How Were Calories Discovered and Quantified?

November 16, 2025 •

3 min read

The modern concept of a calorie has its roots in 18th-century chemistry and physics, long before it became a household term for dieting. The story of how were calories discovered involves key scientific developments in the measurement of heat, the study of combustion, and the understanding of energy conversion in both chemical reactions and biological organisms.

Quick Summary

The discovery of calories traces back to Antoine Lavoisier's 18th-century experiments with ice calorimeters and respiration, establishing the link between combustion and biological energy production. Later, French chemist Nicolas Clément named and formally defined the unit for engineers in the 1820s.

Key Points

Early Calorimetry: Quantifying heat began with Antoine Lavoisier and Pierre-Simon Laplace using an ice calorimeter in the 1780s.
Respiratory Combustion: Lavoisier's experiments showed animal respiration was a slow combustion process, foundational to understanding metabolism.
Formal Definition: Nicolas Clément first defined the calorie unit in the 1820s, initially for engineers.
Food Energetics: Max Rubner applied calorimetry to nutrition, confirming food energy corresponds to animal metabolism.
Bomb Calorimeter: Developed by Pierre Berthelot and applied to food by Wilbur Atwater, this device precisely measures food energy.
Kilocalorie vs. Calorie: Confusion exists between the small calorie (gram-calorie) and the large Calorie (kilocalorie), used for food energy.
Atwater System: Wilbur Atwater developed the system for assigning average caloric values to macronutrients, still used today.

The Founding Fathers of Calorimetry

Joseph Black and Antoine Lavoisier

Scientific interest in heat and energy grew in the 18th century. Scottish chemist Joseph Black introduced the concepts of latent and specific heat around 1761, paving the way for later calorimetry. Building on this, French chemist Antoine Lavoisier and mathematician Pierre-Simon Laplace created the first ice calorimeter in the 1780s. This device measured heat by quantifying melted ice. They used it to show that respiration in a guinea pig was similar to combustion, comparing the animal's heat production to burning charcoal. Lavoisier coined the term 'calorimeter'.

The First Definition: Nicolas Clément

While Lavoisier provided the experimental basis, the term 'calorie' was formally defined by French chemist Nicolas Clément between 1819 and 1824. In lectures, he defined the calorie as the heat needed to raise the temperature of 1 kilogram of water by 1°C. This definition, corresponding to the modern kilocalorie, circulated among engineers and was later recognized in dictionaries, though it created confusion with the smaller calorie unit.

Refinement and Expansion in the 19th Century

The Mechanical Equivalent of Heat

In the 1840s, James Prescott Joule showed that mechanical work could generate heat, establishing the conservation of energy and demonstrating that heat is a form of energy. His work proved energy could be converted between forms.

The Birth of Nutritional Science

Applying calorimetry to nutrition advanced with German physiologists like Carl Voit and his student Max Rubner in the 1880s. Rubner built an advanced calorimeter and showed that the energy from burning food matched the energy from animal metabolism. His work solidified the understanding that bodies get energy from food's caloric content, establishing modern nutritional science.

Wilbur O. Atwater and the Bomb Calorimeter

The bomb calorimeter, developed by Pierre Eugene Berthelot in the 1870s, precisely measures food's energy content. It works by igniting a food sample in a sealed container within a water bath and measuring the water's temperature change. American chemist Wilbur Olin Atwater popularized this for nutrition in the late 19th century, analyzing many foods and developing the Atwater system of assigning caloric values to macronutrients.

The Lingering Confusion of 'Calorie'

Confusion has persisted due to the 'small calorie' (cal) and the 'large calorie' (Cal or kcal).


Feature	Small Calorie (cal)	Large Calorie (kcal or Cal)
Definition	Heat for 1 gram of water by 1°C.	Heat for 1 kilogram of water by 1°C.
Relationship	1000 small calories = 1 large calorie.	1 large calorie = 1000 small calories.
Usage Context	Chemistry/physics, largely replaced by joule.	Nutrition, simplified to "calorie" on labels.
Energy Equivalence	4.184 joules.	4.184 kilojoules.

Marcellin Berthelot suggested using a capitalized 'Calorie' for the larger unit in the 1870s. This is common today, but scientists often prefer the joule (J), the SI unit for energy.

Conclusion: A Legacy of Measurement

From early ice calorimeters to modern bomb calorimeters, the discovery and measurement of calories have been vital to understanding heat, energy, and nutrition. The term's evolution reflects the interdisciplinary nature of science. Despite some historical confusion in terminology, the ability to quantify food energy remains a cornerstone of nutrition science.

Further Research

For those interested in a more in-depth look at the evolution of food energy units, including the work of Atwater, the following article provides extensive historical context: Does the history of food energy units suggest a solution to 'calorie confusion'?.

Frequently Asked Questions

A calorie (cal) raises 1 gram of water by 1°C. A kilocalorie (kcal or Cal) raises 1 kilogram of water by 1°C. Food labels use kilocalories but call them calories.

Antoine Lavoisier and Pierre-Simon Laplace invented the first ice calorimeter in the 1780s.

Early scientists like Max Rubner measured heat from burning food or from animal respiration to link food's chemical energy to metabolism.

A bomb calorimeter is a device that measures the heat of combustion of a substance, like food, by igniting it in a sealed chamber and measuring the temperature change of a water bath.

Food label calories use the Atwater system's average macronutrient values, which don't account for individual digestion or metabolism variations, fiber content, or non-energy nutrient use.

The link between calories and nutrition strengthened in the late 19th century through the work of scientists like Max Rubner and Wilbur O. Atwater.

Lavoisier and Laplace studied respiration in a guinea pig to show it was a process like combustion, comparing its heat and gas exchange to burning charcoal to understand metabolism.