Calorie

The calorie is a unit of energy that comes from old ideas about heat. It tells us how much energy is in food and how our bodies use that energy. The small calorie measures the energy needed to raise the temperature of a tiny amount of water by one degree. In everyday life, especially when we talk about food, the word “calorie” usually means a larger amount of energy called a kilocalorie. This is why food labels show calories — they tell us how much energy we get from eating.

In science, the calorie is not the main unit we use anymore. Scientists now prefer to use the joule, which is the standard unit of energy. But the calorie is still very important in nutrition and food science. It helps us understand how much energy different foods give us and how much our bodies need to stay healthy.

One small calorie is exactly equal to 4.184 joules, and a kilocalorie (or large calorie) equals 4184 joules. This helps scientists and doctors measure energy in food and understand how our bodies use it every day.

History

The word "calorie" comes from a Latin word meaning 'heat'. It was first used as a way to measure heat energy by a scientist named Nicolas Clément in the early 1800s. Later, other scientists started using the same word for a smaller amount of heat energy.

In time, some people began using a big "C" to show the larger amount of heat, which we now call a kilocalorie. This helped avoid confusion between the two different sizes of calories. Today, scientists mainly use a unit called the joule to measure energy, but the calorie is still sometimes used, especially when talking about food energy.

Definitions

The "small" calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. This definition can change a little depending on the air pressure and starting temperature of the water.

Before 1948, scientists used a value of 4.1833 international joules for this measurement. Today, the standard is 4.184 J, which keeps the meaning the same as before.

Name	Symbol	Conversions	Definition and notes
Thermochemical calorie	cal_th	≡ 4.184 J ≈ 0.003964 BTU ≈ 1.162×10⁻⁶ kWh ≈ 2.611×10¹⁹ eV	The amount of energy equal to exactly 4.184 J (joules) and 1 kJ ≈ 0.239 kcal.
4 °C calorie	cal₄	≈ 4.204 J ≈ 0.003985 BTU ≈ 1.168×10⁻⁶ kWh ≈ 2.624×10¹⁹ eV	The amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.
15 °C calorie	cal₁₅	≈ 4.1855 J ≈ 0.0039671 BTU ≈ 1.1626×10⁻⁶ kWh ≈ 2.6124×10¹⁹ eV	The amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure. Experimental values of this calorie ranged from 4.1852 to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.
20 °C calorie	cal₂₀	≈ 4.182 J ≈ 0.003964 BTU ≈ 1.162×10⁻⁶ kWh ≈ 2.610×10¹⁹ eV	The amount of energy required to warm one gram of air-free water from 19.5 to 20.5 °C at standard atmospheric pressure.
Mean calorie	cal_mean	≈ 4.190 J ≈ 0.003971 BTU ≈ 1.164×10⁻⁶ kWh ≈ 2.615×10¹⁹ eV	Defined as 1⁄100 of the amount of energy required to warm one gram of air-free water from 0 to 100 °C at standard atmospheric pressure.
International Steam Table calorie (1929)		≈ 4.1860 J ≈ 0.0039683 BTU ≈ 1.1630×10⁻⁶ kWh ≈ 2.6132×10¹⁹ eV	Defined as 1⁄860 "international" watt hours = 180⁄43 "international" joules exactly.
International Steam Table calorie (1956)	cal_IT	≡ 4.1868 J ≈ 0.0039683 BTU = 1.1630×10⁻⁶ kWh ≈ 2.6132×10¹⁹ eV	Defined as 1.163 mWh = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).

Usage

In North America, when talking about food and nutrition, people mainly use the "large" unit called the calorie, written with a lowercase "c" and the symbol "cal". Sometimes, scientists use a different unit called the kilojoule (kJ) instead. Most American nutrition experts like to use kilocalories, while many scientists prefer kilojoules. In most other countries, experts usually prefer kilojoules over kilocalories.

In Europe, food labels show energy in both kilojoules and kilocalories, written as "kJ" and "kcal". In China, only kilojoules are used.

This unit is often used to talk about the energy in food. For example, fats have 9 kilocalories per gram, while carbohydrates and proteins have about 4 kilocalories per gram. Alcohol in food has 7 kilocalories per gram. The large unit is also used to talk about how many calories people should eat each day.

Eating in a controlled way, known as dieting, can help decrease, maintain, or increase body weight, and prevent or treat diseases like diabetes and obesity. Since losing weight depends on eating fewer calories, different kinds of calorie-controlled diets can be effective.

In chemistry and physics, the term "calorie" usually means the smaller unit, with the large unit called a kilocalorie (kcal). It is used to measure the energy released in chemical reactions or changes in states of matter. However, the joule (J) and its multiples, like the kilojoule (kJ), are becoming more common.

History

Definitions

Usage

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