bomb Calorimetry Origin: Early food energy measurements involved burning food in a bomb calorimeter and measuring the temperature change of water, a process perfectly suited to the calorie's definition.

The Origins of the Calorie vs. Joule

The use of calories instead of joules in nutrition is largely due to historical factors. The calorie, defined in the early 19th century based on the heat required to raise the temperature of water, became the unit of choice for early food energy measurements. W.O. Atwater later popularized the "large" Calorie (kilocalorie) for nutritional science. The joule, a later SI unit defined by mechanical work, measures the same energy but had a different origin and application.

Early Nutritional Science and the Bomb Calorimeter

Early methods for determining food energy relied on the bomb calorimeter. This device measured the heat released when food was burned, directly using the calorie unit. This established the calorie's role in nutritional science.

Practicality, Perception, and Modern Calculation

Several factors explain the calorie's continued use:

Consumer Familiarity

People are widely familiar with calories for tracking food intake, making a switch to kilojoules (which yield larger numbers) potentially confusing. The simple concept of the calorie is well-understood.

The Atwater System: Indirect Calculation

Modern food energy values are often calculated using the Atwater system, which applies average calorie (and kilojoule) conversion factors to the macronutrient content of food. Key factors per gram are:

Carbohydrates: 4 kcal (16.7 kJ)
Protein: 4 kcal (16.7 kJ)
Fat: 9 kcal (37.6 kJ)
Alcohol: 7 kcal (29.3 kJ)

This system reinforces the use of calories in labeling.

Calorie vs. Kilojoule: A Comparison


Feature	Calorie (kcal)	Kilojoule (kJ)
Unit Type	Non-SI unit linked to heat.	Official SI unit linked to mechanical work.
Application	Primary in nutrition, especially in the US.	Used in science and required on labels elsewhere.
Magnitude	Larger unit, resulting in smaller numbers on labels.	Smaller unit (1 kcal ≈ 4.184 kJ), resulting in larger numbers.
Calculation Method	Often uses the Atwater system.	Also uses macronutrient equivalents, expressed in kJ.
Consumer Perception	Familiar and widely understood.	Less familiar to many, potentially confusing.

International Approaches and Dual Labeling

Many countries outside the US use dual labeling, showing both kilojoules and kilocalories on food packaging. This balances scientific standardization with consumer familiarity.

The Conclusion: Familiarity Trumps Standardisation

The main reasons why food is not exclusively measured in joules are the historical establishment of the calorie and its strong hold in consumer awareness, particularly in the US. Despite the joule being the standard SI unit, changing long-standing habits and regulations is challenging. Dual labeling in other regions serves as a compromise between global scientific standards and local practices. Understanding both units provides a more complete picture of food energy.

Frequently Asked Questions

A 'Calorie' with a capital 'C' refers to a kilocalorie (1,000 small 'calories'). The energy listed on food labels is always in kilocalories, but it is often colloquially referred to as just 'calories'.

The joule is the official SI unit for all energy, defined in terms of fundamental physical properties like force and distance. It is not specifically tied to heat, making it a universal standard for various energy types.

Most food calorie counts are calculated indirectly using the Atwater system, which applies average energy values to the macronutrients (protein, fat, carbohydrates) in a food product.

Many countries use dual labeling to accommodate both consumers familiar with the traditional calorie system and those who prefer the official SI unit, the kilojoule.

Yes, it is possible and already done in many countries. Since calories and joules both measure energy, it is a straightforward conversion (1 kcal = 4.184 kJ).

A complete switch to joules is possible in the long term, especially as global standards become more integrated. However, the deep-seated cultural and regulatory history of the calorie makes any such transition a slow and gradual process.

Yes, early nutritional scientists used a bomb calorimeter to burn food samples and directly measure the heat energy released, which formed the basis for caloric values.