One letter apart, and the two scales agree at exactly one temperature in the whole range.
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.
Identical in construction and different in scale; a number without its letter is ambiguous and sometimes dangerously so.
| Degree Celsius symbol | Degree Fahrenheit symbol | |
|---|---|---|
| Water freezes at | 0 °C | 32 °F |
| Water boils at | 100 °C | 212 °F |
| Is it an SI unit? | Accepted for use with the SI, and defined by the BIPM | No. The SI does not define it; NIST treats it as customary |
| Where it is used | Most of the world, and all scientific work | The United States and a few other jurisdictions |
| They agree at | −40 °C | −40 °F — the only coincident point |
| Relationship (from Symbolwise's records) | Commonly confused with Degree Fahrenheit symbol — Identical in construction and different in scale; a number without its letter is ambiguous and sometimes dangerously so. | Commonly confused with Degree Celsius symbol — One letter apart, and the scales agree at only one temperature. |
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.
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 2026-07-31
Tertiary reference. Used for origin narrative and for units the SI does not define, never for an official-definition claim.
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 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.
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.
The becquerel counts nuclear decays — one per second — and says nothing about harm. The sievert weights absorbed energy by the kind of radiation delivering it, and so estimates biological effect. A becquerel of one isotope and a becquerel of another can carry very different risk, which is exactly why exposure limits and medical doses are stated in sieverts and never in becquerels.