Réaumur scale

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This article is about the temperature scale. For other uses, see Réaumur (disambiguation).

**Réaumur temperature conversion formulae**
	from Réaumur	to Réaumur
Celsius	[°C] = [°Ré] × ⁵⁄₄	[°Ré] = [°C] × ⁴⁄₅
Fahrenheit	[°F] = [°Ré] × ⁹⁄₄ + 32	[°Ré] = ([°F] − 32) × ⁴⁄₉
Kelvin	[K] = [°Ré] × ⁵⁄₄ + 273.15	[°Ré] = ([K] − 273.15) × ⁴⁄₅
Rankine	[°R] = [°Ré] × ⁹⁄₄ + 491.67	[°Ré] = ([°R] − 491.67) × ⁴⁄₉
For temperature intervals rather than specific temperatures, 1 °Ré = 1.25 °C and 1 °Ré = 2.25 °F Comparisons among various temperature scales

The Réaumur scale (°Ré, °Re, °R), also known as the octogesimal division,^[1] is a temperature scale in which the freezing and boiling points of water are set to 0 and 80 degrees respectively. The scale is named after René Antoine Ferchault de Réaumur, who first proposed it in 1731.

Réaumur’s thermometer was constructed on the principle of taking the freezing point of water as 0°, and graduating the tube into degrees each of which was one-thousandth of the volume contained by the bulb and tube up to the zero mark. It was the dilatability of the particular quality of alcohol employed which made the boiling point of water 80°. Mercurial thermometers, the stems of which are graduated into eighty equal parts between the freezing and boiling points of water, are not Réaumur thermometers in anything but name. Réaumur may have chosen the octogesimal division because the number 80 could be halved 4 times and still be an integer (40, 20, 10, 5); the number 100, for instance, could only suffer this process twice (50, 25).

The Réaumur scale saw widespread use in Europe, particularly in France and Germany as well as Russia, such as in works of Dostoyevsky and Tolstoy. But by the 1790s, France chose the Celsius scale for the metric system over the Réaumur measurements.^[2] Today it is only of historical significance except that it is still used in the measuring of milk temperature in cheese. It is used in some Italian dairies making Parmiggiano-Reggiano and Grana Padano cheeses and in Swiss Alp cheeses.