Was ist ein/eine Calorie (cal)?
The calorie (symbol: cal), also known as the small calorie or gram calorie, is a unit of energy defined as the amount of heat needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. The thermochemical calorie is exactly 4.184 joules.
Small Calorie vs. Food Calorie
The small calorie (cal, lowercase) must be distinguished from the kilocalorie or food Calorie (Cal or kcal), which equals 1,000 small calories. When nutrition labels say "Calories" (capitalized), they mean kilocalories. A food item listed as "200 Calories" contains 200,000 small calories or 200 kilocalories. This ambiguity is a persistent source of confusion.
Multiple Definitions
Several slightly different calorie definitions exist: the thermochemical calorie (exactly 4.184 J), the International Table calorie (exactly 4.1868 J), and the 15-degree calorie (approximately 4.1855 J, defined at 14.5-15.5 degrees Celsius). The thermochemical calorie is most common in chemistry, while the International Table calorie appears in engineering contexts.
Etymology
Latin Root
The word "calorie" comes from the Latin "calor" meaning heat. The term was introduced into French as "calorie" by physicist Nicolas Clement in 1824 during his lectures at the Conservatoire des Arts et Metiers in Paris. Clement used it to describe the unit of heat energy required to raise one kilogram of water by one degree Celsius — what we now call the kilocalorie.
Confusion from the Start
The calorie's history has been marred by confusion from its inception. Clement defined it as a kilogram-based unit, but later scientists redefined it on a gram basis, creating the small calorie. The old kilogram-based definition persisted as the kilocalorie or food Calorie. This nomenclature confusion, rooted in the 19th century, persists to this day.
Scientific Adoption and Displacement
The calorie was widely adopted in 19th-century chemistry and physics as a convenient unit of heat. It remained standard until the SI system designated the joule as the preferred energy unit in 1960. The calorie has since been gradually displaced in scientific contexts but remains culturally entrenched in nutrition.
Precise Definition
The thermochemical calorie is defined as exactly 4.184 joules. The International Table calorie is defined as exactly 4.1868 joules. In practice, the difference between these definitions is negligible for most purposes.
Key Conversions
1 cal = 4.184 J (thermochemical) = 0.004184 kJ = 0.001 kcal = 0.00116222 Wh = 0.003968 BTU. Inversely: 1 J ≈ 0.239 cal, 1 kJ ≈ 239 cal, 1 kcal = 1,000 cal exactly, 1 BTU ≈ 252 cal.
Temperature Dependence
The original experimental calorie — the heat to raise 1 gram of water by 1 degree Celsius — varies slightly with temperature because water's specific heat capacity varies. At 15 degrees Celsius, it is about 4.1855 J; at 20 degrees, about 4.182 J. The thermochemical calorie (4.184 J) was fixed to avoid this ambiguity.
Geschichte
Nicolas Clement (1824)
French physicist Nicolas Clement introduced the calorie concept in his lectures in 1824, defining it as the heat needed to raise one kilogram of water by one degree Celsius. This kilogram calorie is what we now call the kilocalorie.
Adoption in Chemistry
In the mid-19th century, chemists adopted a gram-based calorie (the "small calorie") as more convenient for laboratory work, since experiments typically involved grams rather than kilograms of material. This created the persistent confusion between the gram calorie and the kilogram calorie.
Wilbur Atwater and Food Calories
American chemist Wilbur Atwater, in the 1890s, conducted calorimetry experiments that established the caloric values of proteins, fats, and carbohydrates (the Atwater system). His work popularized the kilocalorie (using the capitalized "Calorie") as the unit for food energy. This convention became standard in American nutrition.
The Joule's Challenge
The SI system designated the joule as the standard energy unit in 1960, and the CGPM has recommended abandoning the calorie in favor of the joule since 1948. Despite this, the calorie has proven remarkably resilient, surviving in nutrition, chemistry, and everyday speech. The transition from calories to joules remains incomplete six decades after SI adoption.
Aktuelle Verwendung
Chemistry
The calorie (small calorie) still appears in chemistry, particularly in thermochemistry and calorimetry. Specific heat capacities are sometimes given in cal/(g·°C), where water's value is approximately 1 cal/(g·°C) by definition — a convenient reference point. However, SI units (J, kJ) are preferred in modern chemical literature.
Nutrition Reference
The food Calorie (kilocalorie) dominates nutrition in the US and many other countries. The small calorie itself rarely appears on food labels but is embedded in the food Calorie system. When a dietitian says "count your calories," they mean kilocalories, not small calories.
Historical Scientific Literature
Older scientific papers, textbooks, and reference works (pre-1970s) frequently use calories. Researchers reading historical literature must be comfortable converting between calories and joules. Bond energies, heats of reaction, and specific heats may be given in calories in older sources.
Calorimetry
The science of calorimetry (measuring heat) retains "calorie" in its very name, even when modern calorimeters report results in joules. Bomb calorimeters, differential scanning calorimeters, and isothermal titration calorimeters are named for the calorie but typically output data in joules.
Everyday Use
The small calorie (cal) is rarely used directly in everyday life — the food Calorie (kcal) dominates. However, understanding the small calorie helps make sense of the nutritional unit.
The Water Connection
The calorie is most intuitive in terms of heating water: 1 calorie raises 1 gram of water by 1 degree Celsius. To heat a cup of water (about 250 g) from room temperature (20°C) to boiling (100°C) requires about 20,000 calories = 20 kcal. This simple relationship is why the calorie was originally defined.
Energy in Perspective
One small calorie is a very small amount of energy. A match head burning releases about 1,000 calories (1 kcal). A candle flame produces about 20 calories per second. A person at rest generates about 20 calories per second of body heat.
Scientific Literacy
Understanding the distinction between cal and kcal is part of basic scientific literacy. When a textbook states that the heat of vaporization of water is 540 cal/g, it means 540 small calories per gram (= 2,260 J/g). When a food label says 200 Calories, it means 200,000 small calories.
In Science & Industry
Thermochemistry
Historically, thermochemical data was tabulated in calories. The heat of combustion of methane is 213 kcal/mol (891 kJ/mol), and bond dissociation energies were historically given in kcal/mol. The relationship 1 kcal/mol ≈ 4.184 kJ/mol is one of chemistry's most-used conversion factors.
Specific Heat Capacity
Water's specific heat capacity of 1 cal/(g·°C) is the defining property of the calorie. Other materials are often compared to this reference: aluminum has a specific heat of 0.215 cal/(g·°C), iron 0.108 cal/(g·°C), and ethanol 0.58 cal/(g·°C). In SI, water's specific heat is 4.184 J/(g·°C).
Calorimetric Methods
Calorimetry — the measurement of heat in chemical reactions and physical changes — is named after the calorie. A bomb calorimeter measures heat of combustion, a differential scanning calorimeter measures phase transitions, and an isothermal titration calorimeter measures binding energetics. While modern instruments output joules, the field's nomenclature reflects its calorie-era origins.
Atmospheric Science
Solar radiation and atmospheric energy budgets are sometimes expressed in calories per square centimeter per minute (the "langley" = 1 cal/cm²). The solar constant is about 2 cal/(cm²·min) or 1.96 langleys per minute. This CGS-era unit persists in some atmospheric science databases.
Interesting Facts
The calorie was almost replaced by the joule in all contexts when the SI system launched in 1960, but nutrition proved resistant — over 60 years later, calories still dominate food labels worldwide.
One small calorie (4.184 J) is roughly the energy of a housefly in flight — enough to raise 1 gram of water by 1 degree Celsius but not much else.
The confusion between calories and kilocalories has led to at least one documented medical error: a patient was prescribed 2,000 calories per day but received 2,000 kilocalories — the correct amount happened to match, but the unit confusion could have been dangerous.
Water's specific heat capacity of 1 cal/(g·°C) is unusually high compared to most substances, which is why water is so effective at moderating temperatures in climate, cooking, and cooling systems.
Wilbur Atwater, who established food calorie values in the 1890s, spent days sealed inside a human calorimeter — a room-sized device that measured all the heat his body produced while eating specific diets.
The calorie is one of only a few pre-SI units that remains in active use despite being officially deprecated since 1948 — a testament to the power of cultural momentum in measurement systems.
A single raisin contains about 1,090 small calories (1.09 kcal), meaning you could theoretically heat about 1 liter of water by just over 1 degree Celsius by burning one raisin.
The energy to evaporate one gram of water (540 calories) is about 2.5 times the energy to heat it from freezing to boiling (100 calories), which is why sweating is such an effective cooling mechanism.
Regional Variations
Universal Chemistry Unit
The small calorie (cal) appears in chemistry worldwide, though it is being gradually replaced by the joule. Both units are understood internationally.
Nutritional Confusion
In the US, "Calorie" (capitalized) on food labels means kilocalorie. In the EU, labels show both kJ and kcal. In Australia and New Zealand, kJ is primary. In most Asian countries, kcal appears on labels. The small calorie (cal) itself almost never appears on food packaging.
Scientific Publishing
Most scientific journals now require SI units (joules), but many allow calories in parentheses for historical comparison. Some chemistry and biochemistry journals still accept cal or kcal as primary units, particularly for thermochemical data that has been traditionally tabulated in calories.