RGB to CMYK

What this tool is used for

• Producing a print starting point from a screen colour.
• Getting ink percentages to quote to a printer.
• Comparing a screen palette against print requirements.
• Converting a digital brand colour for print collateral.
• Checking roughly how much ink a colour will need.

Frequently Asked Questions

Why is the black channel calculated first?

Because K is defined as whatever all three inks have in common: K = 1 − max(R, G, B). Extracting that shared amount as one cheap black ink instead of three coloured ones is the entire reason the K plate exists — it uses less ink and gives a truer neutral.

What does the divide-by-(1 − K) step do?

It re-scales the remaining colour into the range still available after the black has been laid down. Without it, every value would be dimmed twice — once by its own ink and again by K. When K is 1 the divisor is zero, so that case short-circuits to 0/0/0/100.

Why does my vivid screen colour turn out muted?

Because it is outside the CMYK gamut. Saturated RGB greens and blues are produced by emitted light and simply cannot be mixed from four inks — the conversion returns the closest printable colour, which is duller. There is no formula that fixes this; the colour does not exist on paper.

Is this conversion reversible?

Round-tripping RGB → CMYK → RGB usually returns your starting colour when the input was in gamut, because both directions use the same algebra. That is not true of a profile-based conversion, where out-of-gamut colours are mapped onto the gamut boundary and the original is genuinely lost.

What is K short for?

Key. The black plate is the key plate that the other three are registered against, and it carries most of the image detail. The common explanation that K avoids clashing with the B in RGB or Blue is a plausible-sounding back-formation — the printing term predates the acronym.

Common errors and gotchas

• Expecting the print result to match the screen, which it cannot without a managed profile.
• Ignoring total ink coverage limits, which presses enforce.
• Converting black text to a four-colour mix rather than pure black, which prints soft and misregisters.
• Assuming a vivid screen colour is achievable in ink, when many are not.
• Treating the conversion as authoritative rather than as a starting point for a proof.