Temperature scales explained — and why the conversion is affine
Celsius, Fahrenheit, Kelvin, Rankine, Réaumur. The first three are common; the last two are special-purpose.
Five scales appear on this site. They all measure the same physical quantity, but their reference points differ.
Celsius (°C)
The most common worldwide, used by every country except the US.
- 0 °C = freezing point of water (at 1 atm) - 100 °C = boiling point of water (at 1 atm) - same size as the kelvin
Fahrenheit (°F)
Used in the US for weather, cooking, body temperature.
- 32 °F = freezing point of water - 212 °F = boiling point of water - size is smaller (180 °F span water-ice to water-boil)
Kelvin (K)
The SI base. Same size as Celsius, but 0 K = absolute zero (−273.15 °C). This is the scale used in physics, chemistry, and engineering where negative temperatures don't make sense.
Rankine (°R) and Réaumur (°Ré)
Rankine is Fahrenheit-size, but starting at absolute zero (0 °R = −459.67 °F). Used in some US engineering disciplines (thermodynamics in particular). Réaumur was used historically for cheese-making and is making a small comeback in some European artisan contexts: 0 °Ré = freezing, 80 °Ré = boiling.
The conversion math
Temperature is affine — it's not just a multiplier; it's a multiplier plus an offset. To go from Celsius to Fahrenheit:
F = C \times \frac{9}{5} + 32
That term +32 is the offset (the zero points differ). You can't convert temperature correctly with a single multiplication factor.
Convert on this site: celsius to fahrenheit.