RGB to CMYK
Every page on this search runs the same arithmetic and most of them let you believe it is a print preview. This one shows the arithmetic instead. Type RGB 11, 122, 133 and you get C 91.7 M 8.3 Y 0.0 K 47.8, next to the two lines that produced it: K = 1 - max(R,G,B)/255, then C = (1 - R/255 - K) / (1 - K). Hex works with or without the hash, 3 and 4 digit shorthand is expanded, and an alpha channel is dropped with a line saying so. It runs backwards as well: those four percentages, exactly as printed, convert back to rgb(11, 122, 133), for every colour. Beside the swatch sit the two things the algebra can honestly tell you, the total ink the four percentages add up to, flagged when it runs past the 240% a press starts refusing, and a gamut rule of thumb for bright colours that need two process inks over 50%, and one thing it cannot: no ICC profile is applied, so these numbers are channel values to read and carry, not a forecast of ink on paper.
Gamut, rule of thumb: no flag. This is not one of the bright, heavily inked colours process inks visibly fall short of. Still a rule of thumb about the colour, not a measurement of your press.
three channels in, four percentages out, computed here with no colour management of any kind.
RGB 11, 122, 133 is C 91.7%, M 8.3%, Y 0.0%, K 47.8%, 147.8% total ink, no ICC profile applied.
- C 91.7 M 8.3 Y 0.0 K 47.8
- C 0.0 M 100.0 Y 100.0 K 0.0
- C 0.0 M 0.0 Y 0.0 K 100.0
Both directions run on the arithmetic printed beside the result, in this browser, with nothing uploaded and no account. Hex reads with or without the hash, 3 and 4 digit shorthand is expanded, and an alpha channel is dropped with a line saying so, because CMYK has no alpha. A channel outside 0 to 255, a percentage outside 0 to 100 or a box past 64 characters is named and refused rather than quietly corrected. The four percentages add up to the total ink shown, which this algebra keeps under 300% and never limits: past 240%, where a press starts refusing ink, the panel says so. The gamut line beside it is a rule of thumb about bright colours needing two process inks over 50%, not a measurement of any press. The direction and what you type stay in this browser, and the Start over button above the tool forgets them.
Common questions
- Why are these numbers different from the CMYK in Photoshop or Illustrator?
- Because those applications separate through an ICC profile and this page does not. A profiled separation asks which press, which ink, which paper, then decides how much black to generate and caps the total ink, and it returns different numbers for every profile you pick. This page runs the device independent algebra instead: K from the brightest channel, then cyan, magenta and yellow from what is left. RGB 11, 122, 133 comes out here as C 91.7 M 8.3 Y 0.0 K 47.8. Use that to read and carry channel values. When the job is going on a press, let the print shop separate the file with their own profile.
- What is the formula it uses?
- K = 1 - max(R,G,B)/255, then C = (1 - R/255 - K) / (1 - K), with M from G and Y from B the same way, and the percentages are those fractions times 100. Black is the one colour the second line cannot answer, because K is 1 and the division is by zero, so RGB 0, 0, 0 is defined directly as C 0 M 0 Y 0 K 100. Going the other way the inverse is R = 255 * (1 - C) * (1 - K). Both lines are printed in the panel next to the result, so nothing about the conversion is hidden.
- Can it convert CMYK back to RGB or hex?
- Yes, two of the four direction buttons do exactly that, and the trip is lossless in this tool. The percentages as printed, rounded to one decimal, convert back to the same 0 to 255 channels for every colour: C 91.7 M 8.3 Y 0.0 K 47.8 returns rgb(11, 122, 133) and #0B7A85. The CMYK panel prints that return trip on its own line, so you can see it landed where it started. A rounding to one decimal can move a channel by at most a quarter of a step, which is never enough to reach the next whole number, and the test suite sweeps the colour cube to prove it. Switching direction also carries the colour across into the boxes, so you can flip between the two sides without losing it.
- What does the gamut line mean when it flags a colour?
- It is a rule of thumb about the colour, not a measurement of any press. A colour is flagged when its brightest channel is 200 or more out of 255 and the algebra asks for two process inks over 50%, which is the shape of the colours ink on paper visibly falls short of: the RGB primaries, bright green, bright orange, screen reds. RGB 0, 255, 0 is flagged and comes out as C 100.0 M 0.0 Y 100.0 K 0.0, while orange RGB 255, 165, 0 is not, at C 0.0 M 35.3 Y 100.0 K 0.0. A flagged colour still converts; it simply prints duller than the swatch on your screen.
- Why does a plain grey become black ink only?
- Because this mapping pulls everything achromatic into the K channel. #808080 comes out as C 0.0 M 0.0 Y 0.0 K 49.8: the brightest channel sets K, and once K is removed the three chromatic channels have nothing left to carry. A profiled separation usually does the opposite and builds some of that grey from cyan, magenta and yellow, which is why its K reads lower than the number here. It is also why nothing here caps the ink: RGB 0, 0, 3 asks for 298.8% total coverage, and the panel says so, because a press starts refusing somewhere around 240%.
- Which hex codes does it accept?
- Three, four, six or eight digits, with or without the leading hash, in either case. #0b7 is expanded to #00BB77 and the expansion is shown, and the alpha pair in #0B7A8580 is dropped with a line saying so, because CMYK has no alpha channel. Anything else is named rather than guessed at: a five digit code is refused with the lengths that do work, a stray letter is quoted back, and a channel outside 0 to 255 or a percentage outside 0 to 100 is refused rather than quietly clamped to fit.
- Is anything uploaded, and does it remember what I typed?
- Nothing is uploaded and there is no account: the conversion is arithmetic that runs in this browser tab, and the tool makes no network request at all. The direction you chose and the values you typed are kept in this browser alone so a reload opens where you left off, and the Start over button above the tool forgets them. A sub-page that opens in its own direction never writes that choice over the one you made yourself.
The straight algebraic conversion, computed from the channels you enter and printed with its formula. It is device independent, which is the honest limit: no ICC profile, no ink, no paper and no press is taken into account, so use it to convert and read channel values, never to predict what a printer will put on the page.