RGB to CMYK

The honest version first

Converting RGB to CMYK without an ICC profile is not really a conversion.

RGB describes light being emitted. CMYK describes ink absorbing it. There is no fixed relationship between the two — the same CMYK values produce different colours on different presses, different papers and different inks. The translation is a colour-management problem, not an arithmetic one.

What this page does is the standard device-independent formula: the one every tool uses when it has no profile to work from. It gives you a reasonable starting point and it is not what will come off the press.

So when is this useful

• Getting a rough idea of the ink mix.
• Filling in a field that wants approximate CMYK values.
• Checking whether a colour is obviously out of gamut — a vivid RGB green or a bright orange that simply cannot be made from CMYK inks. If the conversion produces something that looks visibly duller than your original, that is real: the colour is outside what four-colour printing can reproduce.

For anything actually going to print, convert in software that knows the destination profile — and expect the numbers to differ from these.

Rich black

One practical thing the formula will not tell you. Pure k: 100 prints as a flat, slightly washed dark grey over a large area. Print designers use rich black — K at 100 with some cyan, magenta and yellow beneath it — to get a black that looks black.

The exact recipe depends on the press. It is never what an RGB-to-CMYK formula produces, because pure black in RGB has no colour information to distribute.

Nothing is uploaded

Converted in this page.