Temperature measures the degree of hotness or coldness. Convert between Celsius, Fahrenheit, and Kelvin.
Temperature conversions between Celsius, Fahrenheit, and Kelvin come up in weather, cooking, and science, and they behave differently from every other conversion on this site because the scales don't share a common zero point. Getting the formula direction right, and knowing when not to apply it at all, matters more here than for length or mass.
Converting length or mass is a single multiplication: multiply centimetres by a factor to get inches, and the relationship holds at any value including zero, because zero centimetres really is zero inches. Temperature does not work that way. The formula from Celsius to Fahrenheit is F equals C times 9/5 plus 32, and that added 32 means the two scales are offset from each other, not just scaled differently. Zero on one scale does not land on zero on the other: 0 Celsius, where water freezes, is 32 Fahrenheit, nowhere near zero.
This matters most when converting a temperature difference rather than a specific reading. A rise of 10 degrees Celsius is a rise of 18 degrees Fahrenheit, found by multiplying only by 9/5, with no plus-32 involved, because the offset positions the scale's zero point and has nothing to do with a gap between two readings. Applying the full formula to a temperature change instead of an absolute reading produces a number that is wrong by exactly 32.
Celsius and Fahrenheit meet at exactly one point: -40 degrees, where both scales read the same number. It falls directly out of the algebra: setting C equal to C times 9/5 plus 32 and solving gives C equals -40, the single value where the scale factor and the offset cancel out exactly. It's a useful sanity check when converting by hand, since any conversion that doesn't pass through -40 correctly on both sides has an error somewhere in the formula.
Kelvin is not offset the way Celsius and Fahrenheit are offset from each other. It is anchored to absolute zero, the coldest temperature theoretically possible, the point at which particles hold minimum thermal energy. Absolute zero sits at 0 Kelvin, equal to -273.15 Celsius, and because Kelvin starts at a true physical zero rather than an arbitrary reference point like the freezing point of water, it is the scale science actually uses for calculations involving energy and gas laws.
Converting Celsius to Kelvin is the one temperature conversion that is a simple addition rather than a scaling problem: add 273.15 to a Celsius reading to get Kelvin, with no multiplication involved, because the two scales use the same size of degree and differ only in where they place zero.