Convert °Ré to °C degrees Réaumur ↔ degrees Celsius Free Online Secure
Type a value in degrees Réaumur and read it in degrees Celsius at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert °Ré to °C (degrees Réaumur ↔ degrees Celsius)
| °Ré | °C |
|---|---|
| -32 | -40 |
| -8 | -10 |
| 0 | 0 |
| 8 | 10 |
| 16 | 20 |
| 20 | 25 |
| 29.6 | 37 |
| 80 | 100 |
Where these units come from
Fahrenheit set his zero in 1724 at the coldest thing he could make reliably — a brine of ice, water and salt — and put body heat near 96, so that his scale could be divided and divided again. Celsius published his in 1742 the other way up, with 0 for boiling and 100 for freezing; it was turned around after his death, which is why he is remembered for a scale he never used. Kelvin is the only one of the four that begins where there is nothing left to take away: absolute zero. Since 2019 it is defined by fixing the Boltzmann constant rather than by the triple point of water, so the kelvin no longer depends on a particular substance. Rankine is Fahrenheit's degree counted from absolute zero, and Réaumur, with 80 degrees between freezing and boiling, survives mostly in old recipes and in a few cheese dairies.
Where you meet them
Celsius runs weather, cooking and medicine almost everywhere; Fahrenheit does the same work in the United States and keeps a foothold in oven settings and in American recipes. Kelvin belongs to physics, to the colour temperature of lamps and screens, and to anything cold enough that negative numbers would be a nuisance. Rankine appears in American thermodynamics, and Réaumur mainly in historical texts — which is exactly why an old recipe can mislead someone reading it as Celsius.
Questions people ask
What is the Réaumur scale?
A scale invented by René Réaumur in 1730 in which water freezes at 0 °Ré and boils at 80 °Ré. To get Celsius, multiply by 1.25. It was widely used in parts of Europe in the eighteenth and nineteenth centuries and is now almost entirely historical, so you meet it mainly in old books, recipes and instruments.
Why can't I just double the number to double the temperature?
Because three of these scales have arbitrary zeros. Twenty degrees Celsius is not twice as hot as ten; it is ten degrees further along a scale whose zero was chosen for the convenience of water. Only kelvin and Rankine begin at absolute zero, where there is genuinely nothing left, so only on those two does doubling the figure mean doubling the thermal energy. That is also why every conversion here adds or subtracts as well as multiplying.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Translating a classic: Réaumur degrees into Celsius
A translator working on a nineteenth-century novel reaches the line where a coachman says that tonight the frost will reach thirty degrees. Is that bitter, or merely brisk? The answer depends on the scale, and in a Russian, French or German original of that period the scale is very likely Réaumur. To convert °Ré to °C, multiply by 1.25, which is 5/4: 0 °Ré is 0 °C, 16 °Ré is 20 °C and 80 °Ré is 100 °C. The calculator above multiplies as you type, and the sections below look at the problem from the editor's chair: what to keep, what to convert and how to explain it to a reader.
You will find the formula with worked examples, a short profile of each scale, a comparison, a long set of editorial cases and the slips to avoid. The arithmetic runs in your browser, and what you type is not sent anywhere.
How to convert °Ré to °C: multiply by 1.25
The formula
The formula is °C = °Ré × 5/4, or °Ré × 1.25. So 12 °Ré gives 15 °C, 24 °Ré gives 30 °C and −8 °Ré gives −10 °C. To go back, multiply by 0.8. The scales share a zero at the freezing point of water, so nothing needs to be added or subtracted.
Worked examples
A summer day of 20 °Ré is 25 °C. A hard frost of −16 °Ré is −20 °C. A bitter night of −30 °Ré is −37.5 °C. Boiling water at 80 °Ré is 100 °C, and a normal body temperature of 29.6 °Ré is 37 °C.
Why only a multiplication
Both scales put the freezing point at 0. A Réaumur degree is a quarter larger than a Celsius degree, because eighty of them span the same range as a hundred Celsius degrees.
A quick sanity test
For positive values, Celsius is bigger than Réaumur. If a “convert °Ré to °C” calculation result is smaller than the input, you used 0.8 where 1.25 belongs.
The Réaumur scale
Eighty degrees between freezing and boiling
Introduced in 1730 by the French naturalist René-Antoine Ferchault de Réaumur, the scale was widely used in Europe until the nineteenth century. It is written °Ré, and it appears in novels, memoirs, letters and almanacs.
Where it appears
Classic literature, historical letters, old weather diaries and antique thermometers.
The Celsius scale
One hundred degrees between freezing and boiling
Celsius sets 0 at freezing and 100 at boiling. It is the scale a modern reader has in mind, so it is the one into which a translator or editor puts historical temperatures.
Where it appears
Modern editions, subtitles, museum labels and reference works.
Réaumur against Celsius
One Réaumur degree is 1.25 Celsius degrees, so a change of 8 °Ré is a change of 10 °C. The scales agree only at 0. At 80 they differ by 20 degrees, and at −30 by 7.5. For frost the gap grows with the cold: −20 °Ré is −25 °C, which a reader would feel as a much harder winter than the number suggests.
The multiplication is exact, so no rounding enters the conversion. The usual slips are inverting the factor and confusing °R with Rankine.
Where the conversion shows up
Three strategies for translators
An editor can keep the original number and add a note, replace it with Celsius, or give both in the text. A frost of "thirty degrees" might become "thirty degrees below, Réaumur, about −37 °C" in a scholarly edition, or "nearly forty below" in a popular one. The choice depends on the readers, but every option begins with a “convert °Ré to °C” calculation.
Footnotes and glossaries
A short note at the first mention works for most books: "Temperatures are in degrees Réaumur; multiply by 1.25 for Celsius." Later mentions can stay unchanged if the reader has the rule. A glossary entry at the back can give a few reference points: 0 Réaumur is freezing, 80 is boiling, 16 is a comfortable room at 20 °C.
Subtitles and audio
Film adaptations and audiobooks cannot carry footnotes. A subtitle that says "thirty degrees of frost" will leave modern viewers unsure. Adaptors convert silently, so that the screen shows a cold that matches the story. A person who checks a period script must therefore know the scale of the source.
Deciding which scale the author used
Clues help. If the setting is Russia, Germany, Austria or France before about 1850, Réaumur is likely. If the setting is Britain or America, Fahrenheit is likely. If a character says that water boils at eighty degrees, the question is answered. Check the edition's introduction, which may state the scale, and compare with the weather in the story: twenty degrees of frost in Celsius is cold, in Réaumur it is severe.
Travel writing
Travellers of the eighteenth and nineteenth centuries reported temperatures of the places they visited, often in the scale of their home country. A French traveller in the Alps writing that the air was at four degrees below zero means −5 °C. A German explorer noting thirty-five degrees of heat in the tropics, if in Réaumur, would mean nearly 44 °C, a figure so extreme that a modern editor may suspect another scale.
Plausibility checks
When in doubt, apply the plausibility test. Convert the number as Réaumur and as Celsius and ask which is credible in that place and season. Thirty-five degrees of heat on the Baltic coast in July is implausible in Réaumur, which is 43.75 °C, but plausible in Celsius. The check rules out the wrong reading in most cases.
Dealing with ambiguous °R
The symbol °R is used for Réaumur in older texts and for Rankine in engineering. In a literary or weather context it is Réaumur. In a text about steam engines or gases in English units it is probably Rankine. If the text is about both, the context of the number tells: 80 °R for boiling water is Réaumur, 672 °R for boiling water is Rankine.
Spreadsheet formulas
Put Réaumur values in column A and use =A2*1.25 in column B. For the reverse, use =B2*0.8. Keep the original value, the converted value and a column for notes. If you are working on a long text, a spreadsheet of all temperature mentions with page numbers helps keep conversions consistent.
Rounding in a translation
The conversion is exact, but in prose it should be rounded. −18.75 °C reads as "nearly −19 degrees" or "about −19 °C" and 31.25 °C as "about 31 °C". A reader does not need decimals, and the original had none. Keep a precise figure in notes only if the passage relies on it.
A mental shortcut
Add a quarter. For 20 °Ré, a quarter is 5, so 25 °C. For 12 °Ré, 3 more gives 15 °C. For 30 °Ré, 7.5 more gives 37.5 °C. For the reverse, take away a fifth. The shortcut is exact and quick enough to use while reading.
Proofreading a converted passage
After converting, read the passage aloud with the new numbers and ask whether the weather feels right for the scene. A winter journey at minus 4 degrees Celsius would hardly justify the characters' alarm, while minus 25 does. Check also that each unit label is in place and that notes and text agree. A second reader looking only at temperatures catches the slips that a first pass misses.
Reference ladder
−32 °Ré is −40 °C. −24 °Ré is −30 °C. −16 °Ré is −20 °C. −8 °Ré is −10 °C. 0 °Ré is 0 °C. 12 °Ré is 15 °C. 24 °Ré is 30 °C. 32 °Ré is 40 °C. A ladder like this speeds up proofreading.
Gaps and ranges in a text
An author who writes that the temperature swung from five below to eight above is describing a range of thirteen Réaumur degrees, which is 16.25 °C. Converting both ends gives −6.25 °C and +10 °C. The width of the range matters for the scene, and a “convert °Ré to °C” calculation on both ends shows it at once.
Relation to Fahrenheit
From Réaumur to Fahrenheit the rule is °F = °Ré × 2.25 + 32. So 16 °Ré is 68 °F. In an English edition of a Russian novel, a translator may prefer Fahrenheit for American readers, giving 20 °Ré as 77 °F. Which unit to choose depends on the target audience.
Common phrases and their meaning
"A frost of ten degrees" in Réaumur is −12.5 °C. "A frost of fifteen degrees" is −18.75 °C. "Twenty-five degrees of frost" is −31.25 °C, and "twelve degrees of heat" is 15 °C. "Twenty degrees of heat" is 25 °C, and "thirty degrees in the shade" is 37.5 °C, a heat wave. A “convert °Ré to °C” calculation for each phrase gives the translator a fixed table to work from.
Temperature differences
A drop of 8 degrees Réaumur is a drop of 10 degrees Celsius. "The temperature rose by five degrees" is 6.25 °C. Because the scales share a zero, differences convert exactly like absolute values. Be careful, though, with the word "degree" without a scale: it may refer to Fahrenheit in English texts.
Writing the unit properly
Write °Ré with the accent, or °Re when the accent is not available. Put a space between number and symbol. In the first mention, spell it out: degrees Réaumur. In reference works, adding Celsius in brackets is helpful: 20 °Ré (25 °C). Avoid °R alone, which can mean Rankine.
When not to convert
Some editions keep the original scale on purpose, especially when the author's own commentary on the weather, such as a diary that records readings daily, is the document of interest. A scholarly edition will transcribe and then give the conversion in a separate column. A popular edition may convert silently with a general note at the start.
A short worked passage
An original sentence reads: "In the morning it was eight degrees below zero, by midday only two, and by evening again twelve." In Réaumur this is −10 °C, −2.5 °C and −15 °C. A translator who leaves the numbers risks the reader thinking it was mild. A “convert °Ré to °C” calculation for each figure makes the day's cold clear.
Audiobooks and narration
A narrator reading a translated novel aloud cannot pause for a footnote. Producers agree on a rule in advance: either every temperature is converted to Celsius in the script, or none is and the story gives enough context. A short script check with a table of the book's temperature mentions, built using a “convert °Ré to °C” calculation for each, keeps the recording consistent from chapter to chapter.
School editions
Editions for schools often add a note box when a classic text uses Réaumur. The box says what the scale is, shows the factor 1.25 and gives two examples from the text. Teachers use it to connect literature with physics: students convert a frost of twenty degrees and discuss why the number alone does not convey the cold unless one knows the scale.
Historical fiction
Authors writing novels set in the nineteenth century face the opposite problem. A character in 1850 would think in Réaumur or Fahrenheit, not Celsius. Writers may use local units in dialogue and give the modern equivalent in narration, or avoid numbers and use descriptions. Either way, the author needs a conversion table to keep the weather consistent with the period.
Period dramas and games
Screenwriters and game designers working on period pieces want plausible weather. A line about twenty degrees of frost in a Russian setting can be rendered as minus 25 degrees Celsius for the audience. Designers of historical simulations may keep Réaumur on the in-game thermometer as colour, with a tooltip showing Celsius. The conversion is trivial, but the choice affects the feel.
Letters and diaries in print
Editors of published letters face hundreds of weather remarks. A common approach is to leave the original figures in the text and add an appendix table: "Réaumur to Celsius: −20 is −25, −10 is −12.5, 0 is 0, 10 is 12.5, 20 is 25". Readers can then convert at a glance. The table saves hundreds of footnotes and keeps the text clean.
Museum labels
A label for an antique thermometer should show the original scale and the Celsius equivalents: "scale: Réaumur, range −30 to +50 °Ré (−37.5 to +62.5 °C)". Visitors then see both. If the instrument has two scales, such as Réaumur and Fahrenheit, the label can list all three. Curators often write a “convert °Ré to °C” calculation into their templates to avoid errors.
Cooking from old books
Old cookbooks sometimes specify heat for sugar or milk in degrees Réaumur. A syrup "at 30 degrees" is 37.5 °C, much cooler than the boiling stages of candy making. Anyone trying to reproduce a historical recipe has to check whether the figure refers to temperature or to another measure, such as density in degrees Baumé, which look similar on the page.
The confusion with Baumé and others
Old texts also use other degree scales: Baumé for the density of liquids, Brix and Oechsle for sugar content in juice. A figure of thirty degrees could belong to any of them. Context tells: if the sentence is about weather, it is Réaumur or Fahrenheit, but in a winery or sugar works it may be density. Reading with care avoids converting the wrong kind of degree.
Medical cases in old journals
Case histories sometimes give body temperature in Réaumur: "the patient's heat reached 31 degrees" is 38.75 °C, a fever. A normal 29.6 °Ré is 37 °C. Readers of medical history who meet these figures without a note could assume something much milder. A translator should convert with a “convert °Ré to °C” calculation and keep the original in brackets.
More values for convert °Ré to °C
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- -273.15 °Ré = -341.4375 °C
- -100 °Ré = -125 °C
- -20 °Ré = -25 °C
- 10 °Ré = 12.5 °C
- 15 °Ré = 18.75 °C
- 25 °Ré = 31.25 °C
- 30 °Ré = 37.5 °C
- 50 °Ré = 62.5 °C
Common slips when you convert °Ré to °C
Using 0.8 instead of 1.25
From Réaumur to Celsius the number grows. If 20 °Ré became 16, the factor was inverted.
Adding 32
Réaumur and Celsius share a zero. Do not add 32, which belongs to Fahrenheit.
Assuming the scale
Do not assume Réaumur or Celsius without checking the period and setting. A quick plausibility test saves many errors.
Quick questions
What is the method for a “convert °Ré to °C” calculation?
Multiply the Réaumur value by 1.25.
What is 20 °Ré in degrees Celsius?
25 °C.
What is 80 °Ré in degrees Celsius?
100 °C.
What is 0 °Ré in degrees Celsius?
0 °C.
Is my figure stored or sent?
No, it stays on your device.
Where to go next
Other temperature tools are listed here: Convert °Ré to °F, Convert K to °C, Convert °F to °C, Convert °C to °F, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L, Convert tbsp to tsp. For the opposite direction, use Convert °C to °Ré.
The takeaway: a “convert °Ré to °C” calculation multiplies by 1.25 and labels the result °C, with the original kept beside it. The figure you enter is processed on your device only.