Kelvin to Celsius (K to C)

Kelvin to Celsius is C = K - 273.15. Written the way it computes, that is 1 times the Kelvin figure minus 273.15, and the two parts do different jobs: the degree size is identical in both scales, so there is no rescaling to do at all, and the 273.15 accounts for them having different zero points.

That second term is what makes temperature different from every other conversion. Length, mass and volume units all share a zero (zero centimeters is zero inches), so converting one is a single multiplication. Kelvin and Celsius do not, so the conversion is affine rather than scalar and needs an offset as well as a ratio. It is the reason a temperature and a temperature difference convert by different rules: a rise of one degree Kelvin is a rise of exactly one degree Celsius, because an interval scales with the ratio and takes no offset at all.

Worked through at a value people look up: 273.15 K x 1 - 273.15 = 0 °C.

Going back, Celsius to Kelvin is 1 times the Celsius figure plus 273.15. Note what does not happen here: with a scalar conversion the two factors are reciprocals and knowing one gives you the other, but an affine conversion has two constants and the reverse offset is not the negative of the forward one unless the slope is 1.

For mental arithmetic: Subtract 273.

Kelvin figures arriving in everyday life usually come from one of two places, and only one of them is a temperature. Physics and chemistry use it as an absolute temperature: 0 K is absolute zero, 77 K is liquid nitrogen, 273.15 K is ice melting. Lighting uses the same unit for color temperature (a 2700 K bulb is warm white and a 5000 K one is daylight), and that number is not the temperature of anything in the room. It is the temperature a black body would have to be to glow that color, which is why converting it to Celsius gives a meaningless 2426.85 C. If the kelvin figure came off a light bulb box, this conversion is not the one you want.

On precision: the 273.15 is exact by definition: the kelvin and the degree Celsius were fixed against each other, so nothing in this conversion is a measurement. What it cannot do is add precision the input never had. A weather reading of 273.15 K is good to about a degree, and reporting 0 °C from it implies a thermometer nobody owns. The converter scales its decimals to the magnitude of the input for that reason.