 # Convert Temperatures - Fahrenheit, Celsius, Kelvin

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

## General Information on Temperature Units

To define the temperature scales, two fixed points are 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 temperature-­dependent 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&aos;unités). The officially derived SI unit is the degree Celsius.

## Operating Tips for the Calculator 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.

## Degree Celsius (°C)

The common unit for temperature in German-speaking countries is measured in degree Celsius (°C). Anders Celsius, a Swedish astronomer, mathematician and physicist, defined this temperature division named after him, "degree 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 degree Celsius and water boiling at 100 degrees Celsius. And to measure the temperature, mercury­ thermo­meters are often still in use. Celsius thus forms a so-called empirical temperature scale, i.e., an arbitrary determination of the magnitude of the temperature by indicating the temperature in relation to a reference value. For degree Celsius, the arbitrary determination, as described, is between0 degree and 100 degrees, depending on the aggregate ­state of the water. While Kelvin is the base unit for temperature in the international system of units SI (Système international d'unités), degree Celsius (°C) is an officially derived SI unit. As a derived SI unit, the degree Celsius (°C) is officially approved in many European countries, especially in Germany, Austria and Switzerland.

## 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 scale ­intervals of the Kelvin scale correspond to those of the Celsius scale. The Celsius­ temperature is therefore, according to its modern definition, the temperature of the Kelvin scale with a numerical value that is 273.15 lower. For example, 0 Kelvin corresponds to a temperature of minus 273.15 degrees Celsius. Alternatively, a temperature of 20 degrees Celsius equals 293.15 Kelvin. Thus, the conversion formula from Celsius to Kelvin is as follows:

Conversion formula for 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.

## Degree Fahrenheit (°F)

In the USA, the Fahrenheit scale with the unit degree Fahrenheit (°F) is still very common. Daniel Gabriel Fahrenheit was a German physicist who 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 -17.8 degrees Celsius, which he defined as the zero point of his scale, i.e., 0 degree 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 conversion ­method to today's Kelvin and Celsius degrees, 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). With Celsius and Kelvin, there are 100 degrees, i.e., 100 scales, between the freezing point and the boiling point of water, whereas with Fahrenheit, there are 212 − 32 = 180 scales. Therefore, the conversion from Fahrenheit to, e.g., Celsius is done 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 scale­ marks, i.e., the division into 180 instead of 100 degrees up to the boiling point of water. Thus, the conversion ­formula from Celsius to Fahrenheit is as follows:

Conversion formula for 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, including Belize, the Bahamas and the Cayman Islands.

## Degree Rankine (°Ra)

In the USA, the Rankine scale, with degree Rankine (°Ra) as the 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 absolute zero of -273.15 degrees Celsius. Unlike the Kelvin scale, however, the Rankine scale uses the finer scale division of the Fahrenheit scale, which has 180 instead of 100 graduation marks from the freezing point to the boiling point of water. Therefore, the conversion from Rankine to, e.g., Celsius is carried out on the one hand via the shift from 273.15 degrees to absolute temperature­ zero and on the other hand via the number of individual scale graduations, i.e., 1.8 times the graduation mark. Thus, the conversion formula from Celsius to Rankine is as follows:

Conversion formula for 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 degree Rankine or -459.67 degrees Fahrenheit. Degree Rankine is not an SI unit, so it is not part of the SI (Système international d'unités) system.

## 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 Celsius 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

This page of the 'Temperatures' category was last edited or reviewed by Michael Mühl on February 20, 2023. It corresponds to the current status.

### Changes in this category "Temperatures"

• Publication of the topic Convert Temperatures together with the corresponding texts.