Celsius to Kelvin (C to K)

Celsius to Kelvin is K = C + 273.15. Written the way it computes, that is 1 times the Celsius figure plus 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. Celsius and Kelvin 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 Celsius is a rise of exactly one kelvin, because an interval scales with the ratio and takes no offset at all.

Worked through at a value people look up: 20 °C x 1 + 273.15 = 293.15 K.

Going back, Kelvin to Celsius is 1 times the Kelvin figure minus 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: Add 273. The .15 matters in a laboratory and nowhere else.

This is the one temperature conversion that is a pure shift. The degree is the same size in both scales (the kelvin was defined that way on purpose), so there is no ratio to apply and a difference of 1 C is a difference of exactly 1 K. That makes it the only pair here where intervals and values convert by the same rule. One point of style, because it is the most common error in writing: there is no degree sign on a kelvin figure and it is not called a degree. Water boils at 373.15 K, not 373.15 degrees Kelvin. The unit is named after Lord Kelvin and lower-cased as a unit, capitalized as a symbol.

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 20 °C is good to about a degree, and reporting 293.15 K from it implies a thermometer nobody owns. The converter scales its decimals to the magnitude of the input for that reason.