Divide each channel by 255. K = 1 - max(R, G, B). Then C = (1 - R - K) / (1 - K), and the same for M and Y with G and B.
Example: rgb(255, 87, 51) is cmyk(0%, 66%, 80%, 0%).
RGB makes color with light and has a wider range of bright colors than CMYK ink. This tool uses the standard mathematical formula, which does not include a printer color profile. Printed results depend on the paper, the ink and the press, so always check a proof for important print work.
Not necessarily. Very bright screen colors, such as vivid blues and greens, often look duller in print.
K stands for key, the black ink plate.
Not always. Those programs convert with color profiles, so their numbers can differ from this simple formula.