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Temperature units and conversion of temperature units


Learn about temperature, temperature scales, temperature units and the conversion of temperature units.

 Explanation of the pictures

The graph above shows a brief comparison between different temperature units at some known temperature values. Of course, room temperature can vary, and a person's body temperature is not always 37 ° C (98.6 ° F). The value at which boiling water evaporates also depends on the air pressure and is not always exactly 100 ° C. But in a real Finnish sauna the temperature is at least 100 ° C (212 ° F) if it has been heated up properly ... :-)

A while ago I wrote a blog post about pressure units and I thought it would be a good idea to do something similar about temperature units and their conversion.

What is temperature

Temperature is an intense quantity that the energy state of a matter indicates. All matter consists of atoms and molecules that are in constant motion, vibrating or rotating. Even if it is a complex topic, to put it simply, the more you move, the higher the material temperature. The temperature of an object can be defined as the average kinetic energy of its atoms and molecules, a definition of temperature that is relatively easy to understand. Kelvin is the unit of fundamental physical quantity called thermodynamic temperature (T) and is currently defined as one 273.16th of the thermodynamic temperature of the Triple point of water (exactly 0.01 ° C or 32.02 ° F).

But what is "hot" and what is "cold"?

It's all pretty relative; In other words, these terms - hot and cold - are neither particularly precise nor scientific. We therefore need something more specific for specifying the temperature. Various different temperature scales and units have been developed over the past centuries. And since different temperature scales were used in different regions of the world, are
various different scales are still in use today. The specifications of some old temperature scales were initially not very precise (such as human body temperature); however, specific, accurate reference points and information were later established.

There is no real upper limit to high temperatures and it is possible to reach very high temperatures. The temperature on the sun's surface is, for example, 5800 Kelvin, whereas the temperature inside the sun can reach up to 13.6 million Kelvin.

At the bottom, however, there is a very precise limit, namely the absolute zerowhich corresponds to the lowest possible temperature. Absolute zero is a theoretical state that is practically unattainable in reality. Theoretically, all atomic movements would come to a standstill almost completely and only the quantum mechanical zero point energy would remain. The absolute zero corresponds to a temperature of 0 Kelvin, -273.15 ° Celsius or -459.67 ° Fahrenheit. It's pretty cold in space and the average temperature of the universe is below 3 Kelvin.

But now let's take a look at a few of the most common temperature scales and units.

International temperature scales

The thermodynamic temperature is very difficult to measure and various international temperature scales have been published for measurement in practice:

  • ITS-27; International temperature scale from 1927
  • IPTS-48; Practical International Temperature Scale from 1948
  • IPTS-68; Practical International Temperature Scale from 1968
  • ITS-90; International temperature scale from 1990

In addition, other scales were used; For example the PLTS-2000 for the optimized measurement of very low temperatures in the range from 0.9 mK to 1 K (provisional low temperature scale of 2000).

The current one is based on an international agreement ITS-90 scale on the previously mentioned thermodynamic temperature (T). This scale defines the methods for calibrating a particular type of thermometer, designed to ensure that the results are accurate and reproducible around the world. It is also assumed that the numerical values ​​are so close to the
actual thermodynamic temperature (T) as possible at the time. The methods for implementing the ITS-90 temperature scale include fixed points and functions for interpolating the temperatures between the fixed values.

The ITS-90 scale provides the following fixed points:

Temperature units

Kelvin (K)

Kelvin is the basic unit for temperature in the SI system (International system of units). The abbreviation for the unit Kelvin is K (without degree or degree symbol). The Kelvin unit was first introduced by William Thomson (Lord Kelvin) in 1848.

As mentioned earlier, Kelvin is currently defined as one 273.16th of the thermodynamic temperature of the triple point of water, with absolute zero being 0 K. The size of one Kelvin corresponds to the size of one degree Celsius. The melting point of water is 273.15 K (the triple point of water is 273.16 K).

Kelvin is widely used in science and technology. This unity is in everyday life
but rather uncommon. In the ITS-90, the Kelvin temperature is indicated by the symbol T90 indicated (with the T as a capital letter).

A new Kelvin definition is expected to be published in the near future, based on the link between Kelvin and the Boltzmann constant, as a continuation of the project to define all SI units on the basis of fundamental physical constants.

Celsius (° C)

Celsius is a unit for temperature in the SI system derived from the basic unit Kelvin. The abbreviation of Celsius is ° C (degrees Celsius) and the size of one degree Celsius corresponds to the size of one Kelvin. The unit as well as the currently used Celsius scale was first introduced by Andreas Celsius, a Swede, in 1742. The two main reference points on the Celsius scale were the freezing point of water (or the melting point of ice), defined as 0 ° C and the boiling point of water, which was set at 100 ° C.

The melting point of water is a relatively precise figure (provided that it is purified water that has been properly stirred beforehand), but the boiling point of water is not as accurate in practice because the boiling temperature is very high is influenced by the air pressure. Since Celsius is an SI unit derived from Kelvin, it is also linked to the ITS-90 and has the symbol t90 indicated (with lowercase t). According to the ITS-90 lies
the melting point of water just below 0 ° C and the boiling point at normal air pressure at around 99.974 ° C.

The Celsius unit is more suitable for everyday use than the Kelvin and is very popular around the world, although not so much in the US. A degree Celsius is often referred to simply as a degree.

Fahrenheit (° F)

The abbreviation for Fahrenheit is ° F. The Fahrenheit scale was introduced by a Dutchman named Gabriel Fahrenheit in 1724. The two main reference points on this scale are the freezing point of water, set at 32 ° F, and the normal human body temperature of 96 ° F.

In practice it is easy to see that the temperature of a human body is not a particularly precise definition.

The Fahrenheit scale was later redefined, which now fixes the melting point of water at exactly 32 ° F and the boiling point at exactly 212 ° F. On the revised scale, normal human body temperature is around 98 ° F.

Fahrenheit has been replaced by Celsius as the unit of temperature in many regions, but Fahrenheit is still used in the United States, the Caribbean, and parallel to Celsius in Australia and the UK.

Rankine (° R, ° Ra)

The abbreviation of the Rankine scale is ° R or ° Ra. The Rankine scale was developed by the Scotsman William Rankine in
The year 1859 is presented, that is, only a few years after the Kelvin scale. The reference point on the Rankine scale is the absolute zero from 0 ° R, as well as on the Kelvin scale. A Rankine degree is the same as a Fahrenheit degree, but the zero point is completely different. The freezing point of water is 491.67 ° Rankine.

The Rankine scale is not particularly common. It used to be used in the USA in certain technical areas. However, the NIST nowadays advises against the use of Rankine.

Réaumur (° Ré, ° Re)

The Réaumur scale was introduced by Réne de Réaumur in 1730. Serve as reference points the freezing point of water at 0 ° Ré and the boiling point at 80 ° Ré.

The Réaumur scale was used in some parts of Europe and Russia. However, it has practically not been in use since the end of the last century.

Conversion of temperature units

The table below contains calculation formulas for converting measured temperature values ​​from one unit to another.

Unit calculator for temperature units

I am well aware that the conversion table presented above is not one of the "most user-friendly" ones ...

We have provided a free and easy-to-use unit converter for temperature units on our homepage, which are used to convert between the five different units listed above
Temperature units is used. We hope you find this conversion tool useful.


Download this article as a PDF file for free; to do this, click on the image below:


Beamex temperature calibration products

Please also take a look at the new Beamex MC6-T temperature calibrator. Click the image below for more information:

Check out other temperature calibrators and temperature calibration products that Beamex offers on our website:


I hope this post is helpful to you.

With best regards,



Original blog post: Temperature units and temperature unit conversion
Published: February 21, 2018
Posted by Heikki Laurila.