Temperature is the odd one out. Every other conversion on this site is a simple multiplication, because zero means zero in both units. Celsius and Fahrenheit disagree about where zero sits, so the conversion needs an offset as well as a ratio — which is why you cannot convert a temperature by scaling it, and why 20 °C is not twice 10 °C in any meaningful sense.
The formulas:
°F = (°C × 9/5) + 32°C = (°F − 32) × 5/9K = °C + 273.15
The 9/5 comes from the scales' spacing. Between freezing and boiling water there are 100 Celsius degrees and 180 Fahrenheit degrees, and 180/100 is 9/5. The 32 is simply where Fahrenheit put freezing.
Quick reference
The table follows whichever units you select above.
The one that crosses over
−40 is the same temperature on both scales — the only point where they meet. It is a useful sanity check: if your conversion does not agree at −40, the formula is wrong.
Anchors worth knowing
0 °C = 32 °F— water freezes10 °C = 50 °F— a cool day20 °C = 68 °F— room temperature28 °C ≈ 82 °F— properly warm37 °C ≈ 98.6 °F— body temperature100 °C = 212 °F— water boils at sea level
For rough mental conversion from Celsius, double it and add 30. For 20 °C that gives 70 against a true 68; for 30 °C it gives 90 against 86. Close enough to decide on a jacket, not close enough for an oven.
Where people go wrong
Differences convert differently from readings
A rise of 10 °C is a rise of 18 °F, not 50 °F. When you are converting a temperature difference rather than a temperature, use only the ratio and drop the offset. This trips up a lot of spreadsheets.
Kelvin has no degree sign
It is written 300 K, not 300 °K. Kelvin is an absolute scale, so it needs no reference point to be a degree above. Absolute zero is 0 K, which is −273.15 °C.
Oven temperatures are not precise. Recipes round hard — 180 °C is usually printed as 350 °F, though it converts to 356. Gas Mark 4 is the same setting again. Domestic ovens routinely run 10 to 20 degrees off their dial anyway, so an oven thermometer will tell you more than any conversion.
Common questions
What is 100 °F in Celsius?
37.8 °C — just above body temperature, which is why 100 °F is the informal threshold for a fever in countries using Fahrenheit.
Why does the United States still use Fahrenheit?
Largely inertia. The US formally adopted the metric system as its preferred system in 1975 but made the switch voluntary, and everyday weather and cooking never followed. American science, medicine and industry use Celsius and Kelvin throughout.
Is there a temperature where the scales agree?
Yes, exactly one: −40. Below and above that they diverge in opposite directions.