The important thing to know before you copy anything is that this conversion has no colour profile behind it. It applies the standard textbook formula, which assumes theoretically perfect inks printing on theoretically perfect white paper. No real press works that way, and the difference is not small for saturated colours.
Use it to get a sensible starting point, to sanity check numbers someone has sent you, or to understand roughly where a colour sits in ink terms. Do not use it for print proofing, for matching a spot colour, or for anything a client will sign off on. Those need colour managed software with the destination ICC profile loaded and a physical proof to look at.
Why an RGB to CMYK converter cannot be exact
RGB describes emitted light and CMYK describes absorbed light, but that is only the surface of the problem. The deeper issue is that CMYK is not one colour space. It is a family of them, one per combination of ink set, paper stock, press and rendering intent, and the same four percentages produce measurably different colours across those combinations.
A colour managed conversion resolves this by consulting an ICC profile that contains measurements from a real press printing real ink on real paper. It knows that this cyan on this coated stock absorbs a particular amount of red, and it compensates. A formula has none of that information and cannot invent it.
The formula also has no concept of gamut. It will happily return four percentages for a brilliant screen green that no combination of process inks can reproduce, and the numbers will look perfectly reasonable right up until the job comes off the press looking dull.
- Reasonable: getting a rough ink build for a colour so you can talk to a printer about it.
- Reasonable: checking whether a colour is near neutral or heavily saturated in ink terms.
- Reasonable: producing placeholder values while artwork is still being designed.
- Unsafe: proofing, spot colour matching or brand sign off.
- Unsafe: assuming the numbers will print as they appear on your display.
The arithmetic behind the numbers
Each channel is divided by 255 to give a fraction. The key value is one minus the largest of those three fractions, which pulls out as much of the colour as possible into the black plate. That is called maximum grey component replacement, and it is the simplest of several strategies a real workflow might use.
The three ink values then come from how far each channel falls short of that brightest channel, normalised by one minus key. A pure red produces zero cyan, full magenta, full yellow and no key, which is exactly what the model predicts and roughly what a press produces.
Pure black is the one special case. When all three channels are zero the key calculation would divide by zero, so the result is defined directly as no colour ink and 100 percent key. Real print work usually replaces that with a rich black build, adding supporting inks so a large area does not look washed out.
Understanding cmyk gamut limits
Four process inks cover noticeably less of visible colour than a modern display. The gap is worst in saturated blues, vivid greens, bright oranges and anything approaching a neon. Those colours have no ink equivalent at all, so the conversion returns the closest thing it can compute rather than telling you the request was impossible.
The practical symptom is a colour that looks brilliant in the browser and flat on paper. If a brand colour has to work in both media, the safe approach is to pick it inside the print gamut first and accept that the screen version will be less exciting than it could have been.
Total ink coverage is a second limit. Presses generally cap the sum of the four plates somewhere near 300 percent, because more ink than that cannot dry properly on the sheet. The naive formula never exceeds 300 by construction for a fully saturated colour, but hand edited values easily can, so it is worth checking.
Turning a screen colour to ink percentages in practice
Start from the hex code or channel values you already use on the web. Convert them here, then hand the resulting build to whoever is running the print job and ask them to convert properly against the real profile. Their number will differ from yours, and theirs is the one to trust.
If you are producing artwork yourself, set the document colour mode to CMYK with the correct profile before you place any assets, rather than designing in RGB and converting at the end. Converting late is where most colour surprises come from, because every element gets remapped at once with no chance to adjust individually.
Where a colour absolutely must match across print and screen, the usual answer is a spot ink rather than a process build. A spot colour is mixed to a specification rather than simulated by four plates, which removes the gamut problem entirely at the cost of an extra plate on the press.
Reading the output this tool gives you
The four percentages are rounded to whole numbers, which is how print specifications are normally written. Whole percent resolution is well below what any press can hold, so nothing useful is lost in that rounding.
The swatch beside them is drawn using your original RGB colour, not a simulation of the ink. Simulating ink appearance requires the profile the tool does not have, and drawing a fake proof would be worse than drawing none, because it would look convincing.
The coverage readout adds the four plates together so you can spot builds that are heavy on ink before you send them anywhere. Anything over roughly 300 percent should be discussed with the printer.
| Colour type | Conversion quality | Why |
|---|---|---|
| Near neutral greys | Good | Little ink, little room for the model to err |
| Light tints | Good | Low coverage keeps ink interaction small |
| Deep saturated blues | Poor | Cyan absorbs more red than the formula assumes |
| Vivid greens and oranges | Poor | Outside the process ink gamut entirely |
| Solid black | Special case | Real work uses a rich black build instead |
How to convert RGB to CMYK
- 1
Enter your screen colour
Type a hex code or set the red, green and blue sliders until the swatch matches the colour you use on screen.
- 2
Read the four ink values
Cyan, magenta, yellow and key appear as whole percentages, along with the total ink coverage.
- 3
Copy the build
Take the four values as a comma separated string for a specification, or individually for a design application.
- 4
Have it converted properly
Ask your printer to run the same colour through the real destination profile, and treat their numbers as authoritative.
Related tools worth bookmarking
Sources and further reading
- ICC: colour profile specificationsThe International Color Consortium specifications that define how device colour is measured and mapped.color.org
- W3C: a standard default colour space for the internetThe original sRGB definition, which is the colour space every screen value on this page is assumed to be in.w3.org
- MDN: CSS colour valuesReference for every colour notation the web platform accepts, including the device dependent ones.developer.mozilla.org
Frequently asked questions
Common questions about the rgb to cmyk converter.
Treat it as an estimate only. Without an ICC profile for the actual ink, paper and press the numbers are a textbook approximation, and saturated colours in particular will print noticeably duller than the screen preview suggests. Always get a physical proof before signing anything off.
Four process inks cover a smaller range of colour than a display emits, and vivid greens sit well outside that range. The formula still returns four numbers because it has no gamut check, but no ink combination reaches that colour, so the printed result will be visibly duller.
It is the choice of how much of a colour to render with black ink instead of equal amounts of the three coloured inks. This tool uses the simplest maximum replacement strategy, which pulls as much as possible into the key plate and keeps total ink coverage low.
For small text, yes, because a single plate avoids registration problems that make edges look fuzzy. Large solid black areas usually use a rich black build with supporting inks, since a single black plate looks weak and uneven across a big shape.
Because it is applying a real profile rather than a formula. The document working space, the destination profile and the rendering intent all affect the result, which is precisely the information a profile free conversion does not have access to.
No. Everything is computed by JavaScript in the page you are looking at, with no upload and no network request, so an unreleased palette or a confidential brand colour never leaves your own device.