Hex to CMYK Converter
CMYK describes ink on paper rather than light on a screen, so this conversion is an approximation by nature. It is the right starting point for a print job and the wrong thing to send to a press without proofing.
How the conversion works
Normalise the channels to 0–1. Key (black) is 1 minus the largest channel. Each of cyan, magenta and yellow is then how far that channel falls short of the maximum, scaled by what is left after the key.
If key is 1 the color is black and the other three are zero.