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Both reduce to a bare letter c in plain text, and both appear in scientific writing.
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.
Both reduce to a bare letter c and appear together in scientific writing.
Drawn from the visual attributes recorded for each symbol. Only traits the two records disagree about are listed.
| Trait | Degree Celsius symbol | Speed of light symbol |
|---|---|---|
| Built from | circles and letter like | letter like |
| Degree Celsius symbol | Speed of light symbol | |
|---|---|---|
| What it is | A unit of temperature | A fundamental constant |
| Written form | Degree sign then capital C | Italic lower-case c, alone |
| Value | A scale, not a value | Exactly 299 792 458 m/s by definition |
| Survives plain text? | Loses the degree sign | Loses the italic |
| Relationship (from Symbolwise's records) | Commonly confused with Speed of light symbol — Both reduce to a bare letter c and appear together in scientific writing. | Commonly confused with Degree Celsius symbol — Both reduce to a bare letter c and appear in the same scientific writing. |
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.
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
CODATA Internationally Recommended Values of the Fundamental Physical Constants
NIST / CODATA Task Group on Fundamental Constants · retrieved 2026-08-01
Authoritative for the recommended value and uncertainty of a physical constant, and for whether that value is exact by definition or measured. It fixes values; it does not decide which letter denotes which quantity, and no record cites it for a symbol's meaning.
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.
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.
Look for the horizontal line. Fire is a plain upward triangle; air is the same triangle with a bar drawn across its upper part. Dropping the bar is the commonest error in reproductions of the element set, and it turns air into fire.
The same rule as fire and air, inverted. Water is a plain downward triangle; earth is the same triangle with a bar across its lower part. The four element signs are one designed set: two point up, two point down, and within each pair the bar is the only difference.