Each channel is R = 255 x (1 - C) x (1 - K), and the same for G with M and B with Y, where C, M, Y and K are fractions from 0 to 1.
Example: cmyk(0%, 66%, 80%, 0%) becomes rgb(255, 87, 51).
This is a mathematical conversion without a printer color profile. A CMYK color from a real print job can look different on screen. For exact color matching, use your design software with the correct ICC profile.
rgb(0, 0, 0), black. cmyk(0, 0, 0, 0) is rgb(255, 255, 255), white.
Percentages from 0 to 100.