Temperature Converter
The only common conversion where you cannot simply multiply, because two of the scales do not start from the same zero.
The formulas
The offset of 32 is what makes temperature conversion different from every other unit conversion. A temperature difference of 10 °C is 18 °F, but a temperature of 10 °C is 50 °F. Confusing the two is the classic error — and it matters, because a weather forecast of “5 degrees warmer” means something different in each scale.
The useful mental shortcut: double the Celsius figure and add 30. For 20 °C that gives 70 °F against a true 68 — close enough for deciding what to wear, and increasingly inaccurate at the extremes.
Where the scales came from
Fahrenheit (1724) set zero at the coldest temperature Daniel Fahrenheit could reliably reproduce — a brine of ice, water and ammonium chloride — and 96 at roughly human body temperature. The scale was later adjusted so water froze at exactly 32 and boiled at 212, which left body temperature at the odd-looking 98.6.
Celsius (1742) was originally inverted: Anders Celsius put 0 at boiling and 100 at freezing. It was reversed shortly after his death. Its appeal is obvious — two easily reproducible reference points, a hundred degrees apart.
Kelvin (1848) starts at absolute zero, the point at which particles have minimum possible thermal motion. It uses the same degree size as Celsius, so conversion is pure addition. There is no “degree” symbol: it is 300 K, not 300 °K.
Reference points worth knowing
| Point | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| The scales cross | −40 | −40 | 233.15 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20 | 68 | 293.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils (sea level) | 100 | 212 | 373.15 |
| Oven, moderate | 180 | 356 | 453.15 |
Minus forty is the one temperature where Celsius and Fahrenheit agree, which is a genuinely useful thing to remember and a good check that a conversion formula is right.
Frequently asked questions
Why does the US still use Fahrenheit?
Inertia, essentially. The US formally adopted metric as its preferred system in 1975 but made conversion voluntary, and everyday weather reporting never switched. American science and medicine use Celsius and Kelvin routinely; it is the public-facing use that persists.
Is it 'degrees Kelvin'?
No. Kelvin is an absolute scale, so the unit is simply the kelvin: 300 K, written without a degree symbol and with a lowercase k when spelled out. Celsius and Fahrenheit take degrees because their zeros are arbitrary.
What is Rankine used for?
It is an absolute scale using Fahrenheit-sized degrees, still found in some US engineering work, particularly thermodynamics and aerospace, where absolute temperature is needed but the surrounding calculations are in imperial units.