What the formula does
Screens make color by adding red, green and blue light to black. Printers do the opposite: they lay cyan, magenta and yellow ink on white paper, and each ink absorbs one of the light primaries. Cyan absorbs red, magenta absorbs green and yellow absorbs blue. Black ink (K, for "key") is added because the three colored inks together make a muddy brown rather than a clean black, and because one ink is cheaper and dries faster than three.
The conversion on this page is the textbook one used by most online tools and by CSS's device-cmyk() fallback:
- Divide R, G and B by 255 so each is between 0 and 1.
- K = 1 − the largest of the three.
- C = (1 − R − K) ÷ (1 − K), and the same pattern for M (with G) and Y (with B).
- Multiply by 100 for percentages.
Example: the HexaCalc vermilion, rgb(217, 72, 28)
- R = 0.851, G = 0.2824, B = 0.1098
- K = 1 − 0.851 = 0.149
- C = (1 − 0.851 − 0.149) ÷ 0.851 = 0
- M = (1 − 0.2824 − 0.149) ÷ 0.851 = 0.6682
- Y = (1 − 0.1098 − 0.149) ÷ 0.851 = 0.8710
Result: C 0%, M 66.8%, Y 87.1%, K 14.9%, usually written C0 M67 Y87 K15.
More conversions to compare
| Color | RGB | CMYK (rounded) |
|---|---|---|
| Pigment teal | 15, 118, 110 | C87 M0 Y7 K54 |
| Saffron | 242, 179, 61 | C0 M26 Y75 K5 |
| Steel blue | 51, 102, 153 | C67 M33 Y0 K40 |
| Mid gray | 128, 128, 128 | C0 M0 Y0 K50 |
| Pure red | 255, 0, 0 | C0 M100 Y100 K0 |
Notice that a formula conversion always sets at least one of C, M or Y to zero. That is a giveaway: real press separations almost never look like that, because they use some of all four inks for smoother results.
Why your printer's numbers will differ
The formula assumes perfect inks on perfect paper, which do not exist. A print shop converts with an ICC profile that describes a specific press, ink set and paper. Common ones are Coated FOGRA39 or FOGRA51 in Europe, GRACoL 2013 and U.S. Web Coated (SWOP) v2 in North America, and Japan Color 2011 in Japan.
The same RGB color run through different profiles can differ by 10 to 20 points per ink. Profiles also use gray component replacement, swapping some of C, M and Y for K in neutral areas. That saves ink and keeps grays from drifting when the press is slightly off.
So use these numbers for:
- a quick spec sheet or a brief to a designer;
- matching a color in software that only accepts CMYK input;
- understanding roughly which inks a color needs.
Do not use them as final separations for a print job. Export from a layout program (InDesign, Affinity Publisher, Scribus) with the printer's profile, or send RGB files and let the printer convert.
Out-of-gamut screen colors
Many bright screen colors simply cannot be printed with process inks. Saturated blues like #0000FF, electric greens and neon oranges fall outside the CMYK gamut. The formula still gives you numbers, but the printed result will be duller than what you see. If a brand color is in that territory, printers use a spot color (a premixed ink such as a Pantone shade) instead of building it from CMYK.
Black: plain versus rich
rgb(0, 0, 0) converts to C0 M0 Y0 K100. On paper, 100% K alone looks like a dark charcoal, especially over large areas. For big black backgrounds, designers often specify a rich black such as C60 M40 Y40 K100, which adds depth. Keep small body text at plain K100: rich black on fine type shows color fringes when the plates are not perfectly aligned.
Total ink coverage
The tool shows the sum of all four percentages. Presses have a maximum, often called TAC (total area coverage). Typical limits are about 300% to 340% on coated stock and around 240% to 260% on newsprint. The formula never exceeds 200% for a single color, so this mostly matters once you start building rich blacks or receive separations from elsewhere.
Related tools
Going back the other way is the CMYK to RGB converter. If your starting point is a code like #D9481C, the HEX to CMYK converter does both steps at once. The color converter lists CMYK next to every screen format.
FAQ
Is CMYK conversion reversible?
With the formula on this page, yes: converting back returns the original RGB, give or take rounding. With ICC profiles, no. Colors outside the printable gamut are compressed, and that information is lost.
Why is K sometimes 0 for a dark color?
It shouldn't be with this formula. K is driven by the brightest channel, so any color whose brightest channel is below 255 gets some K. Pure, fully bright hues such as #FF0000 are the ones with zero black.
Should I design in CMYK from the start?
Only for print-only work where you know the printer's profile. For anything that will appear on screen as well, design in RGB and convert at export.