How to convert temperatures
Temperature scales measure the same physical quantity (the average thermal energy of molecules), but with different reference points and divisions. The most commonly used conversions:
Celsius to Fahrenheit
°F = °C × 9/5 + 32
Example: 25 °C × 9/5 + 32 = 45 + 32 = 77 °F
Fahrenheit to Celsius
°C = (°F − 32) × 5/9
Example: (98.6 − 32) × 5/9 = 66.6 × 5/9 ≈ 37 °C (human body temperature).
Celsius to Kelvin
K = °C + 273.15
Kelvin is the unit of the International System (SI). It does not use the degree symbol (°) and starts at absolute zero. That is why it is the official scale in Physics and Chemistry.
Why does Kelvin start at absolute zero?
Absolute zero (−273.15 °C or 0 K) is the temperature at which all molecular motion ceases. By physical convention it is impossible to go below it. Kelvin starts there because it simplifies thermodynamics formulas (e.g. the ideal gas law PV = nRT).
Other, rarer scales
- Rankine (°R): Kelvin on the Fahrenheit scale. Used in thermal engineering in the US. Conversion:
°R = °C × 9/5 + 491.67 - Réaumur (°Ré): divides 0–100 °C into 80 parts instead of 100. Formerly used in Europe in the cheese and wine industries. Conversion:
°Ré = °C × 4/5
Frequently asked questions
Why is 0 °C equal to 32 °F?
Fahrenheit chose as zero the lowest temperature he could produce in the laboratory (a mixture of ice, salt and ammonium chloride) and 96 °F as body temperature. Because of that arbitrary calibration, the freezing point of water ended up at 32.
Which scale is the most widely used in the world?
Celsius is the most common in almost the entire world. Fahrenheit dominates only in the United States. Kelvin is universal in science.
Can a temperature be below 0 K?
No. Absolute zero (0 K = −273.15 °C) is the theoretical minimum of temperature. Not even the coldest experiments ever performed have reached it.
Why does water boil at 100 °C?
By definition. The Celsius scale was created using the melting point (0) and boiling point (100) of water at sea level (1 atm) as references. At higher altitudes water boils below 100 °C because the pressure is lower.
Where do 5/9 and 9/5 come from in the formulas?
The interval between the freezing and boiling points of water is 100 °C or 180 °F (212 − 32). The ratio between the scales is 180/100 = 9/5 (and its inverse, 100/180 = 5/9).
Is there a maximum temperature?
Theoretically yes — the Planck temperature (~1.42 × 10³² K). Above it, the laws of physics as we know them stop working.