How CMYK becomes RGB
Screens and printers make colour in opposite ways. A screen starts black and adds red, green and blue light, which is why RGB is called an additive model. A printer starts with white paper and lays down cyan, magenta and yellow inks, each of which absorbs (subtracts) part of the light bouncing off the page, with a fourth black ink, called key or K, doing the heavy lifting in dark areas. To preview an ink recipe on screen, turn each percentage into a fraction from 0 to 1 and work out how much of each light channel survives:
The pairing is no accident. Cyan ink absorbs red light, so the more cyan you print, the less red reaches your eye. Magenta absorbs green, yellow absorbs blue, and black dims all three channels at once, which is why (1 − K) appears in every line.
Worked examples
Going the other way
Converting RGB to CMYK starts by asking how much black ink can do on its own. The brightest of the three channels sets the black level, and whatever each channel still lacks after black is accounted for becomes coloured ink:
Magenta and yellow follow the same pattern using green and blue. This is called maximum black generation, because it uses as much K and as little coloured ink as possible. Real prepress software often holds some black back and lets the colours mix a richer dark instead, which is one of several reasons two tools can give you different CMYK numbers for the same on-screen colour. Pure black is the special case: when R, G and B are all 0, the inks are simply set to cmyk(0%, 0%, 0%, 100%).
Why the on-screen preview is approximate
Here is the honest caveat: there is no single, universal CMYK. The formula above describes an idealised set of inks on idealised paper. A real print job depends on the specific ink set, the coating and whiteness of the stock, the press calibration and the standard being targeted (European coated work often uses the FOGRA39 profile, American work SWOP). Your monitor complicates things from the other side, because it shows colour through its own profile, usually sRGB, and no monitor can show you how ink soaks into uncoated paper. Some vivid RGB colours, especially bright greens, oranges and electric blues, sit outside the CMYK gamut entirely and will print duller no matter what numbers you use.
So treat this tool as it is intended: a fast, reliable preview of the mathematical relationship, perfect for mocking up a print colour on screen, matching a logo across media or sanity-checking values in a PDF. For anything going on a press, convert through the ICC profile your print house recommends in professional software, and ask for a printed proof before a big run. Everything here runs in your browser, and nothing you type is uploaded anywhere.
Frequently asked questions
What is CMYK?
CMYK stands for cyan, magenta, yellow and key, which is the printing name for black. They are the four ink colours used in most colour printing, each given as a percentage from 0 to 100. Unlike a screen, which adds light, inks subtract light from white paper, so CMYK is called a subtractive colour model.
What formula does this converter use?
It uses the standard device-independent formula: R = 255 multiplied by (1 minus C) multiplied by (1 minus K), with the same pattern for green from magenta and blue from yellow, where each ink value is a fraction from 0 to 1. It is the common quick conversion used by design tools for previews.
How does the RGB to CMYK direction work?
First K is found as 1 minus the largest of R, G and B divided by 255. Then each ink is the leftover after black is removed, for example C = (1 minus R divided by 255 minus K) divided by (1 minus K). If the colour is pure black, C, M and Y are all set to 0 and K to 100.
Why does my printed colour not match the screen preview?
Because there is no single real-world CMYK. The printed result depends on the ink set, the paper and the press profile, and your screen has its own colour profile too. This converter uses the simple mathematical relationship, which is great for previews but not a substitute for proofing with your printer's ICC profile.
Why is there a separate black ink at all?
In theory full cyan, magenta and yellow make black, but in practice they make a muddy dark brown and soak the paper with three layers of ink. A dedicated black ink is cheaper, dries faster and keeps small text sharp, which is why it does most of the work in dark areas.