Guide
RGB vs CMYK
Your screen makes colour by emitting light; paper makes it by absorbing light. They are not two encodings of the same thing, which is why a print never quite matches the monitor — and why no converter can promise it will.
Adding light, subtracting light
A monitor starts black and adds red, green and blue light until it reaches white. Paper starts white and adds ink that subtracts wavelengths until it reaches something close to black. The two systems run in opposite directions, and they do not cover the same set of colours: there are colours a screen can emit that no combination of cyan, magenta, yellow and black ink can absorb its way to.
That set of reachable colours is called a gamut, and the practical question is never “what is the CMYK value of this colour” but “is this colour inside the gamut of the press and paper I am printing on”. When it is not, something has to give, and the software decides what — usually by pulling the colour to the nearest reachable one, which is why saturated blues arrive looking slightly purple.
Why every free converter disagrees
Search for an RGB to CMYK converter and you will find dozens, each returning confident numbers, and no two agreeing. They are not broken — they are answering a question that has no answer without more information.
Almost all of them apply the same textbook formula: subtract each channel from one, take the smallest result as black, scale the rest against it. It is arithmetic, and it is reversible, which makes it look authoritative. What it assumes is a press with perfect inks, no dot gain, no ink spreading into the paper fibres, and no coverage limit. Real cyan ink is not a pure cyan filter; real paper absorbs and scatters; real presses put down more ink than the file specifies as the dot expands on contact.
A genuine conversion needs an ICC profile — a measured description of how a specific combination of press, ink and paper actually behaves. US Web Coated (SWOP) v2 is the common North American reference, Coated FOGRA39 the European one, and Japan Color 2001 Coated is standard in Japan. The same RGB value converted through each of these gives three different CMYK results, and all three are correct for their own press.
This is why we do not offer a converter
Everything else on this site returns a number that is exactly right — a paper size, a scaling percentage, a grammage. A CMYK conversion without a profile would be the one tool here that quietly guesses. Rather than add it and bury a disclaimer, the useful version is this page: what to send instead, and which colours to check before you commit.
Colours that do not survive
If a design depends on any of these, see a proof before committing to a run — or design around them.
| Colour | What happens | Why |
|---|---|---|
| Saturated blue | Shifts towards purple or navy | Cyan and magenta together cannot reach the blue a backlit screen emits |
| Bright green | Goes flat and slightly grey | Requires cyan and yellow at strengths that muddy each other on paper |
| Neon and fluorescent | Loses all its glow | Those colours rely on emitted or UV-reactive light, which ink cannot do |
| Vivid orange | Dulls noticeably | Sits between magenta and yellow primaries, so neither reaches it fully |
| Pure screen black | Prints as dark grey | Black ink alone reflects light; large areas need a rich black mix |
When a brand colour has to be exact — a logo, a corporate identity — the answer is not a better conversion but a spot colour: a pre-mixed ink, specified by a Pantone or similar reference, printed as its own plate. It costs more and it is the only way to guarantee the same colour across different presses and papers.
What to do instead
- Ask the printer what they want. Most modern workflows prefer RGB with an embedded profile and convert at their end, using the exact profile for the press and stock. Converting early discards information they could have used.
- If they specify a profile, use that one. Not “CMYK” generically — the named profile. Convert once, at the end, from your master RGB file.
- Soft-proof before you commit. Simulate the target profile on screen and turn on the gamut warning to see which areas cannot be reproduced.
- Set blacks deliberately. Rich black for large areas and display type, plain 100% black for body text.
- Judge under sensible light. A print seen under warm domestic bulbs and the same print under daylight are visibly different objects.
Colour is the part of prepress with the least certainty. The parts with the most — size, scale, resolution, bleed — are where a file is usually actually wrong, and they are worth checking first: the image quality analyser for resolution, bleed for the trim, and the calibration test for whether your own printer is telling the truth about scale.
Frequently asked questions
Why does every free RGB to CMYK converter give a different answer?
Because there is no single correct answer to give. A conversion is only defined once you specify the paper, the ink set and the press behaviour, which is what an ICC profile encodes: US Web Coated (SWOP) v2 in North America, Coated FOGRA39 across much of Europe, Japan Color 2001 elsewhere. Free converters skip that step and apply a naive formula — subtract each channel from one, pull out the common minimum as black — which produces mathematically tidy numbers that assume perfect inks, no dot gain and no paper absorption. None of those assumptions holds on a press.
So should I convert to CMYK before sending a file?
Ask your printer, and default to no. Most commercial printers today prefer receiving RGB files with the profile embedded, because their workflow converts using the exact profile for the press and paper the job is going on — which is better information than you have. Converting early throws away colour you cannot get back and locks in assumptions that may not match the press. The exception is when a printer explicitly specifies a CMYK profile: then convert to that profile, not to “CMYK” in the abstract.
What is rich black and when do I need it?
Black ink alone on a large area reflects enough light to read as dark grey next to a photograph. Rich black adds the other inks underneath — a common recipe is 60 cyan, 40 magenta, 40 yellow, 100 black — giving a visibly deeper black. Use it for large solid areas and headline type; use plain 100% black for body text, where four inks would need to register perfectly against each other and small type shows any misalignment as coloured fringing.
What is total ink coverage and why does it matter?
The sum of all four ink percentages at any one point. Presses impose a ceiling — commonly around 300% on coated stock and 240–260% on uncoated or newsprint — because beyond it the paper cannot absorb the ink fast enough: sheets set off against each other, dry slowly, or cockle. Building a black as 100/100/100/100 gives 400% and will be rejected or silently altered. This is one of the reasons a naive formula conversion causes trouble: it has no concept of a coverage limit.
Why does my print look darker than my screen?
A monitor emits light and paper reflects it, so the brightest white a print can manage is the paper itself under whatever light you are standing in. Screens are also usually far brighter than the viewing conditions of the print. Turning display brightness down towards 120 cd/m² and judging proofs under neutral daylight closes most of the gap — and if colour accuracy genuinely matters, a hardware-calibrated monitor is the only reliable answer.
How do I see the difference before printing?
Soft-proofing. Photoshop, Illustrator, InDesign and Affinity all simulate a target profile on screen — in Photoshop it is View, Proof Setup, then Custom with your printer’s profile, and Ctrl/Cmd+Y to toggle. It is an approximation, but it shows you which colours are about to shift before you commit to a print run. A gamut warning overlay in the same menu marks the areas that cannot be reproduced at all.