Celsius to Réaumur Converter
Convert degrees Celsius to degrees Réaumur with our free online temperature converter.
Quick Answer
1 Celsius = 0.8 degrees Réaumur
Formula: Celsius × conversion factor = Réaumur
Use the calculator below for instant, accurate conversions.
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All conversion formulas on UnitsConverter.io have been verified against NIST (National Institute of Standards and Technology) guidelines and international SI standards. Our calculations are accurate to 10 decimal places for standard conversions and use arbitrary precision arithmetic for astronomical units.
Celsius to Réaumur Calculator
How to Use the Celsius to Réaumur Calculator:
- Enter the value you want to convert in the 'From' field (Celsius).
- The converted value in Réaumur will appear automatically in the 'To' field.
- Use the dropdown menus to select different units within the Temperature category.
- Click the swap button (⇌) to reverse the conversion direction.
How to Convert Celsius to Réaumur: Step-by-Step Guide
Temperature conversions like Celsius to Réaumur use specific non-linear formulas.
Formula:
°Ré = °C × 4/5Example Calculation:
Convert 10°C: 10 × 4/5 = 8.00°Ré
Disclaimer: For Reference Only
These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.
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View all Temperature conversions →What is a Celsius and a Réaumur?
What Is Celsius?
Degree Celsius (°C) is a unit of temperature on the Celsius scale, a temperature scale originally named "Centigrade" and renamed to honor Swedish astronomer Anders Celsius. It is the most common temperature scale used worldwide, adopted by virtually every country for everyday measurements and scientific work.
The Celsius scale is defined by two fixed points:
- 0°C: The freezing point of water at standard atmospheric pressure (1 atmosphere)
- 100°C: The boiling point of water at standard atmospheric pressure
The scale is divided into 100 equal intervals between these two points, making it a decimal-based (base-10) system that aligns perfectly with the metric system.
Modern scientific definition: Since 1954, Celsius has been defined relative to the Kelvin scale (the SI base unit for temperature):
- °C = K − 273.15
- K = °C + 273.15
This means a change of 1°C equals exactly a change of 1 K, but the zero points differ by 273.15 degrees.
Celsius vs. Other Temperature Scales
Celsius vs. Fahrenheit:
- Celsius: 0°C freezing, 100°C boiling (100-degree range)
- Fahrenheit: 32°F freezing, 212°F boiling (180-degree range)
- Conversion: °F = (°C × 9/5) + 32
- Use: Celsius used globally except US; Fahrenheit used primarily in US
Celsius vs. Kelvin:
- Celsius: Relative scale, can be negative, 0°C = freezing
- Kelvin: Absolute scale, no negative values, 0 K = absolute zero (-273.15°C)
- Conversion: K = °C + 273.15
- Use: Kelvin used in scientific contexts; Celsius for practical applications
Why Celsius is intuitive: The reference points (0°C and 100°C) are based on water phase transitions, which are fundamental to everyday life:
- Below 0°C: Water is solid (ice)
- 0°C to 100°C: Water is liquid
- Above 100°C: Water is gas (steam)
This makes Celsius immediately relatable—anyone who has seen ice melt or water boil understands these reference points.
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:
- Base-8 arithmetic: 80 = 10 × 8, facilitating calculations in the duodecimal and base-8 systems common in 18th-century commerce
- Divisibility: 80 has many factors (1, 2, 4, 5, 8, 10, 16, 20, 40, 80), making fraction conversions easier
- Alcohol expansion: Réaumur's diluted ethanol expanded approximately 8% per 10 degrees, making 80 degrees a natural calibration
- 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
Note: The Celsius is part of the metric (SI) system, primarily used globally in science and trade. The Réaumur belongs to the imperial/US customary system.
History of the Celsius and Réaumur
Anders Celsius and the Original Scale (1742)
In 1742, Swedish astronomer Anders Celsius (1701–1744) proposed a temperature scale based on two fixed points related to water. However, his original scale was inverted from what we use today:
Celsius's original scale (1742):
- 0°: Boiling point of water
- 100°: Freezing point of water
This counterintuitive arrangement had water freezing at the higher number and boiling at the lower number. Celsius chose this orientation possibly because he was primarily interested in measuring cold temperatures in Sweden, making it convenient to have larger numbers for colder conditions.
Why inversion? Some historians believe Celsius wanted to avoid negative numbers when measuring cold Swedish winters. By setting freezing at 100°, he could measure winter temperatures as positive values above 100.
The Reversal: Modern Celsius Scale
Shortly after Celsius's death in 1744, the scale was reversed to its current form, where:
- 0°: Freezing point of water
- 100°: Boiling point of water
Who reversed it? Historical records are unclear, but credit is typically given to one or both:
- Carl Linnaeus (1707–1778): Swedish botanist who worked at Uppsala University with Celsius
- Jean-Pierre Christin (1683–1755): French physicist who independently proposed a similar reversed scale in 1743
The reversed scale proved more intuitive—negative numbers represent below-freezing temperatures, and positive numbers represent above-freezing, aligning with everyday experience.
From "Centigrade" to "Celsius" (1948)
For over 200 years, the scale was commonly known as "Centigrade," from the Latin words:
- "Centi": hundred
- "Grade": steps or degrees
The name described the scale's defining characteristic: 100 equal intervals between freezing and boiling.
The 1948 name change: In 1948, the 9th General Conference on Weights and Measures (CGPM) officially renamed the scale from "Centigrade" to "Celsius" for two important reasons:
- Honor Anders Celsius: Recognize the inventor's contribution to science
- Avoid confusion: The term "centigrade" was also used in French and Spanish to describe angular measurements (1/100th of a right angle), creating potential confusion in scientific contexts
The renaming standardized international terminology, making "Celsius" the official name in all languages and scientific literature.
Adoption into the Metric System (SI)
1954 - SI Integration: The 10th General Conference on Weights and Measures formally adopted Celsius into the International System of Units (SI) in 1954. Celsius was defined relative to the Kelvin scale:
- Kelvin: SI base unit for thermodynamic temperature
- Celsius: Derived unit, defined as K − 273.15
This integration meant Celsius became part of the coherent system of metric units used worldwide for science, engineering, and commerce.
1967-1968 - Definition refinement: The definition was refined to be based on the triple point of water (0.01°C, 273.16 K) rather than ice point and boiling point, providing a more precise scientific standard.
2019 - Modern definition: Following the 2019 redefinition of SI base units, the Kelvin (and thus Celsius) is now defined by fixing the Boltzmann constant, providing an even more fundamental and reproducible definition.
Global Adoption (20th Century)
Throughout the 20th century, Celsius adoption spread globally as countries adopted the metric system:
Early adopters (1790s-1800s):
- France and other European countries adopting metric system
- Gradual spread through scientific community
Mid-20th century (1960s-1980s):
- United Kingdom transitioned from Fahrenheit to Celsius (1960s-1970s)
- Canada adopted Celsius in 1975
- Australia, New Zealand adopted metric/Celsius (1960s-1970s)
- Most former British colonies transitioned to Celsius
Modern status:
- 190+ countries use Celsius as the official temperature scale
- 3 countries primarily use Fahrenheit: United States, Bahamas, Cayman Islands
- Universal in international aviation, shipping, science, and medicine
The US Exception
The United States remains the primary holdout, continuing to use Fahrenheit for:
- Weather forecasts
- Household thermostats
- Cooking temperatures (ovens, recipes)
- Public discourse
However, Celsius is used in US contexts:
- Scientific research (NASA, universities)
- Military
- Medical (increasingly, alongside Fahrenheit)
- International trade and diplomacy
Multiple attempts to convert the US to metric/Celsius (notably in the 1970s) have failed due to cultural resistance, conversion costs, and lack of political will.
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
Common Uses and Applications: degrees Celsius vs degrees Réaumur
Explore the typical applications for both Celsius (metric) and Réaumur (imperial/US) to understand their common contexts.
Common Uses for degrees Celsius
The Celsius scale is the standard temperature measurement in nearly all countries except the United States, and is used extensively across all fields:
1. Weather and Meteorology
The primary temperature scale for weather forecasts, climate data, and meteorological reports worldwide. All international weather organizations use Celsius as the standard.
Weather reporting:
- Daily forecasts (high/low temperatures)
- Heat warnings (above 30-35°C)
- Freeze warnings (below 0°C)
- Wind chill calculations
- Heat index calculations
Climate science:
- Historical temperature records
- Climate change monitoring
- Sea surface temperature measurements
- Atmospheric temperature profiles
- Glacial and polar ice monitoring
Common Conversions:
- Celsius to Fahrenheit (for US weather reports)
- Celsius to Kelvin (for scientific weather data)
2. Domestic and Everyday Use
Daily temperature measurements including thermostats, air conditioning units, water heaters, and personal thermometers in all metric countries.
Household applications:
- Home heating thermostat settings (18-22°C)
- Air conditioning settings (22-24°C)
- Water heater temperature (50-60°C)
- Refrigerator temperature (4°C)
- Freezer temperature (-18°C)
- Baby bath water (37°C)
- Laundry water temperatures (cold, 30°C, 40°C, 60°C, 90°C)
3. Science and Research
Universal standard in scientific research alongside Kelvin. Used in chemistry, biology, physics, earth sciences, and engineering for temperature measurements and calculations.
Why Scientists Use Celsius:
- Easy conversion to Kelvin (K = °C + 273.15)
- Intuitive water-based reference points
- Decimal-based like other SI units
- International standardization
- Direct relationship to Kelvin (1°C = 1 K difference)
Scientific applications:
- Chemical reactions and kinetics
- Material testing and properties
- Biological experiments and incubation
- Environmental monitoring
- Quality control testing
4. Medical and Healthcare
Standard for body temperature measurements, medical equipment calibration, pharmaceutical storage requirements, and clinical guidelines worldwide.
Medical Temperature Guidelines:
- Normal body temperature: 36.5-37.5°C
- Fever threshold: Above 38°C
- Hypothermia risk: Below 35°C
- Hyperthermia emergency: Above 40°C
- Vaccine storage: 2-8°C (refrigerated) or -20°C (frozen)
Medical equipment:
- Digital thermometers
- Incubators and warmers
- Sterilization equipment (autoclaves at 121°C or 134°C)
- Laboratory analyzers
- Blood storage (4°C for whole blood, -80°C for plasma)
Convert medical temperatures with our temperature converter.
5. Culinary and Food Safety
Used for cooking instructions, food storage, and safety guidelines in most countries. Recipe books, ovens, and cooking appliances display temperatures in Celsius.
Food Safety Temperatures:
- Danger zone: 5-60°C (41-140°F) - bacteria multiply rapidly
- Refrigeration: 0-4°C (32-39°F)
- Freezing: -18°C (0°F) or below
- Safe minimum cooking: 75°C (167°F) for most foods
- Poultry: 75°C (167°F) internal temperature
- Ground meat: 71°C (160°F) internal temperature
Common Oven Settings:
- Slow/Low: 120-150°C (248-302°F)
- Moderate: 160-180°C (320-356°F)
- Standard: 180-200°C (356-392°F)
- Hot: 200-230°C (392-446°F)
- Very Hot: 230-250°C (446-482°F)
Use our Celsius to Fahrenheit converter for recipe conversions.
6. HVAC and Climate Control
Standard unit for heating, ventilation, and air conditioning systems in commercial and residential buildings worldwide.
Climate control:
- Programmable thermostats
- Central heating systems
- Air conditioning units
- Heat pumps
- Industrial climate control
- Data center cooling
7. Education
Taught as the primary temperature scale in schools worldwide as part of the metric system curriculum.
Educational contexts:
- Elementary science (water freezing/boiling)
- Chemistry (reaction temperatures)
- Physics (thermodynamics)
- Biology (optimal growth temperatures)
- Geography (climate zones)
8. Aviation and Transportation
International aviation uses Celsius for temperature reporting, along with other metric units.
Aviation applications:
- Outside air temperature (OAT)
- Engine temperature monitoring
- Cargo hold temperature
- De-icing temperature thresholds
- Weather reporting at airports (METAR/TAF)
9. Agriculture and Horticulture
Plant growth:
- Optimal growing temperatures (species-specific)
- Germination temperatures
- Greenhouse climate control
- Frost protection thresholds (below 0°C)
Livestock:
- Barn and shelter temperature monitoring
- Incubation temperatures (poultry)
- Heat stress thresholds
When to Use degrees Réaumur
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.
Additional Unit Information
About Celsius (°C)
Is Celsius the same as Centigrade?
Yes, 'Celsius' and 'Centigrade' refer to the same temperature scale.
History of the name:
- 1742-1948: Called "Centigrade" (from Latin: "centum" = hundred, "gradus" = steps)
- 1948: Officially renamed "Celsius" by the 9th General Conference on Weights and Measures
Reasons for the change:
- Honor Anders Celsius: Recognize the inventor's contribution
- Avoid confusion: "Centigrade" was also used for angular measurements (1/100th of a right angle), causing confusion in French and Spanish scientific literature
Modern usage: "Celsius" is the official and preferred term worldwide, though "Centigrade" is still occasionally heard, especially among older generations.
How does Celsius relate to Kelvin?
The Celsius scale is defined relative to the Kelvin scale, the SI base unit for thermodynamic temperature.
Key relationships:
- K = °C + 273.15 (Celsius to Kelvin)
- °C = K − 273.15 (Kelvin to Celsius)
- 1°C change = 1 K change (same interval size)
Differences:
- Zero points differ: 0°C = 273.15 K
- Kelvin is absolute: No negative values (0 K = absolute zero)
- Celsius is relative: Can be negative (negative values are below water's freezing point)
When to use which:
- Kelvin: Thermodynamics, gas laws, absolute temperature calculations
- Celsius: Everyday measurements, weather, cooking, most practical applications
Use our Celsius to Kelvin converter for instant conversions.
Why is Celsius used so widely?
Celsius is the global standard for several compelling reasons:
1. Intuitive reference points:
- 0°C = water freezes (ice formation)
- 100°C = water boils (steam formation)
- Water is fundamental to life, making these points universally relatable
2. Metric system integration:
- Decimal-based (base-10), like all metric units
- Easy to work with: 100 equal intervals
- Aligns with other SI units
3. Scientific convenience:
- Direct conversion to Kelvin (K = °C + 273.15)
- Same interval size as Kelvin (1°C = 1 K difference)
- International scientific standard
4. Global adoption:
- 190+ countries use Celsius officially
- International weather reporting
- Universal aviation standard
- Medical and healthcare standard
5. Simplicity:
- Negative temperatures = below freezing
- Positive temperatures = above freezing
- Easy to understand and remember
How do you convert Celsius to Fahrenheit quickly?
Quick mental math approximation:
- Multiply by 2
- Add 30
Examples:
- 20°C → (20 × 2) + 30 = 70°F (actual: 68°F, close!)
- 25°C → (25 × 2) + 30 = 80°F (actual: 77°F)
- 10°C → (10 × 2) + 30 = 50°F (actual: 50°F, exact!)
- 0°C → (0 × 2) + 30 = 30°F (actual: 32°F, within 2°)
Accuracy: Within 2-4°F for most common temperatures (0-30°C range)
For exact conversions:
- Formula: °F = (°C × 9/5) + 32
- Tool: Celsius to Fahrenheit converter
Memorize key points:
- 0°C = 32°F (freezing)
- 10°C = 50°F
- 20°C = 68°F
- 30°C = 86°F
- 37°C ≈ 98.6°F (body temperature)
What is a comfortable room temperature in Celsius?
Standard comfortable room temperature: 20-22°C (68-72°F)
Detailed comfort ranges:
- 16-18°C (61-64°F): Cool, good for sleeping
- 18-19°C (64-66°F): Comfortable with warm clothing
- 20-21°C (68-70°F): Ideal for most people during daytime activities
- 22-23°C (72-73°F): Warm and comfortable
- 24-25°C (75-77°F): Getting warm, may need cooling
- Above 26°C (79°F): Uncomfortably warm indoors
Factors affecting comfort:
- Humidity: Higher humidity feels warmer
- Air movement: Fans increase comfort
- Activity level: Exercise generates heat
- Clothing: More clothing allows lower temperatures
- Personal preference: Varies by individual
- Acclimatization: People adapt to local climates
Typical thermostat settings:
- Winter heating: 20°C (68°F)
- Summer cooling: 24°C (75°F)
- Energy savings: Lower in winter (18°C), higher in summer (26°C)
- Office standard: 21-22°C (70-72°F)
At what Celsius temperature does water boil at high altitude?
Water boils at lower temperatures at high altitude because atmospheric pressure decreases:
Boiling point by altitude:
- Sea level (0m): 100°C (212°F)
- 500m (1,640ft): ~98.5°C (209°F)
- 1,000m (3,281ft): ~97°C (207°F)
- 1,500m (4,921ft): ~95°C (203°F)
- 2,000m (6,562ft): ~93°C (199°F)
- 3,000m (9,843ft): ~90°C (194°F)
- 4,000m (13,123ft): ~87°C (189°F)
- 5,000m (16,404ft): ~83°C (181°F)
- 8,849m (29,032ft - Mt. Everest): ~71°C (160°F)
Rule of thumb: Water's boiling point decreases by approximately 1°C for every 300m (or 1°F per 500ft) increase in elevation.
Why this matters:
- Cooking times increase: Food takes longer to cook at lower boiling temperatures
- Pasta, rice, vegetables: May need extra time
- Baking adjustments: Recipes may need modification at high altitude
- Tea/coffee brewing: Lower temperature may affect flavor extraction
Is 20°C hot or cold?
20°C (68°F) is generally considered mild to comfortable—neither hot nor cold.
Context matters:
Indoor temperature:
- Perfect room temperature for most people
- Standard thermostat setting in many countries
- Comfortable for light clothing
Outdoor weather:
- Pleasant spring/fall day
- Light jacket or sweater may be comfortable
- Good weather for outdoor activities
Water temperature:
- Cool for swimming
- Tolerable for active swimming, cold for leisure
- Ocean/lake water at 20°C feels refreshing but cool
Sleeping:
- Slightly warm for optimal sleep
- Most people prefer 16-18°C (61-64°F) for sleeping
Cultural/regional perspectives:
- Tropical residents: May find 20°C cold
- Arctic residents: May find 20°C warm
- Temperate zone residents: Find it comfortable and pleasant
Humidity factor:
- 20°C with high humidity feels warmer
- 20°C with low humidity feels cooler
What temperature is dangerous for humans in Celsius?
Dangerously Cold (Hypothermia) - Body Temperature:
- Below 35°C (95°F): Hypothermia begins, shivering
- 32-35°C (89-95°F): Mild hypothermia, confusion, drowsiness
- 28-32°C (82-89°F): Moderate hypothermia, irregular heartbeat
- Below 28°C (82°F): Severe hypothermia, unconsciousness, life-threatening
- Below 24°C (75°F): Usually fatal
Dangerously Hot (Hyperthermia) - Body Temperature:
- 38°C (100.4°F): Fever/heat stress
- 39°C (102.2°F): Moderate fever
- 40°C (104°F): High fever, medical attention needed
- 41°C (105.8°F): Heat stroke risk, emergency
- 42°C (107.6°F): Critical, organ damage begins
- Above 43°C (109.4°F): Usually fatal without rapid cooling
Environmental Temperature Dangers:
Cold:
- Below -40°C (-40°F): Frostbite in minutes, exposed skin freezes
- -30 to -40°C (-22 to -40°F): Extreme cold, survival difficult
- -20 to -30°C (-4 to -22°F): Very cold, proper protection essential
- Below -10°C (14°F): Frostbite risk on exposed skin
Heat:
- Above 35°C (95°F): Heat stress risk, especially with high humidity
- 40-45°C (104-113°F): Heat exhaustion and heat stroke risk
- Above 50°C (122°F): Survival difficult without shade, water, and cooling
- Above 55°C (131°F): Extreme danger, few minutes of exposure can be fatal
Heat Index (temperature + humidity): High humidity makes temperatures feel hotter and increases danger—40°C with high humidity can be more dangerous than 45°C with low humidity.
Why do Americans use Fahrenheit instead of Celsius?
Historical reasons:
1. Early adoption (1720s):
- Fahrenheit scale invented in 1724 by Daniel Gabriel Fahrenheit
- Adopted in English-speaking world, including American colonies
- Celsius wasn't invented until 1742, after Fahrenheit was established
2. Independence (1776):
- US gained independence before metric system was developed (1790s)
- American infrastructure already built around British Imperial system
- No compelling reason to change at the time
3. Metric system resistance:
- France developed metric system in 1790s
- US chose not to adopt metric officially
- Multiple attempts to convert US to metric have failed (notably 1970s)
Cultural and practical reasons:
1. Cultural inertia:
- Generations of Americans learned Fahrenheit
- Emotional attachment to familiar measurements
- "If it ain't broke, don't fix it" mentality
2. Conversion costs:
- Enormous expense to convert infrastructure
- All weather stations, thermostats, ovens, road signs
- Industrial equipment, scientific instruments
- Education system overhaul needed
3. Perceived precision:
- Fahrenheit has smaller degree increments
- 1°F = 0.56°C (finer granularity)
- Some argue this is more precise for everyday use
Current status:
Fahrenheit domains (US):
- Weather forecasts
- Household thermostats
- Cooking temperatures (ovens)
- Public discourse
Celsius domains (US):
- Scientific research (NASA, universities)
- Military
- Medical (increasingly)
- International trade/diplomacy
Other Fahrenheit users: Only 3 countries primarily use Fahrenheit: United States, Bahamas, Cayman Islands. The rest of the world (190+ countries) uses Celsius.
Practical impact:
- Americans traveling abroad must learn Celsius
- International collaboration requires conversion
- Many Americans now learn both scales
- US is increasingly isolated in temperature measurement
Use our Fahrenheit to Celsius converter to easily switch between scales.
What is normal body temperature in Celsius?
Normal body temperature: 36.5-37.5°C (97.7-99.5°F)
Average: 37°C (98.6°F)
Important factors affecting body temperature:
1. Time of day:
- Morning (6 AM): Lower, around 36.3°C (97.3°F)
- Afternoon/Evening (6 PM): Higher, around 37.3°C (99.1°F)
- Daily variation: About 0.5-1°C difference
2. Measurement location:
- Rectal: 0.5°C (0.9°F) higher than oral (most accurate)
- Oral: Standard reference point
- Ear (tympanic): Similar to rectal if done correctly
- Armpit (axillary): 0.5°C (0.9°F) lower than oral (least accurate)
- Forehead (temporal): Convenient but less accurate
3. Age:
- Infants: Slightly higher (36.6-37.8°C / 97.9-100°F)
- Children: Similar to adults
- Elderly: May be slightly lower (35.8-36.9°C / 96.4-98.4°F)
4. Activity level:
- Rest: Lower baseline temperature
- Exercise: Can temporarily raise to 38-39°C (100-102°F)
- Digestion: Slightly raises temperature
5. Other factors:
- Menstrual cycle (women)
- Time since eating
- Ambient temperature
- Hydration status
- Circadian rhythm
Body temperature guide:
Below normal:
- Below 35°C (95°F): Hypothermia, medical concern
- 35-36°C (95-96.8°F): Mild hypothermia possible
- 36-36.5°C (96.8-97.7°F): Lower end of normal
Normal range:
- 36.5-37.5°C (97.7-99.5°F): Normal healthy range
- 37°C (98.6°F): Classic "normal" temperature (average)
Elevated/Fever:
- 37.5-38°C (99.5-100.4°F): Slightly elevated, monitor
- 38-39°C (100.4-102.2°F): Low-grade fever
- 39-40°C (102.2-104°F): Moderate fever, monitor closely
- Above 40°C (104°F): High fever, seek medical attention
Measurement best practices:
- Wait 30 minutes after eating/drinking/exercise before measuring
- Use same method consistently for comparison
- Digital thermometers most accurate for home use
- For infants: rectal measurement most reliable
How many degrees Celsius is freezing?
Water freezes at 0°C (32°F) at standard atmospheric pressure (sea level, 1 atmosphere).
What "freezing" means:
- 0°C: Temperature at which water transitions from liquid to solid (ice)
- Below 0°C: Water is solid (ice, snow)
- Above 0°C: Ice melts to liquid water
- Exactly 0°C: Water and ice can coexist in equilibrium
This is a defining point: The Celsius scale is specifically defined with 0°C as the freezing point of pure water, making it an intuitive and memorable reference.
Factors affecting freezing point:
1. Salinity:
- Pure water: 0°C (32°F)
- Seawater (~3.5% salt): ~-2°C (28°F)
- Saturated salt solution: ~-21°C (-6°F)
2. Pressure:
- Higher pressure: Slightly lowers freezing point
- Lower pressure: Slightly raises freezing point
- Effect is small: About -0.0075°C per atmosphere
3. Impurities/additives:
- Sugar: Lowers freezing point (ice cream stays soft)
- Alcohol: Significantly lowers freezing point (vodka freezes at -27°C)
- Antifreeze (ethylene glycol): Lowers to -37°C (50/50 mix)
- Road salt (calcium chloride): Melts ice down to -25°C
Weather context:
Freezing conditions:
- Below 0°C: Freezing, snow and ice form, water pipes at risk
- 0 to -5°C: Light freeze, frost forms overnight
- -5 to -10°C: Moderate freeze, icy roads
- Below -10°C: Hard freeze, outdoor activities limited
Near-freezing:
- 0-2°C: Just above freezing, frost possible
- 2-5°C: Cool, generally no freezing concerns
- 5-10°C: Cold but no freeze risk
Freezer temperatures:
- -18°C (0°F): Standard home freezer (well below freezing)
- -20°C (-4°F): Deep freeze
- -40°C (-40°F): Ultra-cold freeze (commercial/research)
Why 0°C matters:
- Frost warnings issued when temperature drops below 0°C
- Roads ice over below 0°C
- Outdoor water pipes freeze below 0°C
- Plants vulnerable to frost damage below 0°C
About Réaumur (°Ré)
What are the freezing and boiling points of water in Réaumur?
Water freezes at 0°Ré and boils at 80°Ré under standard atmospheric pressure (1 atm at sea level). This 80-degree span is the defining feature of the Réaumur scale, compared to Celsius's 100-degree span.
How does Réaumur relate to Celsius?
1 degree Réaumur = 1.25 degrees Celsius (or 5/4 °C)
Conversion formulas:
- °C = °Ré × 5/4 (or °Ré × 1.25)
- °Ré = °C × 4/5 (or °C × 0.8)
Both scales set water's freezing point at 0°, making conversions straightforward multiplication without offset terms.
Is the Réaumur scale still commonly used?
No, it is rarely used today, having been superseded by Celsius throughout Europe during the late 19th and early 20th centuries. However, Réaumur survives in:
- Traditional Italian cheesemaking (Parmigiano-Reggiano DOP specifications)
- Historical document interpretation (18th-19th century literature and science)
- Antique thermometer collecting
- Some traditional European confectionery practices
Modern usage is essentially limited to historical and artisan contexts.
Why did Réaumur choose 80 degrees instead of 100?
Réaumur's 80-degree scale reflected 18th-century practical considerations:
- Alcohol expansion rate: His diluted ethanol expanded approximately 80 units per 1,000 between water's freezing and boiling
- Mathematical divisibility: 80 has many factors (2, 4, 5, 8, 10, 16, 20, 40), simplifying fractional calculations
- Base-8 arithmetic: 80 = 10 × 8, fitting duodecimal and octal systems used in commerce
- Practical precision: 80 divisions provided sufficient resolution without excessive graduations on thermometer tubes
The choice was empirically driven by his instrument's physical properties rather than abstract decimal preference.
Which countries historically used the Réaumur scale?
Primary Réaumur users (1730-1900):
- France: 1730-1794 officially; lingered until 1850s-1870s in practice
- German states/Germany: 1740s-1871 officially; transition through 1900s
- Imperial Russia: 1740s-1917
- Austria-Hungary: 1750s-1870s officially; rural use into early 1900s
- Parts of Italy: Particularly northern regions; survives in traditional dairy
- Switzerland: German-speaking cantons used Réaumur until late 1800s
Countries that NEVER adopted Réaumur:
- Britain (used Fahrenheit)
- United States (Fahrenheit)
- Spain (used regional scales, then Celsius)
- Netherlands (Fahrenheit preference)
How do you convert a Réaumur temperature to Fahrenheit?
Two-step method:
- Convert Réaumur to Celsius: °C = °Ré × 1.25
- Convert Celsius to Fahrenheit: °F = (°C × 9/5) + 32
Direct formula: °F = (°Ré × 9/4) + 32 (or °Ré × 2.25 + 32)
Example: 20°Ré to Fahrenheit
- 20°Ré × 2.25 = 45
- 45 + 32 = 77°F
Why does Italian Parmigiano-Reggiano still use Réaumur?
Tradition and legal protection:
Parmigiano-Reggiano is a DOP (Protected Designation of Origin) product, meaning production methods are legally codified to preserve historical authenticity. Original recipes and techniques from the 18th-19th centuries specified Réaumur temperatures, and DOP regulations maintain these specifications.
Reasons for retention:
- Historical authenticity: Preserves traditional cheesemaking heritage
- Legal documentation: Original consortium rules cited Réaumur
- Artisan identity: Reinforces traditional, non-industrial methods
- Practical continuity: Changing regulations requires bureaucratic process
Modern cheesemakers use Celsius thermometers but convert and reference Réaumur in documentation for DOP compliance.
What does "°R" mean on an antique thermometer?
On European antique thermometers (pre-1900): °R = Réaumur
Check for confirmation:
- Freezing point marked 0°R
- Boiling point marked 80°R (not 100 or 212)
- European origin (French, German, Russian, Italian)
- Pre-1900 manufacture date
On American engineering documents (post-1900): °R = Rankine
- Absolute temperature scale (°R = °F + 459.67)
- Used in US thermodynamics and engineering
Context, origin, and scale markings determine which "°R" is meant.
How do you read historical weather data recorded in Réaumur?
Step-by-step conversion:
- Identify the temperature in Réaumur (e.g., -25°Ré during Napoleon's 1812 Russian campaign)
- Convert to Celsius: °C = °Ré × 1.25
- -25°Ré × 1.25 = -31.25°C
- Convert to Fahrenheit if desired: °F = (°C × 9/5) + 32
- (-31.25°C × 1.8) + 32 = -24.25°F
Example: Paris summer 1783 recorded as 28°Ré
- 28°Ré × 1.25 = 35°C = 95°F (significant heat wave)
Can you still buy Réaumur thermometers?
Original antiques: Available from antique dealers, auction houses, and specialty collectors
- Prices: $200-$5,000+ depending on age, condition, maker
- Functionality: Many have degraded alcohol columns (display only)
Modern reproductions: Some specialty scientific instrument makers produce Réaumur thermometers for:
- Museum exhibits and educational purposes
- Traditional cheesemaking (small-scale production for Parmigiano-Reggiano artisans)
- Historical reenactment groups
Dual-scale thermometers: Réaumur/Celsius transition thermometers (1850-1900) are collector favorites, showing both scales side-by-side.
What is the relationship between Réaumur and Kelvin?
Kelvin (absolute thermodynamic scale):
- K = °C + 273.15 (absolute zero at 0 K)
Réaumur to Kelvin:
- Convert Réaumur to Celsius: °C = °Ré × 1.25
- Add 273.15: K = (°Ré × 1.25) + 273.15
Direct formula: K = (°Ré × 5/4) + 273.15
Example: 0°Ré (water freezing)
- (0 × 1.25) + 273.15 = 273.15 K
Example: 80°Ré (water boiling)
- (80 × 1.25) + 273.15 = 100 + 273.15 = 373.15 K
Why did Réaumur use alcohol instead of mercury?
Réaumur chose diluted ethanol (alcohol-water mixture) over mercury for several reasons:
Technical advantages:
- Consistent expansion: Alcohol's thermal expansion was more linear than wine spirits
- Visibility: Clear liquid + dye was easier to read than opaque mercury
- Larger expansion coefficient: Alcohol expanded more per degree, improving readability
- Lower freezing point: Alcohol mixture remained liquid well below 0°C
Practical considerations:
- Safety: Less toxic than mercury for household use
- Cost: Cheaper to produce than mercury thermometers
- Availability: Alcohol was readily available in 18th-century France
Limitations:
- Boiling point: Alcohol limited upper temperature range (compared to mercury's 357°C boiling point)
- Evaporation: Over decades, alcohol could slowly evaporate through glass, affecting calibration
People Also Ask
How do I convert Celsius to Réaumur?
To convert Celsius to Réaumur, enter the value in Celsius in the calculator above. The conversion will happen automatically. Use our free online converter for instant and accurate results. You can also visit our temperature converter page to convert between other units in this category.
Learn more →What is the conversion factor from Celsius to Réaumur?
The conversion factor depends on the specific relationship between Celsius and Réaumur. You can find the exact conversion formula and factor on this page. Our calculator handles all calculations automatically. See the conversion table above for common values.
Can I convert Réaumur back to Celsius?
Yes! You can easily convert Réaumur back to Celsius by using the swap button (⇌) in the calculator above, or by visiting our Réaumur to Celsius converter page. You can also explore other temperature conversions on our category page.
Learn more →What are common uses for Celsius and Réaumur?
Celsius and Réaumur are both standard units used in temperature measurements. They are commonly used in various applications including engineering, construction, cooking, and scientific research. Browse our temperature converter for more conversion options.
For more temperature conversion questions, visit our FAQ page or explore our conversion guides.
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Last verified: February 19, 2026