If Celsius uses °C, why is kelvin simply K? The two scales use equal-sized intervals, but their modern unit symbols follow different conventions: degree Celsius keeps the degree sign; kelvin does not.
That is part of the notation itself, not a typographic flourish. Adding or removing one mark can turn a current unit expression into a historical or incorrect one.
Why Celsius has ° but kelvin does not
| Quantity | Unit name | Unit symbol | Example |
|---|---|---|---|
| Celsius temperature | degree Celsius | °C | 20 °C |
| Thermodynamic temperature | kelvin | K | 293.15 K |
The kelvin is an SI base unit. Its name is kelvin and its symbol is the ordinary capital letter K. In 1967, the CGPM replaced the older unit name “degree Kelvin” and symbol °K with “kelvin” and K.
The degree Celsius keeps its historical name
The unit of Celsius temperature is the degree Celsius, symbol °C. The size of one degree Celsius is equal to the size of one kelvin, but the Celsius scale has a different zero point.
The relation is exact: 0 °C = 273.15 K. Temperature differences are simpler: a difference of 5 °C has the same magnitude as a difference of 5 K.
What happened to °K
Historical scientific documents may use °K or the phrase “degree Kelvin.” Those forms were once official. They are no longer the modern SI notation.
If you are quoting or transcribing a historical source, preserving °K may be the right editorial choice. If you are writing current technical text, use K.
The 1967 resolution allowed the older usages temporarily, which helps explain why °K continued to appear in instruments, textbooks, and archived papers after the change. A 1980 CIPM note later requested that the symbols °K and deg no longer be permitted, closing that transition in official usage.
Kelvin uses the ordinary letter K
Unicode also includes U+212A KELVIN SIGN, K. Unicode normalization treats it as canonically equivalent to U+004B, and the standard directs authors to the ordinary letter instead. The distinction is covered in Kelvin Sign K vs Letter K.
So modern kelvin notation does not require a special “Kelvin glyph.” The unit symbol is simply capital K used according to SI rules.
As with other unit symbols, K is not pluralized and does not take a period except as normal sentence punctuation. Write 1 K and 20 K, not “Ks.”
Where the spaces go
In SI writing, a space separates the numerical value from the unit symbol: 20 °C and 293.15 K. This number-unit boundary is a recurring feature of measurement symbols; inside °C, however, the degree sign and C stay together because they form one unit symbol.
Plane angles use a different spacing convention: 20° has no gap before the degree sign. The character is still U+00B0, but its role in an angle is not the same typographic construction as the unit symbol °C.
Forms you may see, but should not introduce
- 20° K adds a degree sign to the kelvin symbol.
- 20°K uses the historical degree-Kelvin style.
- 20 C omits the degree sign from the Celsius unit symbol.
- 20 ℃ uses U+2103 DEGREE CELSIUS, a compatibility character that Unicode recommends replacing with the ordinary °C sequence in normal use.
The last form can still represent Celsius correctly to a reader; its issue is Unicode representation, not the temperature itself. See ℃ vs °C for that distinction.
Absolute temperature and temperature intervals
The Celsius and kelvin scales have the same interval size but different zero points. Consequently, 20 °C = 293.15 K, while 20 K is a much lower absolute temperature.
For a temperature difference or interval, the numerical values can match. An increase from 20 °C to 25 °C is a difference of 5 °C, and that interval has the same magnitude as 5 K. The BIPM explicitly allows temperature differences to be expressed in either kelvins or degrees Celsius.
Lowercase k means something else
Lowercase k is not a kelvin variant at all: it is the SI prefix kilo, meaning 103. A generic lowercase operation can therefore turn a valid temperature symbol into a different measurement token.
The related k vs K guide explains why this matters in spreadsheets, databases, APIs, and OCR-cleanup workflows.
What data cleanup must not change
Temperature strings are surprisingly easy to damage with generic cleanup. Lowercasing K changes the symbol, while stripping the space from 20 °C changes the standard formatting even if a human reader can still guess the meaning.
Structured data is safest when the number and unit are stored separately. If the value must remain plain text, preserve the original case and unit boundary instead of running ordinary prose transformations over it.
The current-text rule
For current notation, the rule is short: °C for degree Celsius, K for kelvin. The degree sign is part of the Celsius symbol, not a generic marker that belongs in every temperature unit.