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Convert Temperatures | Fahrenheit Degrees Celsius Kelvin

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The temperature units in use or still known today were introduced by various scientists in the course of the last 300 years. The respective temperature scales were named after them. Here we present the temperature units degrees Celsius (°C), Kelvin (K), degrees Fahrenheit (°F), degrees Rankine (°Ra) and degrees Réaumur (°Ré) as well as their conversion into the respective other temperature. You can easily convert all these temperatures into each other with the following temperature calculator.

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Further information on the most common temperature units

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General Information on Temperature Units

To define the temperature scales, two fixed points were usually defined that occur in nature and whose values can be reproduced by experiments. The respective scale distance, as the fineness of the division of the temperature scale between these two fixed points, was then evenly divided on the basis of a temperature­dependent substance or process property, such as mercury.

In the meantime, for inter­national unification, Kelvin has been established as the base unit for temperature in the inter­national system of units SI (Système international d'unités). Degrees Celsius is the officially derived SI unit. However, other temperature units that do not belong to the SI units, such as Fahrenheit and Rankine, are also in use. For historical reasons, the temperature units Fahrenheit (°F) and Rankine (°Ra) are used primarily in the Anglo-American world.

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Operating aid for the calculator

Temperature Converter Enter the value of the temperature to be converted and select from and into which temperature unit this value is to be converted. The temperature converter provides the common SI units as well as the most common Anglo-American temperature units for the calculation. Further information on the various temperature units can be found behind the help buttons of the temperature calculator marked with a question mark.

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Degrees Celsius (°C)

The common unit for temperature in German-speaking countries is measured in degrees Celsius (°C). Anders Celsius, a Swedish astronomer, mathematician and physicist defined this temperature division named after him, degrees Celsius, in the 18th century. He chose the melting point and boiling point of water as fixed points for the temperature scale and divided the change in volume of mercury between these points into 100 equal parts. So today we are used to water freezing at 0 degrees Celsius and water boiling at 100 degrees Celsius. And to measure the temperature, mercury­silver­thermo­meters are often still in use. Celsius thus forms a so-called empirical temperature scale, i.e. an arbitrary determination of the order of magnitude of the temperature by specifying the temperature in relation to a reference value. For degrees Celsius, the arbitrary determination lies, as described, at 0 degrees and 100 degrees depending on the aggregate­state of the water. While Kelvin is the base unit for temperature in the inter­national SI (Système international d'unités) system of units, degrees Celsius (°C) is an officially derived SI unit. As a derived SI unit, degrees Celsius (°C) is officially permitted in many European countries, especially in Germany, Austria and Switzerland.

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Kelvin (K)

The Kelvin (K) is the inter­nal base unit of temperature. William Thomson (Lord Kelvin), an Irish physicist, introduced this temperature scale at the end of the 19th century. Its unit, the Kelvin, has been the SI unit of temperature in its present form since 1968. The zero point of today's Kelvin scale is at absolute zero, i.e. the coldest, theoretically possible temperature. This corresponds to minus 273.15 degrees Celsius. The scale­intervals of the Kelvin scale correspond to those of the Celsuis scale. The Celsius­temperature is therefore, according to its modern definition, the temperature of the Kelvin scale with a numerical value that is 273.15 smaller. Thus, for example, 0 Kelvin corresponds to a temperature of minus 273.15 degrees Celsius. Or a temperature of 20 degrees Celsius corresponds to 293.15 Kelvin. Thus the conversion formula from Celsius to Kelvin is as follows:

Conversion formula Celsius to Kelvin
Temperatur in K = Temperatur in °C + 273.15

The Kelvin is the base unit for temperature in the SI (Système international d'unités) inter­national system of units. All other temperature units and temperature scales in the metric system are derived from the Kelvin.

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Fahrenheit (°F)

In the USA, the Fahrenheit scale with the unit degrees Fahrenheit (°F) is still very common. Daniel Gabriel Fahrenheit was a German physicist and developed his temperature scale in 1714. Fahrenheit used as the zero point of his scale the lowest temperature he could produce with a mixture of ice, water and ammonia. This was minus 17.8 degrees Celsius, which he defined as the zero point of his scale, i.e. 0 degrees Fahrenheit. As a second and third fixed point, Fahrenheit defined the freezing point of normal water at 32 degrees Fahrenheit and the body temperature of a healthy person at 96 degrees Fahrenheit. In conjunction with the improved accuracy of measurements in the 19th  century and the conversion­method to today's Kelvin and degrees Celsius, the current fixed points of the Fahrenheit scale are at the freezing point (32 degrees Fahrenheit) and the boiling point of water (212 degrees Fahrenheit). Whereas with Celsius and Kelvin there are 100 degrees, i.e. 100 scales, between the freezing point and the boiling point of water, with Fahrenheit there are 212 − 32 = 180 scales. Therefore the conversion from Fahrenheit to e.g. Celsius on the one hand via the shift of 32 degrees at the freezing point of water and on the other hand via the number of individual scale­strokes, i.e. the division into 180 instead of 100 degrees up to boiling water. Thus, the conversion­formula from Celsius to Fahrenheit results as follows:

Conversion formula Celsius to Fahrenheit
Temperatur in °F = Temperatur in °C × 1,8 + 32

The Fahrenheit scale is only officially used in the USA and its outlying areas, in Belize, the Bahamas and the Cayman Islands.

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Rankine (°Ra)

In the USA, the Rankine scale with the unit degrees Rankine (°R) as a derived unit of the Fahrenheit system is most commonly used in the aerospace industry. William Rankine, a natural scientist, proposed the Rankine scale as a temperature scale in 1859. The Rankine scale has its zero value, like the Kelvin scale, at the absolute zero of minus 273.15 degrees Celsius. However, unlike Kelvin, Rankine uses the finer scale spacing of the Fahrenheit scale, which has 180 graduations instead of 100 graduations from the freezing point to the boiling point of water. Therefore, the conversion from Rankine to e.g. Celsius is done on the one hand via the shift from 273.15 degrees to absolute temperature­zero and on the other hand via the number of individual scale graduations, i.e., 1.8 times the graduation. Thus, the conversion formula from Celsius to Rankine is as follows:

Conversion formula Celsius to Rankine
Temperatur in °Ra = (Temperatur in °C + 273.15) × 1,8

The difference of one degree Rankine (°Ra) is equal to the difference of one degree Fahrenheit, but absolute zero is 0 degrees Rankine or minus 459.67 degrees Fahrenheit. Degrees Rankine is not an SI unit, so it does not belong to the inter­natio­nal units­system SI (Système international d'unités). The Rankine scale was used mainly in Anglophone countries.

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Réaumur (°Ré)

Réaumur, a French naturalist, introduced the Réaumur unit for measuring temperature in 1730. The reference points of the Réaumur scale are - similar to the Cesius scale - the melting point of ice (0°Ré) and the boiling point of water (80°Ré) at normal pressure. However, in contrast to the Celsius scale, Réaumur divided these two values into 80 equal degrees. Thus, the conversion formula from Celsius to Réaumur is as follows:

Conversion formula Celsius to Réaumur
Temperatur in °Ré = Temperatur in °C  × 0.8

Réaumur was in use in Western Europe until the end of the 19th century, but is now almost meaningless as a temperature unit. Réaumur is only used very rarely today, for example in the production of alpine cheese in Switzerland and Italy and for the classification of sugar syrup. Réaumur is not an SI unit, so it is not part of the SI (Système international d'unités) inter­national system of units.

Source information

As source for the information in the 'Temperatures' category, we have used in particular:

Last update on November 25, 2022

The pages of the 'Temperatures' category were last editorially reviewed by Michael Mühl on November 25, 2022. They all correspond to the current status.

Previous changes on November 25, 2019

  • Publication of the topic Convert Temperatures together with the corresponding texts.
  • Editorial revision of all texts in this category