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Réaumur (°Ré) to Celsius Converter

Enter a value in réaumur (°ré) to convert it to celsius, fahrenheit.

Result

22.50 °C
72.50 °F

How it is calculated

The Réaumur scale puts the freezing point of water at 0 °Ré and the boiling point at 80 °Ré, so 80 Réaumur degrees span the same range as 100 Celsius degrees. One Réaumur degree is therefore 100 ÷ 80 = 1.25 Celsius degrees, and because both scales start at the freezing point of water there is no offset: °C = °Ré × 1.25. The tool also gives Fahrenheit with °F = °Ré × 2.25 + 32 (2.25 is 1.25 × 1.8). Both results are rounded to two decimals. Negative temperatures are accepted, but a value that converts to below absolute zero (below about −218.52 °Ré, which is −273.15 °C) is physically impossible and shows no result.

Worked example

An old text says a room should be kept at 18 °Ré. Celsius: 18 × 1.25 = 22.50 °C. Fahrenheit: 18 × 2.25 + 32 = 40.5 + 32 = 72.50 °F. As a check, the boiling point 80 °Ré gives 80 × 1.25 = 100 °C and 212 °F, and the freezing point 0 °Ré gives 0 °C and 32 °F.

Try this example →

Common mistakes

  • Dividing by 1.25 instead of multiplying. A Réaumur degree is larger than a Celsius degree, so the Celsius number is always bigger (for positive values): 18 °Ré is 22.5 °C, not 14.4 °C.
  • Confusing Réaumur with Rankine or Rømer. Each is a different historical scale with its own zero and degree size; this tool is for Réaumur only.
  • Using the Fahrenheit formula (× 1.8 + 32) on a Réaumur number. The multiplier here is 2.25 for Fahrenheit.
  • Reading "°R" as Réaumur when a source means Rankine. If the document is from the English-speaking engineering world, check which scale it is.
  • Rounding the Celsius result to a whole number in cooking or cheese-making instructions where one degree can matter.

About this calculator

The Réaumur scale, devised by French scientist René Antoine Ferchault de Réaumur in 1730, sets water's freezing point at 0°Ré and boiling point at 80°Ré (rather than 100), so it converts to Celsius as °C = °Ré × 1.25 (and to Fahrenheit as °F = °Ré × 2.25 + 32).

Once the dominant temperature scale across much of continental Europe, Réaumur was largely displaced by Celsius in the 19th century, but it still turns up today in older Russian and French scientific literature, some traditional cheese-making and confectionery recipes, and historical texts, where this converter lets readers translate an antique reading into a familiar modern scale.

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