How temperature conversion works
Unlike most unit conversions, temperature is not a simple multiplication: each scale has its own zero point. To convert between any pair of units, the simplest approach is to use Celsius as an intermediate reference. The input value is first turned into Celsius, and that value is then converted into the desired target scale.
Formulas used
Celsius to Fahrenheit: F = C × 9/5 + 32
Celsius to Kelvin: K = C + 273.15
Celsius to Rankine: °R = (C + 273.15) × 9/5
Celsius to Réaumur: °Ré = C × 4/5
Worked example
Water boils at 100°C at sea level. Converting to Fahrenheit, we multiply 100 by 9/5, which gives 180, then add 32, reaching 212°F. In Kelvin, we simply add 273.15 to the 100°C, which results in 373.15 K.
The most common temperature scales
Celsius is used day to day across most of the world, with 0°C marking the freezing point of water and 100°C its boiling point at sea level. Fahrenheit is used mainly in the United States, where water freezes at 32°F and boils at 212°F. Kelvin is the scale used in science, starting at absolute zero, the lowest temperature theoretically possible, with no negative values. Rankine and Réaumur are less common today but still show up in specific technical and historical contexts.
A temperature change is not the same as a reading
A common mistake is applying the full conversion formula, with its offset of 32 or 273.15, when what is actually needed is converting how much a temperature changed. If an oven's temperature rose by 10°C, that rise corresponds to an 18°F increase, not the result of converting 10°C as if it were a reading, which would give 50°F. That is why this page includes a separate calculator just for temperature changes, without applying the offset used in absolute reading formulas.