Convert °Ré to °F degrees Réaumur ↔ degrees Fahrenheit Free Online Secure
Type a temperature in degrees Réaumur and read it in degrees Fahrenheit at once. The relation is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert °Ré to °F (degrees Réaumur ↔ degrees Fahrenheit)
| °Ré | °F |
|---|---|
| -32 | -40 |
| -8 | 14 |
| 0 | 32 |
| 8 | 50 |
| 16 | 68 |
| 20 | 77 |
| 29.6 | 98.6 |
| 80 | 212 |
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.
Is it true that −40 °C and −40 °F are the same?
Yes, and it is the only place the two scales meet. Fahrenheit degrees are smaller than Celsius ones — nine of them to every five — and the two scales start from different zeros; the one temperature where the difference in size and the difference in starting point cancel out exactly is minus forty. Below that, the Fahrenheit number is the lower of the two.
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.
Great-grandfather's letters: Réaumur degrees into Fahrenheit
Many American families keep a bundle of letters, a diary or a bible with notes in the margin from relatives who came from Germany, Switzerland, Russia or the Austrian lands. Somewhere in them the writer remarks on the weather: "this winter we had twenty degrees of frost", "the heat reached twenty-eight". The unit was almost certainly Réaumur, and a descendant in the United States wants to know what it was in Fahrenheit. To convert °Ré to °F, multiply by 2.25 and add 32: 0 °Ré is 32 °F, 16 °Ré is 68 °F and 80 °Ré is 212 °F. The calculator above does both steps as you type, and the sections below help a family historian read such remarks and put them in context.
You will find the formula with worked examples, a short profile of each scale, a comparison, a long set of cases from family history and the slips to avoid. The arithmetic runs in your browser, and what you type is not sent anywhere.
How to convert °Ré to °F: multiply by 2.25, then add 32
The formula
The formula is °F = °Ré × 9/4 + 32, which is °Ré × 2.25 + 32. So 12 °Ré gives 12 × 2.25 + 32 = 59 °F, 29.6 °Ré gives 98.6 °F and −8 °Ré gives 14 °F. To go back, use (°F − 32) × 4/9. The zeros differ, so 32 must be added after the multiplication.
Worked examples
A mild spring day of 12 °Ré is 59 °F. A hot summer day of 25 °Ré is 88.25 °F. A cold night of −10 °Ré is 9.5 °F. A hard frost of −20 °Ré is −13 °F, and boiling water at 80 °Ré is 212 °F.
Why two steps
A Réaumur degree is 2.25 times a Fahrenheit degree, so you multiply. Water freezes at 0 °Ré and at 32 °F, so you add 32 to place the result on the Fahrenheit scale.
A quick sanity test
0 °Ré must give 32 °F and 80 °Ré must give 212 °F. If a “convert °Ré to °F” calculation formula misses either, a step is wrong.
The Réaumur scale
The scale of the old country
Réaumur put 0 at freezing and 80 at boiling. It was the common scale across much of Europe in the eighteenth and nineteenth centuries, which means family papers from those countries often use it. The symbol is °Ré.
Where it appears
Letters, diaries, family bibles, almanacs and local chronicles from the old country.
The Fahrenheit scale
The scale of the new home
Fahrenheit has 32 at freezing and 212 at boiling. In the United States it is the scale of everyday life, so a descendant needs it to feel what the ancestor felt.
Where it appears
American weather, thermostats, kitchens and the family's own records.
Réaumur against Fahrenheit
One Réaumur degree is 2.25 Fahrenheit degrees, so a swing of 4 °Ré is a swing of 9 °F. The numbers differ a great deal: 16 °Ré is 68 °F, and 24 °Ré is 86 °F. The scales agree only at −25.6, a curiosity without practical use.
The formula is exact, so rounding appears only when you display the answer. The common slips are forgetting the 32 and using 1.8, which belongs to Celsius.
Where the conversion shows up
Letters about weather
"We had twenty degrees of frost" in a nineteenth-century letter from the Russian Empire is −25 °C, or −13 °F, a hard winter. "The heat was twenty-five degrees" is 31.25 °C, or 88.25 °F. Without the conversion, an American reader might think the first was a mild day and the second a chilly one. A “convert °Ré to °F” calculation restores what the writer meant.
Emigrant diaries
Diaries of the sea voyage and the first winter often mention temperatures. A day on deck at "eight degrees" is 10 °C, or 50 °F. A night in the prairie sod house at "minus twelve" is −15 °C, or 5 °F. Family historians who convert such lines understand the hardships behind them.
Almanacs and family bibles
Some family bibles and household almanacs carry notes on frosts and harvests. A note that the first frost came when the thermometer showed "two degrees below" means −2.5 °C, or 27.5 °F, a hard freeze for the date. Combined with the date, such notes tell how early or late a season was.
Telling which scale the writer used
Clues help. If the letter comes from Germany, Switzerland, Austria, Russia or France before about 1860, Réaumur is likely. If the writer mentions that water boils at eighty degrees, it is certain. If the numbers in summer are in the twenties and in winter they run to minus twenty, Réaumur is probably the scale, because in Celsius those would be unusual in a temperate country.
School and military records
Schoolmasters' diaries and regimental logs mention cold days when drills were cancelled. "Closed at twelve degrees of frost" is −15 °C, or 5 °F. A conversion helps compare with American practice, where schools close in much colder weather in some states and in milder weather in others.
Photographs and thermometers
Old family photographs sometimes show a wall thermometer with two scales. Réaumur on the left and Fahrenheit or Celsius on the right is common. If you own an antique thermometer from the old country, check that 0 on the Réaumur side lines up with 32 °F and that 80 °Ré lines up with 212 °F. It confirms the scale and the instrument's quality.
Spreadsheet formulas
Place Réaumur values in column A and use =A2*2.25+32 in column B. For the reverse, use =(B2-32)*4/9. For differences, use =A2*2.25 with no 32. Add columns for the date, the source and a note. Keep the original number beside the converted one, so another researcher can check your work.
Rounding and precision
The formula is exact, but family records are rarely precise. A letter that says "about twenty degrees" does not justify 77.0 °F. Write "about 77 °F". When the text gives exact degrees, a rounded Fahrenheit value to the nearest degree is plenty.
A mental shortcut
Double the number, add a quarter of the result, then add 32. For 12 °Ré: 24 plus 6 is 30, plus 32 gives 62, close to the exact 59. A better shortcut is to multiply by 2.25 directly: 12 times 2.25 is 27, plus 32 is 59. For rough impressions, double and add 30 gets you within a few degrees.
Landmarks to memorise
0 °Ré is 32 °F, 4 °Ré is 41 °F, 8 °Ré is 50 °F, 12 °Ré is 59 °F, 16 °Ré is 68 °F and 20 °Ré is 77 °F. Every 4 °Ré adds 9 °F. With this ladder you can read most letters without a calculator.
Reference ladder
−32 °Ré is −40 °F. −24 °Ré is −22 °F. −16 °Ré is −4 °F. −8 °Ré is 14 °F. 0 °Ré is 32 °F. 24 °Ré is 86 °F. 29.6 °Ré is 98.6 °F. 32 °Ré is 104 °F. 80 °Ré is 212 °F. A ladder like this helps check a transcription.
Checking by reversing
To check any convert °Ré to °F result, subtract 32 and multiply by 4/9. If 12 °Ré became 59 °F, then 59 minus 32 is 27, and 27 times 4/9 is 12. A second test is direction: above −25.6, Fahrenheit is the larger number.
Relation to Celsius
Celsius is 1.25 times Réaumur. 16 °Ré is 20 °C, and 20 °C is 68 °F. A family historian whose source mentions two scales can use Celsius as a bridge. If a letter says "20 degrees Réaumur, or 25 degrees Celsius", the pairing confirms the reading.
Temperature differences
A change of 4 °Ré is a change of 9 °F. A fall of 8 degrees overnight is 18 °F. Differences are multiplied by 2.25 without adding 32. This matters in letters that describe a sudden turn in the weather: "it dropped twelve degrees in a night" is 27 °F.
Hot summers and the harvest
Writers often mention the heat of harvest. "Twenty-eight degrees in the shade" in Réaumur is 35 °C, or 95 °F, a heat wave. "Eighteen degrees" is 22.5 °C, or 72.5 °F, a pleasant day. The family historian can compare these with records of the same year from other sources to see whether the letter exaggerates.
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 a family history, show the original and the conversion: "twenty degrees of frost (−20 °Ré, −13 °F)". Avoid °R alone, which readers might take as Rankine.
Sharing your findings
When you publish a family history or post transcriptions online, add a short note about the scale and the factor. A line such as "Temperatures in the letters are in Réaumur; Fahrenheit equivalents are given in brackets" helps cousins who read the text later. A table of landmarks at the back, built with a “convert °Ré to °F” calculation, is an easy addition.
Learning from the weather
Combining a letter's weather note with the date and the place lets you check it against modern records, such as the frosts of a particular winter. A cold snap described as "twenty-two degrees" in January 1850 is −27.5 °C, or −17.5 °F. If regional records show the same, the letter is a reliable witness.
Letters from the prairie to the old country
Emigrants wrote back too, describing American weather to relatives who knew only Réaumur. A farmer in Kansas might report "the thermometer reached a hundred degrees" in Fahrenheit, and a cousin at home, reading it as Réaumur, would imagine a heat that would boil water. Replies often asked: "Which degrees do you mean?" A “convert °Ré to °F” calculation in the opposite direction, from Fahrenheit, answers it: 100 °F is 30.2 °Ré.
Building a family timeline
When you assemble a timeline of an ancestor's life, add the weather where known: a wedding in "a frost of ten degrees" in 1851 is at 9.5 °F, a baptism on a day with "eighteen degrees of heat" at 72.5 °F. These details make the story vivid for younger relatives, and the conversion table can sit in an appendix.
Language clues and translation
If you translate a letter with the help of a dictionary, note the word for degree and any abbreviation such as "R." or "gr. R." after the number. The abbreviation signals Réaumur. Translators add a note on first use, and a family historian can follow the same practice. It helps readers who might otherwise assume Fahrenheit and misjudge the weather.
Immigration papers and ship lists
Immigration papers rarely mention temperature, but accompanying letters and guidebooks do. Guidebooks for emigrants described the climate of destination states for readers who knew only Réaumur: "winters of ten to twenty degrees of frost" meant 9.5 to −13 °F in American terms. A modern reader who finds such a guide in an archive can see how the authors tried to prepare travellers, and a “convert °Ré to °F” calculation makes their advice concrete.
Family heirloom thermometers
An heirloom wall thermometer with a Réaumur scale tells you more than the weather. It shows which country the family came from and what they considered a comfortable room: perhaps a red mark at 14 °Ré, which is 63.5 °F. If the mark says "temperate" at 14 and "warm" at 18, the room standard of the time was cooler than ours, where 68 to 72 °F is typical.
Winters in the old country
"A winter of fifteen degrees" is −18.75 °C, or −1.75 °F, cold but common. "Twenty-five" is −31.25 °C, or −24.25 °F, severe. "Thirty" is −37.5 °C, or −35.5 °F, extraordinary. These can be compared with winters in the American Midwest, where the family settled: the same cold, in different numbers, and a reason that many emigrants did not find the prairie winter a shock.
The Atlantic crossing
Passenger accounts of the voyage mention sea and air temperatures. A cold morning at "four degrees" is 5 °C, or 41 °F, and a warm afternoon at "sixteen" is 20 °C, or 68 °F. Sailing ships crossed in about forty days. A “convert °Ré to °F” calculation for each note helps a descendant imagine what the passengers lived through.
Church records and parish chronicles
Parish chronicles sometimes record the weather on a notable day, such as a funeral or a flood. A burial in "a frost of eighteen degrees" is at −22.5 °C, or −8.5 °F. Such details are easy to miss if you read the number as Celsius or Fahrenheit. Genealogists who transcribe records keep the original and add the conversion in brackets.
Medical notes
Family letters sometimes report a child's fever: "the thermometer showed thirty-one degrees" is 38.75 °C, or 101.75 °F. A normal 29.6 °Ré is 98.6 °F. Read without the scale, 31 could be mistaken for a normal Celsius reading. Context, such as a letter about illness, makes it clear.
Farming and gardening advice
Letters from home often include advice. "Keep the cellar at six degrees" means 45.5 °F in Réaumur, a sensible figure for potatoes and apples. "The seed needs ten degrees in the soil" is 54.5 °F, close to what modern guides give for spring planting. The old advice often holds up when converted.
Cooking and household notes
A handwritten recipe may say "heat the milk to thirty degrees": 30 °Ré is 99.5 °F, body warm, as for yoghurt or cheese starters. "Water not hotter than forty" would be 122 °F, just comfortable for the hands. Such household details come alive once the scale is understood, and a “convert °Ré to °F” calculation for each makes the recipe usable.
Spas and healing waters
Letters from health resorts mention bath temperatures. "The bath is at twenty-eight degrees" is 35 °C, or 95 °F, and "thirty-two" is 40 °C, or 104 °F. A family member sent to take the waters wrote to relatives about the experience, and the figures show how warm the baths were.
More values for convert °Ré to °F
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 = -582.5875 °F
- -100 °Ré = -193 °F
- -20 °Ré = -13 °F
- 10 °Ré = 54.5 °F
- 15 °Ré = 65.75 °F
- 25 °Ré = 88.25 °F
- 30 °Ré = 99.5 °F
- 50 °Ré = 144.5 °F
Common slips when you convert °Ré to °F
Forgetting to add 32
Multiplying by 2.25 alone gives too low a number. 12 °Ré is not 27 °F but 59 °F.
Using 1.8
The factor 1.8 converts from Celsius. From Réaumur the factor is 2.25.
Converting a difference as a temperature
A change of 4 °Ré is a change of 9 °F. Do not add 32.
Quick questions
What is the method for a “convert °Ré to °F” calculation?
Multiply by 2.25, then add 32.
What is 16 °Ré in degrees Fahrenheit?
68 °F.
What is 80 °Ré in degrees Fahrenheit?
212 °F.
What is 0 °Ré in degrees Fahrenheit?
32 °F.
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 °C, Convert °C to °F, Convert °R to °F, Convert °F to K, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L, Convert dm to m. For the opposite direction, use Convert °F to °Ré.
The takeaway: a “convert °Ré to °F” calculation multiplies by 2.25, adds 32 and labels the result °F, with the original kept beside it. The figure you enter is processed on your device only.