Celsius, Fahrenheit and Kelvin: Why Three Scales Exist
Three scales, three stories
Fahrenheit came first, in 1724. Daniel Gabriel Fahrenheit, a Dutch-German physicist and the best thermometer maker of his era, set 0 °F at the temperature of a freezing brine mixture — the coldest point he could reproduce reliably in his laboratory — and placed human body temperature near 96 degrees. On that scale, plain water freezes at 32 °F and boils at 212 °F.
Celsius followed in 1742. Anders Celsius, a Swedish astronomer, wanted a decimal-friendly scale with a clean 100 degrees between freezing and boiling water. Strangely, his original version ran backwards — 0 was boiling and 100 was freezing — and the scale was reversed after his death into the version used by nearly the entire world today.
Kelvin arrived in 1848, when the British physicist William Thomson — later Lord Kelvin — proposed an absolute scale: one whose zero is the lowest possible temperature, where thermal motion bottoms out. That point, absolute zero, sits at −273.15 °C, and the kelvin (K) simply counts Celsius-sized degrees upward from it.
Why the conversion formula looks like that
Between freezing and boiling, water spans 100 Celsius degrees but 180 Fahrenheit degrees (from 32 to 212). The ratio 180/100 reduces to 9/5 — that is the entire origin of the famous 9/5. And because water freezes at 0 °C but 32 °F, a 32-degree shift must be added or removed on top of the scaling.
Kelvin needs no ratio at all: kelvins and Celsius degrees are exactly the same size, so converting between them is a pure shift of 273.15. Each formula below undoes the other, which makes checking your work easy.
°F = °C × 9/5 + 32 • °C = (°F − 32) × 5/9 • K = °C + 273.15Converting 25 °C to Fahrenheit
Scale first: 25 × 9/5 = 45. Then shift: 45 + 32 = 77. So a warm 25-degree day is a 77-degree day.
25 °C = 77 °F
Converting 68 °F to Celsius
Shift first: 68 − 32 = 36. Then scale: 36 × 5/9 = 20.
68 °F = 20 °C
Converting 25 °C to kelvin
Same size degrees, so just add the offset: 25 + 273.15 = 298.15.
25 °C = 298.15 K
Anchor points worth memorizing
A handful of fixed points let you place any temperature instantly in any scale. Memorize the freezing and boiling rows and the rest of daily life interpolates between them:
Two rows deserve a closer look. The crossover point is a mathematical curiosity: −40 is the only temperature where Celsius and Fahrenheit agree, because −40 × 9/5 + 32 = −72 + 32 = −40. And body temperature is a handy sanity check — the familiar 98.6 °F is just 37 °C converted. The old name "centigrade" also clicks here: it means 100 grades, 100 steps from freezing to boiling.
| Reference point | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20 | 68 | 293.15 |
| Human body temperature | 37 | 98.6 | 310.15 |
| Water boils | 100 | 212 | 373.15 |
| The crossover point | −40 | −40 | 233.15 |
Kelvin and absolute zero
The kelvin is the official base unit of temperature in science (SI), because physical laws come out cleanest when temperature is counted from true zero. Gas laws, thermodynamics and even the color temperature of stars — the Sun's surface is about 5,800 K — are written in kelvin.
Two conventions trip people up. First, you write 298 K with no degree sign, because the kelvin is an absolute unit, not a degree marked off on someone's scale. Second, nothing can be colder than 0 K: unlike Celsius and Fahrenheit, the kelvin scale does not extend below its zero. Absolute zero has never been fully reached — lab experiments get within billionths of a degree, and the laws of thermodynamics say the last step is unreachable.
For everyday weather and cooking, the choice is purely local: most of the world reads Celsius, the US reads Fahrenheit, and science quietly runs on kelvin underneath both.