Convert °F to °Ré degrees Fahrenheit ↔ degrees Réaumur Free Online Secure
Type a temperature in degrees Fahrenheit and read it in degrees Réaumur 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 °F to °Ré (degrees Fahrenheit ↔ degrees Réaumur)
| °F | °Ré |
|---|---|
| -40 | -32 |
| 0 | -14.2222222222 |
| 32 | 0 |
| 50 | 8 |
| 68 | 16 |
| 77 | 20 |
| 98.6 | 29.6 |
| 212 | 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
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.
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.
Comparing climates across the old divide: Fahrenheit into Réaumur
When the first weather networks were organised in the eighteenth century, British and American observers recorded temperature in Fahrenheit while their colleagues in France, the German states and Russia used Réaumur. Anyone who wants to set an English diary beside a Continental register must translate one scale into the other. To convert °F to °Ré, subtract 32 and multiply by 4/9: 32 °F is 0 °Ré, 68 °F is 16 °Ré and 212 °F is 80 °Ré. The calculator above does both steps as you type, and the sections below show how this translation worked in early meteorology, in travel writing and in the study of past climates.
You will find the formula with worked examples, a short profile of each scale, a comparison, a long set of cases from the history of weather observation and the slips to avoid. The arithmetic runs in your browser, and what you type is not sent anywhere.
How to convert °F to °Ré: subtract 32, then multiply by 4/9
The formula
The formula is °Ré = (°F − 32) × 4/9. So 68 °F gives 36 × 4/9 = 16 °Ré, 98.6 °F gives 29.6 °Ré and 0 °F gives −14.22 °Ré. To go back, use °F = °Ré × 2.25 + 32. The zeros differ, so the 32 must be subtracted before the scaling.
Worked examples
A London summer day of 77 °F is 20 °Ré. A winter freeze of 14 °F is −8 °Ré. A room at 70 °F is 16.9 °Ré. Boiling water at 212 °F is 80 °Ré, and a bitter −4 °F is −16 °Ré.
Why two steps
On Fahrenheit, water freezes at 32, on Réaumur at 0. So you first remove that offset. The range from freezing to boiling is then 180 Fahrenheit degrees or 80 Réaumur degrees, and the ratio is 4 to 9.
A quick sanity test
32 °F must give 0 °Ré and 212 °F must give 80 °Ré. If a “convert °F to °Ré” calculation formula fails either test, a step is wrong.
The Fahrenheit scale
The scale of British and American observers
Fahrenheit has 32 at freezing and 212 at boiling. Observers in Britain and its colonies used it for diaries, almanacs and weather registers, and it remains the everyday scale in the United States.
Where it appears
British and American diaries, registers and almanacs.
The Réaumur scale
The scale of the Continent
Réaumur has 0 at freezing and 80 at boiling. It was the standard in France, much of Germany, the Habsburg lands and Russia during the eighteenth and nineteenth centuries. The symbol is °Ré.
Where it appears
Continental diaries, registers, books and instruments from that period.
Fahrenheit against Réaumur
One Réaumur degree equals 2.25 Fahrenheit degrees, so a rise of 4 °Ré is a rise of 9 °F. The scales agree at one point only, −25.6, where both give the same number. For ordinary weather the numbers differ widely: 68 °F is 16 °Ré, and 86 °F is 24 °Ré.
The formula is exact, so rounding appears only in the displayed result. The usual slips are skipping the subtraction of 32 and using 5/9, which belongs to Celsius.
Where the conversion shows up
Early weather networks
In 1780 a scientific society based at Mannheim began to send matching instruments to observers across Europe, with a common Réaumur scale and rules for the hours of reading, so that readings could be compared. Observers elsewhere used their own scales. A historian placing a British Fahrenheit reading next to a Mannheim network reading must run a “convert °F to °Ré” calculation first.
Diaries on both sides
American statesmen and farmers kept weather diaries in Fahrenheit, often three times a day. A reading of 86 °F at three o'clock is 24 °Ré, and a morning reading of 5 °F is −12 °Ré. Continental diaries from the same decades give similar patterns in Réaumur, which can be set side by side after conversion.
Comparing regions
Roughly, January in London averages about 41 °F, which is 4 °Ré (a “convert °F to °Ré” calculation gives it at once), while St Petersburg sees about 14 °F, or −8 °Ré. July in London is near 64 °F, or 14.2 °Ré, and in Vienna near 70 °F, or 16.9 °Ré. Such comparisons made in the nineteenth century were done by hand, with printed conversion tables.
Conversion tables in printed books
Almanacs and encyclopaedias often included a table of corresponding degrees: 32 °F is 0 °Ré, 50 °F is 8 °Ré, 68 °F is 16 °Ré, 86 °F is 24 °Ré, 104 °F is 32 °Ré. The pattern repeats every 18 °F and 8 °Ré. Readers used the table to translate observations from foreign books. Today's online tool does the same job in one step.
Spreadsheet formulas
Put Fahrenheit values in column A and use =(A2-32)*4/9 in column B. For the reverse, use =B2*2.25+32. For differences, use =A2*4/9 with no subtraction. Format with one decimal, label the units clearly and write Ré with the accent.
Rounding and precision
Old thermometers were read to a whole degree or a half. Converting 68 °F gives exactly 16 °Ré, but 70 °F gives 16.89, which should be rounded to 16.9. Report converted values with a precision no finer than the original reading, which was about 0.5 °F, or 0.2 °Ré.
A mental shortcut
Subtract 32, then multiply by 0.44. For 68 °F: 36 times 0.44 is 15.84, close to 16. For 104 °F: 72 times 0.44 is 31.7, close to 32. The slight error is under one per cent. A more exact way is to take four ninths: divide by 9, multiply by 4.
Landmarks to memorise
32 °F is 0 °Ré, 50 °F is 8 °Ré, 68 °F is 16 °Ré, 86 °F is 24 °Ré, 104 °F is 32 °Ré and 212 °F is 80 °Ré. Every 18 °F adds 8 °Ré. With this pattern you can place any Fahrenheit reading between two landmarks.
Reference ladder
−40 °F is −32 °Ré. −4 °F is −16 °Ré. 0 °F is −14.2 °Ré. 14 °F is −8 °Ré. 32 °F is 0 °Ré. 59 °F is 12 °Ré. 77 °F is 20 °Ré. 98.6 °F is 29.6 °Ré. 212 °F is 80 °Ré. A ladder like this makes proofreading fast.
Checking by reversing
To check any convert °F to °Ré result, multiply it by 2.25 and add 32. If 68 °F became 16 °Ré, then 16 times 2.25 is 36, and adding 32 gives 68. A second test is the direction: above −25.6, the Réaumur value is lower than the Fahrenheit one.
Relation to Celsius
Réaumur is 0.8 times Celsius. A Fahrenheit reading becomes Celsius by (°F − 32) ÷ 1.8, and Réaumur by multiplying that by 0.8. So 68 °F is 20 °C and 16 °Ré. A historian with all three scales in a source can use the Celsius value as a bridge, and the three results must agree.
Temperature differences
A change of 18 °F is a change of 8 °Ré. A swing of 45 °F is a swing of 20 °Ré. Never subtract 32 from a difference. Daily ranges and seasonal amplitudes, which are differences, convert with the 4/9 factor alone.
Writing the units properly
Write °F and °Ré with a space after the number: 68 °F, 16 °Ré. In a scholarly edition give the original first and the converted figure in brackets: "68 °F (16 °Ré, 20 °C)". Avoid °R alone, which could be read as Rankine, and state the scale at the first mention.
A complete example
A diary entry from 1790 notes: "7 am, 28°; noon, 41°; 9 pm, 33°." If the observer used Fahrenheit, these are −1.8 °Ré, 4 °Ré and 0.4 °Ré, and in Celsius −2.2 °C, 5 °C and 0.6 °C. The conversion shows a cold morning and a mild noon. A “convert °F to °Ré” calculation for each entry builds a table that can be compared with other diaries.
Instruments with two scales
Instrument makers sold thermometers engraved with Fahrenheit on one side and Réaumur on the other. A reading of 95 °F matched 28 °Ré. A collector checks that the two scales line up: 32 °F opposite 0 °Ré and 212 °F opposite 80 °Ré. A thermometer where they do not agree may be a later restoration or a fake. A “convert °F to °Ré” calculation on a few points confirms the match.
Avoiding confusion in a team
When several people transcribe records, decide the process in advance. Record the original value and the scale in separate columns. Run the conversion by formula, not by hand. Review a sample of entries against a known pair, such as boiling at 212 °F and 80 °Ré. A shared checklist helps avoid mixing scales within a project.
Snow, ice and harbours
Harbour records noted the freezing of rivers and bays. A January with a mean near 20 °F, or −5.3 °Ré, kept the Baltic ports closed for weeks, while a mean of 30 °F, or −0.9 °Ré, did not. Ice dates from British and Continental ports reveal the severity of winters, and temperature records help to explain them after a conversion with a “convert °F to °Ré” calculation.
Volcanic summers and cold years
Historians of climate study unusually cold years, such as the one after a major eruption, using temperature records from many countries. A cold June at 55 °F, or 10.2 °Ré, in England and at 9 °Ré, or 11.25 °C, in a Continental register tell the same story from two scales. Putting them together requires a common unit, and the choice is nearly always Celsius.
Letters between scientists
Scientists on opposite sides of the Channel and the Atlantic wrote to one another about experiments, often comparing instruments. A correspondent who writes that a mixture "stood at 50 degrees" must say which scale. An English chemist's 50 °F is 8 °Ré, while a German's 50 °Ré is 62.5 °C. Letters that omit the scale have puzzled editors for generations.
Teaching the history of measurement
A class can reconstruct a conversion table from three fixed points: freezing at 32 °F and 0 °Ré, boiling at 212 °F and 80 °Ré, and a mid-point such as 122 °F and 40 °Ré. Plotting the points gives a straight line, showing that the scales are linearly related. The exercise explains why the conversion needs one subtraction and one scaling factor.
Archives and digital projects
Digital archives that host scanned weather registers add a transcription layer and a converted layer. For a British register, the converted layer might be in Celsius, with Réaumur added if the register compares itself with a Continental one. A project team writes a short note on the conversion method so that later users can trust the numbers and reproduce them with a “convert °F to °Ré” calculation of their own.
Maps of isotherms
Early maps of lines of equal temperature, made in the nineteenth century, had to combine data from many countries. Cartographers converted all readings to one scale before drawing the lines. A line of 50 °F, 8 °Ré or 10 °C is the same line, and maps sometimes label the three. Looking at such a map today, a reader can see how the climate of Europe was first understood as a whole.
Exploration and expedition logs
British expeditions to the Arctic recorded cold in Fahrenheit: −40 °F is −32 °Ré, and −50 °F is −36.4 °Ré. Russian expeditions to Siberia recorded in Réaumur: −40 °Ré is −50 °C, or −58 °F. Comparing the two records requires converting to a common scale, and modern climatologists choose Celsius.
Trade and industry
British engineers selling machines to the Continent supplied instructions with Fahrenheit temperatures, which local agents translated into Réaumur. A boiler running at 250 °F corresponds to 96.9 °Ré, and a drying room at 120 °F to 39.1 °Ré. A mistaken conversion could ruin a batch, so agents kept tables on their desks.
Medical records
An English physician noting a patient's temperature at 102 °F wrote of a fever. On the Continent the same patient would be at 31.1 °Ré. Normal body temperature is 98.6 °F, or 29.6 °Ré. Case histories exchanged between countries were annotated with both scales to prevent misreading.
Horticulture
Gardeners exchanging notes about hothouses compared temperatures: 55 °F is 10.2 °Ré, 65 °F is 14.7 °Ré, and 80 °F is 21.3 °Ré. A British head gardener advising a Russian estate would convert his usual figures with a “convert °F to °Ré” calculation. Winter protection for plants depended on keeping the glasshouse above freezing, 32 °F or 0 °Ré.
Seafaring
Ships' logs recorded air and sea temperatures. A British ship at sea in 60 °F air, which is 12.4 °Ré, logged a sea at 55 °F, which is 10.2 °Ré. Continental logs recorded the same weather in Réaumur. Maritime climatologists converting historical logs to Celsius take care to identify the scale for each fleet.
Plausibility checks
If a source gives a temperature without naming the scale, ask whether the number makes sense in each. A summer reading of 24 is pleasant in Réaumur (30 °C) and chilly in Fahrenheit (−4.4 °C is absurd for summer), so the scale must be Réaumur or Celsius. A winter reading of 14 is mild in Celsius, bitter in Fahrenheit and frosty in Réaumur. The check usually settles the question.
The point where the numbers meet
The two scales give the same number at −25.6. At that temperature both read −25.6, which is −32 °C. It is a quirk, unlike the famous coincidence of Fahrenheit and Celsius at −40. It is of little practical use, but it is a good test for a formula: put in −25.6 °F and you should get −25.6 °Ré.
More values for convert °F 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 °F = -135.622 °Ré
- -100 °F = -58.6667 °Ré
- -20 °F = -23.1111 °Ré
- 10 °F = -9.77778 °Ré
- 15 °F = -7.55556 °Ré
- 25 °F = -3.11111 °Ré
- 30 °F = -0.888889 °Ré
- 50 °F = 8 °Ré
Common slips when you convert °F to °Ré
Forgetting to subtract 32
Multiplying by 4/9 without subtracting gives a wrong result. 68 °F is not 30.2 °Ré but 16 °Ré.
Using 5/9
The factor 5/9 converts to Celsius. For Réaumur use 4/9.
Converting a difference as a temperature
A change of 18 °F is 8 °Ré. Do not subtract 32.
Short answers
How do I convert °F to °Ré?
Subtract 32 and multiply by 4/9.
What is 68 °F in Réaumur?
16 °Ré.
What is 212 °F in Réaumur?
80 °Ré.
What is 32 °F 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 °F to °R, Convert °F to K, Convert °C to °Ré, Convert °Ré to °C, Convert km to mi, Convert mi to km, Convert L to gal, Convert gal to L, Convert m to mm. The reverse direction is Convert °Ré to °F.
In short, a “convert °F to °Ré” calculation means subtracting 32, multiplying by 4/9 and writing the unit beside the answer. Your number is computed locally, with no account and no upload.