Réaumur to Rømer Converter

The Réaumur scale (symbol: °Ré, °Re, or °R) is a temperature scale that divides the interval between the freezing point and boiling point of water into 80 equal divisions under standard atmospheric pressure (1 atmosphere).

Scale Calibration

Fixed Points:

• Freezing point of water: 0 degrees Réaumur (0°Ré)
• Boiling point of water: 80 degrees Réaumur (80°Ré)
• Degree size: Each Réaumur degree = 1.25 Celsius degrees (or 5/4 °C)

Mathematical Relationships:

• Réaumur to Celsius: °C = °Ré × 5/4 (or °Ré × 1.25)
• Celsius to Réaumur: °Ré = °C × 4/5 (or °C × 0.8)
• Réaumur to Fahrenheit: °F = (°Ré × 9/4) + 32 (or °Ré × 2.25 + 32)
• Fahrenheit to Réaumur: °Ré = (°F - 32) × 4/9

Why 80 Degrees?

Réaumur's choice of 80 degrees between water's freezing and boiling points was not arbitrary:

1. Base-8 arithmetic: 80 = 10 × 8, facilitating calculations in the duodecimal and base-8 systems common in 18th-century commerce
2. Divisibility: 80 has many factors (1, 2, 4, 5, 8, 10, 16, 20, 40, 80), making fraction conversions easier
3. Alcohol expansion: Réaumur's diluted ethanol expanded approximately 8% per 10 degrees, making 80 degrees a natural calibration
4. Practical range: Most European weather falls between -20°Ré and +30°Ré, yielding manageable numbers

Thermometric Fluid

Unlike Fahrenheit's mercury thermometers, Réaumur used diluted ethanol (alcohol-water mixture) because:

• Consistent expansion: 80 parts expansion per 1000 parts volume per 10°Ré
• Visibility: Clear alcohol + dye was easier to read than mercury
• Lower freezing point: Alcohol mixture remains liquid far below water's freezing point
• Safety: Less toxic than mercury for household thermometers

The Réaumur scale's 300-year history mirrors the development of scientific measurement, European political changes, and the eventual triumph of the metric system.

René-Antoine Ferchault de Réaumur (1683-1757)

Born in La Rochelle, France, Réaumur was a polymath whose work spanned entomology, metallurgy, and experimental physics. Appointed to the French Academy of Sciences at age 25 (1708), he gained fame for inventing processes to produce opaque "Réaumur porcelain" glass and techniques for steel production.

1730: Creation of the Scale

Réaumur's thermometer research began as an investigation into improving scientific instruments. His 1730 paper to the French Academy, "Règles pour construire des thermomètres dont les degrés soient comparables" ("Rules for Constructing Thermometers Whose Degrees Are Comparable"), proposed:

• Universal standard: All thermometers should use identical calibration points
• Reproducibility: Freezing and boiling water provided reliable fixed points
• Alcohol-based: Diluted ethanol expansion was more linear than wine spirit
• 80-degree scale: Practical for calculation and measurement precision

Réaumur's thermometers quickly became standard in French scientific institutions, replacing inconsistent instruments calibrated to "blood heat" or "deepest winter cold."

European Adoption (1730-1800)

The Réaumur scale spread across continental Europe within decades:

France (1730-1794):

• French Academy of Sciences adopted Réaumur as standard (1732)
• Weather observations recorded in Réaumur at royal observatories
• Public thermometers in Paris showed Réaumur readings
• Pharmacies and hospitals used Réaumur for medication storage

German States (1740s-1871):

• Prussia adopted Réaumur for meteorological observations (1740s)
• German scientific journals published temperatures in Réaumur
• Instrument makers in Nuremberg, Dresden, and Berlin standardized on Réaumur
• Persisted in German-speaking regions until unification standardization

Imperial Russia (1740s-1917):

• Russian Academy of Sciences adopted Réaumur (1740s)
• St. Petersburg and Moscow weather stations used Réaumur exclusively
• Remained official scale until Bolshevik Revolution (1917)
• Russian literature and documents reference Réaumur (Tolstoy, Dostoevsky novels)

Holy Roman Empire/Austria-Hungary:

• Vienna Observatory used Réaumur (1750s-1870s)
• Austrian meteorological network standardized on Réaumur
• Persisted in rural Austria and Hungary into the early 20th century

Competition with Other Scales (1742-1850)

The mid-18th century saw multiple temperature scales competing:

Fahrenheit (1714): Dominated Britain, Netherlands, and English-speaking world Celsius/Centigrade (1742): Proposed by Anders Celsius, initially inverted (100° freeze, 0° boil) Réaumur (1730): Dominant in France, Germany, Russia, Italy

Scientific preference gradually shifted toward Celsius due to:

• Decimal logic: 100 degrees matched metric system's base-10 philosophy
• International standardization: Celsius gained support from international scientific congresses
• Simplicity: 0-100 was conceptually cleaner than 0-80

Decline and Official Abandonment (1794-1900)

France (1794):

• French Revolution's metric system officially adopted Celsius (centigrade)
• Réaumur declared obsolete by Revolutionary government
• Rural France continued using Réaumur into the 1850s
• Instrument makers produced dual-scale thermometers (Réaumur/Celsius) through 1870s

Germany (1871-1880s):

• German unification prompted measurement standardization
• Meteorological services switched to Celsius (1876)
• Industrial and commercial sectors gradually converted (1880s-1900s)
• Last German Réaumur thermometers manufactured circa 1920

Russia (1917-1920):

• Bolshevik Revolution brought metric system adoption
• Soviet government mandated Celsius for all official purposes (1918-1920)
• Complete conversion by 1925

20th Century Survival (1900-Present)

Despite official abandonment, Réaumur persisted in niche applications:

Italian Dairy (1900s-present):

• Parmigiano-Reggiano DOP (Protected Designation of Origin) regulations specify Réaumur
• Traditional cheesemakers measure milk temperature in Réaumur for authenticity
• Consortium rules reference specific Réaumur temperatures for curd formation

Historical Literature:

• 18th-19th century scientific papers require Réaumur conversion
• Historical meteorological data recorded in Réaumur
• Antique thermometer collecting preserves knowledge

Symbolism and Tradition:

• European heritage associations preserve Réaumur knowledge
• Museum exhibits explaining pre-metric measurement systems
• Educational demonstrations of historical scientific practice

Historical Scientific Literature

Researchers studying 18th-19th century European science must convert Réaumur temperatures:

Meteorological Records:

• French, German, Russian weather observations (1730-1900)
• Climate historians reconstructing historical weather patterns
• Agricultural records linking crop yields to temperature data

Industrial Documentation:

• Metallurgy research from German states
• French textile dyeing process documentation
• Russian glass manufacturing temperature logs

Italian Artisan Cheesemaking

The Parmigiano-Reggiano Consortium (Consorzio del Formaggio Parmigiano-Reggiano) maintains traditional Réaumur specifications:

DOP Regulations Referencing Réaumur:

• Milk heating: Must reach 26.4-27.2°Ré (33-34°C) before rennet addition
• Curd cutting: Performed at specific Réaumur temperatures
• Whey separation: Temperature-critical step measured in Réaumur

Traditional cheesemakers use antique or reproduction Réaumur thermometers to maintain authenticity and comply with centuries-old recipes. Modern producers convert Celsius measurements but reference Réaumur in documentation.

Antique Thermometer Collecting

Réaumur thermometers are prized collectibles:

Value Factors:

• Age: 18th-century Réaumur thermometers: $500-5,000+
• Maker: Instruments by famous makers (Fahrenheit, Dollond): $2,000-20,000
• Condition: Working alcohol column increases value significantly
• Provenance: Scientific institution provenance adds premium

Collectors seek:

• Wall-mounted wooden-case thermometers (1750-1850)
• Brass-framed scientific instruments
• Dual-scale Réaumur/Celsius transition models (1850-1900)
• Russian Imperial thermometers with Cyrillic markings

Historical Reenactment and Museums

Living history sites and science museums demonstrate Réaumur thermometers:

• Colonial Williamsburg-style European village recreations
• Science museum historical instrument exhibits
• University physics department antique collections
• Historical society educational programs

Literary and Historical Research

Translators and historians must understand Réaumur references in:

Literature:

• Tolstoy's War and Peace: Russian temperatures in Réaumur
• Goethe's scientific writings: Réaumur measurements
• 19th-century French novels: Weather and fever descriptions

Historical Documents:

• Napoleon's Russian campaign weather logs (1812): -30°Ré cold
• French Revolution period documents
• Industrial Revolution factory records

Traditional European Confectionery

Some traditional European candy makers reference Réaumur in heritage recipes:

• Dutch sugar boiling techniques
• French confectionery historical recipes
• German marzipan production documentation (pre-1900)

Modern practitioners convert to Celsius but may cite Réaumur for historical authenticity.