I'm not quite sure what you're asking, but I'll try and cover all the bases.

Firstly RGB values run from 0 to some arbitrary number depending on the colour depth - how many different colours your image format or editing program can store. A typical colour depth is 8bit per channel. Here the number of red, green and blue values is two to the power of 8, which is 256 so the numbers go from 0 to 255.

Even if you were to convert the 0 to 255 value to a percentage with 0 = 0 and 255 = 100% then the numbers still wouldn't correspond because CMYK is a subtractive colour model whereas RGB is an additive colour model.

CMYK is subtractive because it is used for mixing inks which display different colours by absorbing light whereas RGB is used by monitors which produce light, which gets added together. White in CMYK is produced by the values 0%, 0%, 0%, 0% i.e. don't put any ink down to absorb light. White in RGB is produced by the values 100%, 100%, 100% i.e. output the full amount of all colours of light which mix together to give white light.

CMYK and RGB are thus opposites of each other. So to answer your questions the values aren't equal because of this. But they are directly comparable so you can convert approximately in your head, each C,M,Y, has it's opposite R,G,B, (I'll cover K later)

• 100% Cyan ~ 0% Red (+100% Green, +100% Blue)
• 100% Magenta ~ 0% Green (+100% Red, +100% Blue)

• 100% Yellow ~ 0% Blue (+100% Red, +100% Green)

• 100% Red ~ 0% Cyan (-100% Yellow, -100% Magenta)

• 100% Green ~ 0% Magenta (-100% Cyan, -100% Yellow)
• 100% Blue ~ 0% Blue (-100% Red, -100% Green)

The reality is however a little more complex as it depends on exactly what shade or RGB and the corresponding inks is used as described by the colour space. Whereas monitors use isolated pixels (not directly influencing each other) printers are in most cases dropping ink onto same point and the mixture of inks will look differently compared to inks dropped separately. Calculations between RGB spaces can be done using 3x3 matrix conversions, conversions from RGB to printer CMYK colour spaces cannot be done with matrix no more.

Also due to the limitations of subtractive model printer's CMYK gamut is more limited compared to common RGB colour spaces, printers have more limited brightness of saturated colours.

What about K then?

Again, another practicality, mixing all CMY inks together can yield a dirty dark greenish colour, not black. To combat this printers added another ink: Keyline (black). Now if you want black you just use 100% K. Simple. Not quite, you can make the black a bit darker by added a small amount of magenta, yellow, and a slightly bigger amount of cyan, giving so called rich black. Using more ink makes it darker but risks oversaturating the paper or printing medium. This is not really relevant for Photography, I just included it for completeness. If you're not doing pagesetting work I would just leave the CYMK conversion to the printer who knows about all of this!