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Temperature Scales Information and Conversions

There is more than one scale for measuring temperature, as most know. In alphabetical order the best known are: Celsius, Fahrenheit, Kelvin, Rankine, Reaumur. But are there more?

See below-we get 10 at the last count!

If you choose Centigrade as one of the extras, you are using terminology that is a wee bit out of date. Centigrade was replaced by Celsius in 1948 to honor the person, Anders Celsius a Swedish scientist, who developed the original Centigrade scale in 1742 and it isn't a really different scale, but the change gave us a new one- -there's more to it-see below about scales.

Also, another useful bit of information: since 1990 the unit of temperature in the Absolute Temperature or Kelvin scale is the Kelvin, not the Degree Kelvin. The fine distinction is made because of a very small shift in the definition of the scale reference point from the freezing point of water in the 1968 scale to the triple point of water, a difference of 0.01°C in the 1990 scale.

In the prior scale, The International Practical Temperature Scale of 1968 (IPTS-68), the unit was the Degree Kelvin, or °K.

Care to guess who this scale and its units are named after?

If you answered William Thompson you are correct. Huh? (Hint: Lord Kelvin is an honorary name.)


If you choose to call the International Temperature Scale of 1990, ITS-90, a seperate scale, then there are more than five. We know about Celsius, Fahrenheit, Kelvin, Rankine and, if you check out the graphic below, there's Reaumur, too.

OK, so there's the ITS-90 (six) and before that the IPTS-68 (mentioned above-that's seven).

We have it on good authority (Refererence: NBS Special Publication 300-Volume 2, August 1968) that there were two previous real scales, IPTS-48(eight) and ITS-27(nine) along with a revised 1948 scale in 1960 which was not a change at all in the scale, just a modification in its decription.

Note also that the 1948 scale adopted a change in the name of the units in the official scale from Centigrade or Centesimal, to Celsius.

Therefore it looks like nine scales. A purist might point out that none of these scales are the ideal, thermodynamic or Kelvin temperature scale, which exists in theory and about which many articles have been written.

So, we'll bow to the fact that such a scale exists in a theoretically ideal way and wind up with ten scales, of which, one is ideal and three are not used any more. If there's any comments or feedback, we'd certainly appreciate feedback.

[Note for the purists. We're not counting the provisional temperature scales, although we know of two of them, the Provisional IPTS of 1970 and the new, Provisional Low Temperature Scale of 2000 (PLTS-2000). The latter is for temperatures below 0.65 K, the low limit of ITS-90.]


In this day and age of innumeracy, it is insufficient for many people to provide a table or actual formula for conversion. It is far easier to provide a "calculator" for performing the conversion, and for all too many it is absolutely necessary to have such a tool ready and handy.

We are not providing such a tool on this site out of sheer principle, although we have written a beauty as an introductory exercise in Visual Basic that could be so used. The simple facts are there are many, many such calculators and a wealth of useful (and useless) information about the various scales, their history and quirks available on the Web. Some of each are listed below.

Use them with care lest you get too lazy!

Notice, however, that in an effort to sponsor numeracy, we have provided as the first tool a simple and informative graphic (below) that tells it all without doing the thinking for anyone! The graphic is courtesy of Perfectly Useless Software who seemed to have vanished from the Web after posting this neat graphic.

Featured Links
A manufacturer of phase change or chemical temperature indicators. Most people know them as Temperature Labels, Thermal Crayons, Temperature Stickers, Temperature Paint etc. Like so many other devices, these bear the condition of too many synonyms, not enough standards.

More Temperature Scale Info & Conversion Tools

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