What is the SI unit of temperature with symbol °C?

The SI unit of temperature with the symbol °C is Celsius. More precisely, the official unit name is the degree Celsius, while Celsius is the name commonly used in quiz answers and everyday speech.

The scale was proposed in 1742 by Swedish astronomer Anders Celsius. His original scale was reversed from today’s version: 0 represented water’s boiling point and 100 its freezing point at standard atmospheric pressure. The scale was later inverted, creating the arrangement now familiar worldwide.

Celsius is related directly to the kelvin, the SI base unit for thermodynamic temperature. A temperature of 0 °C equals 273.15 K, while a change of 1 °C is exactly the same size as a change of 1 K. Celsius is not the same as Fahrenheit or Rankine, which use different numerical intervals and reference systems.

The older term centigrade means “hundred steps” and is still encountered occasionally, but degree Celsius is the modern preferred name.