The same interval size and a different starting point — and only one of them takes a degree sign.
A change of one kelvin and a change of one degree Celsius are exactly the same size, so a temperature *difference* may be stated in either. The scales start in different places: 0 K is absolute zero and 0 °C is the freezing point of water, 273.15 K. The written forms differ too — the kelvin takes no degree sign, so 300 K is right and 300 °K is not.
The same interval size, a different origin, and only one of the two takes a degree sign.
Drawn from the visual attributes recorded for each symbol. Only traits the two records disagree about are listed.
| Trait | Degree Celsius symbol | Kelvin symbol |
|---|---|---|
| Built from | circles and letter like | letter like |
| Degree Celsius symbol | Kelvin symbol | |
|---|---|---|
| Zero point | The freezing point of water | Absolute zero |
| Degree sign | Yes: °C, with a space before it | No: 300 K, never 300 °K |
| Size of one unit | Identical to one kelvin | Identical to one degree Celsius |
| Role in the SI | Accepted for use with the SI | The SI base unit of thermodynamic temperature |
| Converting | °C = K − 273.15 | K = °C + 273.15 |
| Relationship (from Symbolwise's records) | Commonly confused with Kelvin symbol — The same interval size, a different origin, and only one of the two takes a degree sign. | Commonly confused with Degree Celsius symbol — Both measure temperature and one kelvin equals one degree Celsius in size, but the scales start in different places and only Celsius takes a degree sign. |
The International System of Units (SI), 9th edition
Bureau International des Poids et Mesures · retrieved 2026-07-31
The defining document for SI unit names and symbols, including the capitalisation and spacing rules. It defines units; it says nothing about how a symbol is encoded in a character set, and nothing about non-SI units except where it lists those accepted for use with the SI.
Guide for the Use of the International System of Units (SI), NIST Special Publication 811
National Institute of Standards and Technology · retrieved 2026-07-31
A national metrology institute's guidance on applying the SI, including symbol formatting. Authoritative for usage rules in the United States; it interprets the SI rather than defining it.
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for origin narrative and for units the SI does not define, never for an official-definition claim.
Check the letter after the degree sign. C is Celsius, on which water freezes at 0 and boils at 100. F is Fahrenheit, on which the same points are 32 and 212. The scales coincide at −40 and nowhere else, so a number without its letter is genuinely ambiguous — and at everyday temperatures the same figure means very different weather.
The degree sign is the discriminator, and it is the thing plain text loses. °C is a temperature and needs its preceding degree sign and a space before the number; italic c is a speed and stands alone in an equation. Where formatting is stripped — a filename, a search box, a plain-text note — they become indistinguishable.
A capital K is the kelvin, a unit of temperature. A lower-case k is the prefix kilo, meaning a factor of one thousand and never standing alone. So kW is a kilowatt and KW is not a real unit symbol at all. NIST's guide lists exactly this kind of case error among the standard mistakes in applying the SI.
Alpha is a Greek letter and stands for a quantity — an angle, a coefficient, a significance level. The proportionality sign is an operator and stands between two quantities to say they keep a constant ratio. The drawings differ in that the proportionality sign's right-hand stroke runs out in an open curve rather than closing into the letter's tail.