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Convert temperatures between the historic Reaumur scale and Celsius instantly. Essential for interpreting antique thermometers, historical scientific texts, and traditional European dairy and cheese-making recipes.
1.25
Multiply °Re to get °C
0 - 80
Reaumur Freeze-Boil Range
1730
Year Scale Was Invented
Formula: °C = °Re × 1.25 | °Re = °C × 0.8
| Reaumur (°Re) | Celsius (°C) | Context |
|---|---|---|
| -218.52 °Re | -273.15 °C | Absolute zero |
| -160 °Re | -200 °C | Extreme cryogenic temperatures |
| -80 °Re | -100 °C | Severe cold reference |
| -14.2 °Re | -17.8 °C | 0 °F (Fahrenheit zero) |
| 0 °Re | 0 °C | Freezing point of water |
| 8 °Re | 10 °C | Cool autumn day |
| 16 °Re | 20 °C | Comfortable room temperature |
| 20 °Re | 25 °C | Warm room / pleasant outdoor |
| 24 °Re | 30 °C | Hot summer day |
| 29.6 °Re | 37 °C | Normal human body temperature |
| 32 °Re | 40 °C | Heat wave temperature |
| 40 °Re | 50 °C | Very hot (desert extreme) |
| 56 °Re | 70 °C | Pasteurization temperature |
| 80 °Re | 100 °C | Boiling point of water |
| 160 °Re | 200 °C | Oven baking temperature |
The Reaumur scale (symbol: °Re or °R) is a historic temperature scale invented by French scientist Rene Antoine Ferchault de Reaumur in 1730. It defines the freezing point of water as 0 °Re and the boiling point of water as 80 °Re at standard atmospheric pressure. Reaumur created this scale while studying the thermal expansion of an alcohol-water mixture, which expanded by 80 parts per 1,000 between those two reference points — hence the 80-degree span.
The Celsius scale (symbol: °C), originally called the "centigrade" scale, was proposed by Swedish astronomer Anders Celsius in 1742. It divides the same freezing-to-boiling range of water into 100 equal degrees. Celsius is now the standard temperature scale in science, medicine, and everyday use throughout most of the world, forming part of the International System of Units (SI).
Because both scales share the same zero point (water freezing) but differ in their boiling points (80 vs. 100), the conversion between them is a simple ratio: °C = °Re × (100/80) = °Re × 1.25. Conversely, °Re = °C × (80/100) = °C × 0.8. This linear relationship — with no offset — makes Reaumur-to-Celsius one of the simplest temperature conversions.
The Reaumur scale was the dominant temperature scale in continental Europe for over a century. It was used by scientists like Lavoisier and Laplace, and was standard in France until the metric system reforms during the French Revolution gradually replaced it with the centigrade (Celsius) scale. Germany and Russia continued using Reaumur thermometers well into the 19th century, and the scale persists today in traditional cheese-making and dairy processes in parts of Switzerland, France, and Italy.
The formula is straightforward: °C = °Re × 1.25 (equivalently, °C = °Re × 5/4). Since both scales start at 0 for the freezing point of water, there is no additive offset — just a simple multiplication.
Question: A vintage French thermometer reads 16 °Re. What is this in Celsius?
Question: A historical medical text records a patient's temperature as 29.6 °Re. Convert to Celsius.
Question: A traditional Gruyere recipe calls for heating milk to 28 °Re. What Celsius setting should you use?
To multiply by 1.25 in your head, add the original number to one quarter of itself. For example: 40 °Re + (40 ÷ 4 = 10) = 50 °C. Or equivalently, divide the Reaumur value by 4, then multiply by 5. Both approaches give exact results since the conversion factor is a clean rational number (5/4).
| Condition | °Re | °C |
|---|---|---|
| Freezer temperature | -16 | -20 |
| Refrigerator | 3.2 | 4 |
| Cool morning | 8 | 10 |
| Spring day | 12 | 15 |
| Room temperature | 16 | 20 |
| Warm room | 20 | 25 |
| Hot summer day | 28 | 35 |
| Human body | 29.6 | 37 |
| Hot bath water | 32 | 40 |
| Process Step | °Re | °C |
|---|---|---|
| Milk storage | 3.2 | 4 |
| Mesophilic culture activation | 24 | 30 |
| Curd setting (soft cheese) | 25.6 | 32 |
| Curd setting (hard cheese) | 28 | 35 |
| Scalding curds (Gruyere) | 36 | 45 |
| Scalding curds (Emmental) | 42.4 | 53 |
| HTST pasteurization | 57.6 | 72 |
| Boiling / sterilization | 80 | 100 |
| Reference Point | °Re | °C |
|---|---|---|
| Absolute zero | -218.52 | -273.15 |
| Coldest recorded air temp (Antarctica) | -71.3 | -89.2 |
| Mercury freezes | -30.8 | -38.5 |
| Fahrenheit zero point | -14.2 | -17.8 |
| Water freezes (Reaumur zero) | 0 | 0 |
| Water boils (Reaumur top) | 80 | 100 |
| Lead melts | 262.6 | 328.2 |
| Iron melts | 1220.8 | 1,526 |
Centuries of European scientific literature — from Lavoisier's chemistry experiments to early meteorological records — recorded temperatures in Reaumur. Historians of science, archivists, and researchers need accurate conversion to interpret these primary sources and compare them with modern data.
Many artisanal cheese recipes from France, Switzerland, and Italy were written using Reaumur temperatures. Producers of Gruyere, Emmental, Comte, and other traditional cheeses sometimes still reference these historical temperatures to maintain centuries-old production methods and flavor profiles.
Collectors and museums frequently encounter 18th- and 19th-century thermometers marked with the Reaumur scale. Understanding the conversion is essential for correctly reading these instruments, verifying their calibration, and placing their readings in a modern context.
Historical weather diaries, agricultural records, and literary references from 18th-century France, Germany, and Russia frequently cite temperatures in Reaumur. Converting these values helps historians reconstruct past climates, understand daily life, and contextualize cultural artifacts.
Unlike Fahrenheit, Reaumur and Celsius both define 0 as the freezing point of water. This means there is no additive offset — only a multiplication factor of 1.25.
The factor 1.25 is exactly 5/4. For integer Reaumur values divisible by 4, this gives perfectly clean Celsius results. For example, 80 °Re × 5/4 = 100 °C exactly.
Since 100 > 80, a given temperature will always have a numerically higher Celsius value than its Reaumur equivalent (for the same sign). Use this as a quick sanity check on your conversions.
Both are sometimes abbreviated as °R. Rankine is an absolute temperature scale (like Kelvin but for Fahrenheit). Always check the context: historical European sources mean Reaumur; thermodynamic texts usually mean Rankine.
0.8 is the factor for Celsius-to-Reaumur (the reverse direction). To go from Reaumur to Celsius, always multiply by 1.25. Mixing these up gives a smaller number when you should get a larger one.
The Reaumur-to-Celsius conversion has no additive constant. If you find yourself adding or subtracting 32, you are likely confusing this with a Fahrenheit conversion formula.
Multiply the Reaumur temperature by 1.25 (or equivalently by 5/4). The formula is °C = °Re × 1.25. For example, 20 °Re × 1.25 = 25 °C. Both scales share the same zero point (the freezing point of water), so the conversion is a simple scaling factor with no offset.
The Reaumur scale was invented by French scientist Rene Antoine Ferchault de Reaumur in 1730. Reaumur was a polymath who made contributions to metallurgy, entomology, and natural history. He designed a thermometer using an alcohol-water mixture whose expansion properties led him to choose 80 degrees as the boiling-point marker.
Reaumur used an alcohol-water thermometric fluid that expanded by 80 thousandths of its volume when heated from the freezing point (0 °Re) to the boiling point (80 °Re) of water. He chose 80 divisions to match this physical expansion, unlike Celsius who chose 100 divisions for decimal convenience.
The Reaumur scale is essentially obsolete for general use. However, it persists in niche applications: some European cheese-making and dairy processes reference traditional Reaumur temperatures, antique thermometer collectors encounter it regularly, and historians need it to interpret 18th- and 19th-century scientific and medical records.
The Reaumur scale was widely used in France from 1730 until the metric reforms of the French Revolution. It also saw extensive use in Germany, Russia, Scotland, and other parts of continental Europe throughout the 18th and 19th centuries. Russia was among the last countries to phase it out, continuing to use Reaumur thermometers into the early 20th century.
Normal human body temperature of 37 °C equals 29.6 °Re. To convert: 37 × 0.8 = 29.6 °Re. In historical medical texts from France and Germany, fevers and body temperatures were commonly recorded on the Reaumur scale, so this is a particularly useful reference point for historians of medicine.
To convert Reaumur to Fahrenheit, use °F = (°Re × 2.25) + 32. All three scales share different reference points: Reaumur goes 0-80, Celsius goes 0-100, and Fahrenheit goes 32-212 between freezing and boiling water. Reaumur and Celsius share a common zero, but Fahrenheit offsets by 32 degrees.
Traditional European cheese-making recipes, especially from France, Switzerland, and Italy, were developed using Reaumur thermometers. Many artisan cheesemakers still reference these historical temperatures for curd heating, rennet activation, and aging processes. Converting accurately preserves centuries of dairy craft knowledge encoded in Reaumur degrees.
This calculator uses the exact conversion factor °C = °Re × 1.25 (5/4). The Reaumur scale is a historical temperature scale and is not used in modern scientific or industrial standards. For critical applications, always verify conversions and consult authoritative references for the specific context you are working with.