Convert an RGB color into the four CMYK inks

Divide each channel by 255, then work the four ink shares out of those three fractions. The brightest channel sets the black, and the other three inks come from it and from their own channel.

Formula: K = 1 − the largest channel, and then C = (1 − R − K) ÷ (1 − K). Magenta works the same way from the green channel and yellow from the blue.

The color rgb(59, 130, 246) divides down to channels of 0.231, 0.510 and 0.965. Blue is the brightest, so black comes out at 0.035, that is 4%. Cyan gives 76% and magenta 47%, while yellow lands at zero because the whole value of the blue channel went into the black. The result is cmyk(76%, 47%, 0%, 4%).

A few values come up more often than the rest. Black rgb(0, 0, 0) comes down to the black ink alone at 100%, white rgb(255, 255, 255) gives four zeros, and the gray rgb(128, 128, 128) is written as 50% black with no other ink involved.

The formula is independent of the press and the paper, which is why it gives the same answer everywhere. What the inks actually work out to on a given press is set by the ICC profile your printer specifies, and the profiles used on both sides of the Atlantic are in the table on this page. To go the other way, use the CMYK to RGB converter.

RGB describes color with light. A screen has dots in three colors and adds their light together, so three full values give white and three zeros give black.

CMYK describes color with ink, and ink works the other way round: it sits on white paper and takes light away from it, so the more ink the darker the result. That is why the primaries stand on opposite sides in the two notations. Full cyan takes red out of the white, and full magenta takes green.

The abbreviation comes from the names of the four inks: cyan, magenta, yellow and K, the black. That last letter stands for the key plate, the one that carries the outlines and the text[1].

Black ink joins the set for several reasons. The key plate holds outlines and text more precisely than three inks can, a hundred percent of cyan, magenta and yellow together makes a dark but imperfect black, and a single black ink costs less and dries faster than three colored ones laid over each other[1].

In everyday work the two turn up in different places. RGB values come from the browser, from Figma and from every color picker on screen. CMYK values sit in a printer's file specification, in the brand book and in the PDF export dialog — wherever the file is heading for paper.

A screen describes color in the sRGB space, which W3C adopted for HTML and CSS[2] and which is therefore the same on every compliant device. Ink has no single scale like that, and the profiles in the next table are what stand in its place.