How to write degrees Celsius
The standard way to write degrees Celsius is two characters: the degree sign ° followed by a capital C. SI style puts a space between the number and the unit (21 °C) and never between ° and C. Do not drop the degree sign, either: "21 C" can be read as 21 coulombs.
For temperatures below zero, a true minus sign − looks better than a hyphen: −5 °C. To type °C, enter the degree sign (Alt + 0176 on Windows, Option + Shift + 8 on a Mac, a long press on 0 on iPhone) and then C. In HTML, write °C.
°C vs ℃
℃ (U+2103, DEGREE CELSIUS) is a single character that squeezes both parts into one glyph. It comes from East Asian character sets, where it fits the square grid of Chinese, Japanese and Korean text. Unicode treats it as a compatibility character, and °C is the better choice for normal use: it works in every font, matches the °F next to it, and is what people search for.
The same goes for ℉ (U+2109) and for K, the kelvin sign (U+212A), which is a duplicate of the capital letter K. Kelvin never takes a degree sign: write 300 K.
Converting Celsius and Fahrenheit
From Celsius to Fahrenheit, multiply by 9/5 (1.8) and add 32: 25 °C × 1.8 + 32 = 77 °F. To go back, subtract 32 and multiply by 5/9: (77 °F − 32) × 5/9 = 25 °C. For kelvin, add 273.15 to the Celsius value.
The scale is named after Swedish astronomer Anders Celsius, who proposed it in 1742. His original version ran backward, with 0 for boiling water and 100 for freezing, and was flipped soon after his death. It was widely called "centigrade" until 1948, when the name Celsius was officially adopted.
Posting about the weather? Pair °C with a sun ☀︎ or snowflake ❄︎.