Convert °C to °Ré degrees Celsius ↔ degrees Réaumur Free Online Secure
Type a value in degrees Celsius and read it in degrees Réaumur 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 °C to °Ré (degrees Celsius ↔ degrees Réaumur)
| °C | °Ré |
|---|---|
| -40 | -32 |
| -10 | -8 |
| 0 | 0 |
| 10 | 8 |
| 20 | 16 |
| 25 | 20 |
| 37 | 29.6 |
| 100 | 80 |
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
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.
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.
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.
Frost of twenty degrees: reading old novels with the Réaumur scale
A nineteenth-century novel says that the frost last night was twenty-five degrees, and the hero shivers. A German travel diary notes eighteen degrees in the shade and a pleasant day. Neither gives a unit, because in much of Europe the unit was understood: the Réaumur scale, which puts the freezing point of water at 0 and the boiling point at 80. To convert °C to °Ré, multiply by 0.8, which is 4/5: 0 °C is 0 °Ré, 20 °C is 16 °Ré and 100 °C is 80 °Ré. The calculator above multiplies as you type, and the sections below help readers of old books, diaries and instruments translate such numbers.
You will find the formula with worked examples, a short profile of each scale, a comparison, a long set of cases from literature and history and the slips to avoid. The arithmetic runs in your browser, and what you type is not sent anywhere.
How to convert °C to °Ré: multiply by 0.8
The formula
The formula is °Ré = °C × 4/5, or °C × 0.8. So 25 °C gives 20 °Ré, 37 °C gives 29.6 °Ré and −10 °C gives −8 °Ré. To go back, multiply by 1.25. The two scales have the same zero, so there is nothing to add or subtract.
Worked examples
A warm summer day of 30 °C is 24 °Ré. A mild room of 20 °C is 16 °Ré. A hard frost of −20 °C is −16 °Ré, and boiling water at 100 °C is 80 °Ré. Body temperature, 37 °C, is 29.6 °Ré.
Why only a multiplication
Water freezes at 0 on both scales. The only difference is the size of the degree: there are 100 Celsius degrees between freezing and boiling, but only 80 Réaumur degrees, so a Réaumur degree is larger, 1.25 °C.
A quick sanity test
For positive temperatures the Réaumur number is always smaller than the Celsius one. If a “convert °C to °Ré” calculation result is bigger than the input, you used 1.25 where 0.8 belongs.
The Celsius scale
The scale of a hundred steps
Celsius divides the range between freezing and boiling water into 100 parts. Proposed by the Swedish astronomer Anders Celsius in 1742, it became the everyday scale of most countries and the scale of science after kelvin.
Where it appears
Thermometers, forecasts, ovens and modern books.
The Réaumur scale
The scale of eighty steps
Réaumur puts 0 at the freezing point of water and 80 at the boiling point. It was introduced in 1730 by the French naturalist René-Antoine Ferchault de Réaumur. It lasted in much of Europe into the nineteenth century and has left traces in literature, diaries and, reportedly, in a few traditional trades.
Where it appears
Old novels, travel diaries, weather registers and antique thermometers.
Celsius against Réaumur
One Réaumur degree equals 1.25 Celsius degrees, so a rise of 8 °Ré is a rise of 10 °C. Both scales agree only at the freezing point of water. At −40, Celsius and Fahrenheit coincide, but Réaumur does not: −40 °C is −32 °Ré.
The multiplication is exact, so no rounding enters the conversion. The only common slip is to apply the factor in the wrong direction.
Where the conversion shows up
Novels and memoirs
Writers in Russia, Germany and France used degrees Réaumur well into the nineteenth century. A frost of 20 degrees in a novel is −25 °C, a heat of 28 degrees is 35 °C, and a pleasant spring day of 12 degrees is 15 °C. A modern translator may convert the numbers or leave them, so a reader benefits from a “convert °C to °Ré” calculation when the cold or heat matters to the plot.
Travel diaries and weather registers
Diarists noted temperature as part of daily life. "Eighteen degrees in the shade" in a German diary is 22.5 °C. Researchers reconstructing past climates collect such notes and convert them with a “convert °C to °Ré” calculation, taking care with the place and time of day. Many registers recorded morning, noon and evening values, which average to a daily figure.
Instruments
Antique thermometers often carry two scales, for example Réaumur on one side and Fahrenheit on the other. A reading of 16 °Ré on the first matches 68 °F on the second and 20 °C in modern terms. Collectors use the ratio 4 to 5 between divisions to spot a faked or mislabelled instrument.
Hothouses and gardens
Gardening handbooks of the period gave greenhouse temperatures in Réaumur: a temperate house at 8 to 10 degrees, which is 10 to 12.5 °C, and a stove house for tropical plants at 15 to 20 degrees, which is 18.75 to 25 °C. Head gardeners at great estates kept registers of daily temperatures, which survive in estate archives. A modern conservator restoring a historic glasshouse converts such numbers with a “convert °C to °Ré” calculation to set the heating.
Ships and logs
Ship logs from French, German and Russian vessels recorded air and sea temperatures in Réaumur. A sea at 14 degrees is 17.5 °C, and an air temperature of 24 is 30 °C. Maritime historians who compile climate data from logs standardise all entries to Celsius, a process that begins with identifying the scale used by each ship's nation.
Spreadsheet formulas
Place Celsius values in column A and use =A2*0.8 in column B. For the reverse, use =B2*1.25. For differences, the same factors apply. Format with one decimal and put the unit in the header, writing Ré clearly so it is not confused with Rankine.
Rounding and precision
The factor is exact: 37 °C is 29.6 °Ré, not 30. Old sources were typically read to the nearest degree or half degree, so converting back gives the same precision: 20 °Ré is 25 °C, give or take 0.6. Do not report more digits than the original reading supported.
A mental shortcut
Take away a fifth. For 30 °C, a fifth is 6, so 24 °Ré. For 45 °C, a fifth is 9, so 36 °Ré. For the reverse, add a quarter: 20 °Ré plus 5 is 25 °C. The shortcut is exact because 4/5 is 1 minus a fifth and 5/4 is 1 plus a quarter.
Landmarks to memorise
0 °C is 0 °Ré, 10 °C is 8 °Ré, 20 °C is 16 °Ré, 37 °C is 29.6 °Ré, 50 °C is 40 °Ré and 100 °C is 80 °Ré. With these in mind, you can place any number from an old text between two known values.
Reference ladder
−40 °C is −32 °Ré. −30 °C is −24 °Ré. −20 °C is −16 °Ré. −10 °C is −8 °Ré. 0 °C is 0 °Ré. 25 °C is 20 °Ré. 40 °C is 32 °Ré. 100 °C is 80 °Ré. A ladder like this helps check a figure at a glance.
Checking by reversing
To check any convert °C to °Ré result, multiply it by 1.25 and see whether you return to the start. If 25 °C became 20 °Ré, then 20 times 1.25 is 25. A second test is the size: the Réaumur number is always four fifths of the Celsius one.
Relation to Fahrenheit
From Réaumur to Fahrenheit the rule is °F = °Ré × 2.25 + 32. So 16 °Ré is 68 °F. On antique dual-scale thermometers, 0 °Ré lines up with 32 °F and 80 °Ré with 212 °F, which gives a handy way to check that the instrument has been calibrated correctly.
Temperature differences
A rise of 10 °C is a rise of 8 °Ré. A fall of 25 °C is a fall of 20 °Ré. Because both scales share a zero, differences convert like absolute values. This differs from Fahrenheit, where an offset of 32 applies only to absolute temperatures.
Writing the unit correctly
The symbol is °Ré, sometimes written °R or °Re. The shorter forms are ambiguous because °R is also used for Rankine. In modern text, write °Ré with the accent and a space after the number. In quoted old text, keep the author's form and add the Celsius value in brackets.
Reading a thermometer with two scales
Look for the freezing point on both scales. If both show 0 there, one is Celsius and the other Réaumur. Count the divisions between freezing and boiling: 80 means Réaumur and 100 means Celsius. Eight Réaumur divisions line up with ten Celsius ones. With that ratio you can read any dual-scale thermometer without a calculator.
Telling the scale from context
If a source says water boils at 80 degrees, it is Réaumur. If it says 212, it is Fahrenheit. A frost of "twenty degrees" in a Russian text from the nineteenth century is most probably Réaumur, which is −25 °C. Dates and countries are clues, and a “convert °C to °Ré” calculation on the plausible reading tells you whether the number makes sense.
Why Celsius won
Celsius fitted the decimal system and fixed points were easy to reproduce. During the nineteenth century, countries adopted it one by one, and in the twentieth century the metric reforms completed the shift. By the middle of the century Réaumur had vanished from official use, though not from old books.
Museums and catalogues
Catalogue entries for antique instruments list the scale and range, such as "Réaumur, −30 to +50". A curator adds "(−37.5 to +62.5 °C)" so that visitors can understand. Auction houses do the same. If you buy or sell an antique thermometer, check that the range matches the scale before assuming it is original.
A worked reading exercise
A diary entry says: "Morning, 4 below; noon, 3 above; evening, 1 below." In Réaumur this is −5 °C, 3.75 °C and −1.25 °C. The day was cold at the ends and mild at midday. Without the conversion, the reader might picture a milder winter than the writer lived through.
Poets, painters and weather
Letters and notebooks of poets and painters often mention the weather in passing. A painter writing that it was ten degrees warm in the studio may mean 12.5 °C, enough to work in a cold climate. Editors of collected letters add conversions in notes so that the reader understands what the writer felt. A “convert °C to °Ré” calculation makes the passing remark concrete.
Teaching with old units
History and physics teachers can use Réaumur to show how measurement standards develop. Students convert a frost of 15 degrees Réaumur to Celsius, −18.75 °C, and to Fahrenheit, −1.75 °F, and see that three scales describe one reality. The exercise also highlights why modern science settled on fixed points and a decimal scale.
Letters from the front
Soldiers' letters in wars of the nineteenth century complain of cold in plain numbers: "last night twelve degrees below". In Réaumur this is −15 °C. Historians of military campaigns collect such remarks to reconstruct conditions, and they check the national habit of the writer before they trust the conversion.
Shops and signs
Some old shop thermometers, made as advertising pieces, show Réaumur on the left and Fahrenheit on the right, with the shop's name painted between. Collectors prize them for their typography. A quick check: if the freezing mark on both scales is at the same height, the Réaumur side should reach 80 where the Fahrenheit side reaches 212.
Why eighty degrees
Réaumur diluted alcohol with water until it expanded by 8 per cent between freezing and boiling, and divided the range into 80 steps, one for each tenth of a per cent. The choice tied the scale to the liquid. The Celsius scale, defined by two fixed points and a hundred steps, did not depend on the liquid in the tube, which helped it spread.
A short life of Réaumur
René-Antoine Ferchault de Réaumur lived from 1683 to 1757 and became a leading French scientist of his day. He wrote a multi-volume work on insects, studied the making of steel and tinplate, worked on porcelain and described the artificial incubation of eggs. His thermometer paper appeared in 1730. The scale carries his name, though much of his career went to other subjects.
Other old scales
The eighteenth century produced many scales. Newton's put freezing at 0 and boiling at 33. Rømer's put freezing at 7.5 and boiling at 60, so 100 °C is 60 °Rø. Delisle's counted downward, with boiling at 0 and freezing at 150. Each had its logic, and each survives in old tables. A Réaumur figure is just one of several to untangle.
Cheese, sweets and tradition
Some accounts say that the Réaumur scale lingered in a few traditional trades, such as cheese-making and confectionery in parts of Europe. A craftsman might heat a mixture to 44 degrees, which is 55 °C, as his teacher did. Anyone writing the recipe down for a modern kitchen needs the conversion, and may find the old numbers are round in Réaumur but not in Celsius.
Doctors and fever
Physicians in some countries once recorded fevers in Réaumur. A normal 29.6 °Ré is 37 °C, and a fever of 31 °Ré is 38.75 °C. Case notes from hospitals of the nineteenth century may mix scales between wards. A historian of medicine converts them with a “convert °C to °Ré” calculation before comparing patients.
Polar and mountain journals
Expedition journals recorded extreme cold. A low of −30 °Ré is −37.5 °C, and −40 °Ré is −50 °C. Such numbers matter for understanding what the explorers endured. Reports sometimes give the same day in two scales, and a modern editor adds the Celsius figure in brackets.
Baths and spas
Old guides to spas list water temperatures in Réaumur: a tepid bath at 26 to 28 degrees, which is 32.5 to 35 °C, and a warm bath at 30 to 32, which is 37.5 to 40 °C. Springs were described by their degrees. A visitor following a historical guide today converts them, as the water that was warm then is still warm now.
More values for convert °C to °Ré
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 °C = -218.52 °Ré
- -100 °C = -80 °Ré
- -20 °C = -16 °Ré
- 10 °C = 8 °Ré
- 15 °C = 12 °Ré
- 25 °C = 20 °Ré
- 30 °C = 24 °Ré
- 50 °C = 40 °Ré
Common slips when you convert °C to °Ré
Using 1.25 instead of 0.8
From Celsius to Réaumur the number gets smaller. If 20 °C became 25, the factor was inverted.
Adding 32
Réaumur and Celsius share a zero. Do not add 32, which belongs to Fahrenheit.
Confusing °R with Rankine
The symbol °R is ambiguous. In a historical text it is probably Réaumur, in a thermodynamics text Rankine.
Short answers
How do I convert °C to °Ré?
Multiply by 0.8.
What is 25 °C in Réaumur?
20 °Ré.
What is 100 °C in Réaumur?
80 °Ré.
What is 0 °C in Réaumur?
0 °Ré.
Does my number leave my device?
No, the calculation runs in your browser.
More pages and a wrap-up
For other temperature conversions see Convert °C to °F, Convert °C to °R, Convert °F to °Ré, Convert °Ré to °F, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L, Convert m/s to ft/s. The reverse direction is Convert °Ré to °C.
In short, a “convert °C to °Ré” calculation means multiplying by 0.8 and writing the unit beside the answer. Your number is computed locally, with no account and no upload.