CMYK describes how much ink a press lays down. HEX describes how much light a screen puts out. Those are opposite systems, so a conversion between them is a translation with a rounding error baked in, not a lookup with one right answer.
The arithmetic itself is short. Each ink percentage subtracts from a white sheet, black knocks everything down together, and the three surviving values become red, green and blue:
R = 255 x (1 - C) x (1 - K)G = 255 x (1 - M) x (1 - K)B = 255 x (1 - Y) x (1 - K)Run 75, 50, 25, 10 through it and you get 63, 95, 112, which a browser writes as #3F5F70. Every general purpose converter, this one included, uses that formula. It is fast, reversible, and blind to paper, ink and press behaviour.
What the formula gets right and where it drifts
For screen work the result is dependable in one direction: dark, muted, low-saturation mixes land close to what a proof looks like. Navy blues, warm greys, muted greens and anything with a heavy black component translate with little visible surprise.
Drift shows up at the saturated end. 100% process cyan converts to #00FFFF, an sRGB value no ink on paper reaches. Pure yellow becomes #FFFF00 for the same reason. The formula hands back the brightest thing a monitor owns because the maths has nowhere else to go, and the printed sheet lands noticeably duller. Treat those results as a starting point to adjust by eye rather than a match.
No ICC profile is involved here. Real print work resolves CMYK through a profile such as Coated FOGRA39 or US Web Coated SWOP, which accounts for the specific ink set, the paper, and how dots spread once wet. A profile-aware conversion in Photoshop or Illustrator will return a slightly different HEX than this page does, and for brand-critical color that number is the one to trust.
Reading the ink coverage bar
The coverage figure above the copy row adds the four percentages together. It has nothing to do with the HEX output, and it is there because a lot of people arrive at this page while checking a file that is heading to print.
- Under 240% is safe on uncoated and newsprint stock, where paper soaks up ink and dries slowly.
- 240% to 300% suits most coated sheetfed work. The common rich black recipe of 60/40/40/100 sits at 240%.
- Over 300% risks set-off, where wet ink transfers onto the back of the next sheet in the stack. A 100/100/100/100 mix reads 400% and no commercial press will run it clean.
Coverage limits belong to the press, not to a rule book. Ask the printer for their number before a long run.
Practical moments this saves time
A brand book arrives with CMYK builds only and the website needs matching values. Paste each build, copy the HEX, and hand the developer a set that at least starts in the right family. Or a packaging design gets approved and the product page has to echo it, so the label green becomes a button color without a round trip through design software.
It also works in reverse for sanity checks. If a CMYK build converts to something visibly off from the approved swatch, the build is often the problem, not the conversion. Going the other direction is handled by the HEX to CMYK converter.
Four ink numbers in. One web-ready code out. Verify anything critical against a printed proof.
