Result
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Facts and limits of this method
| Black formula | K = 1 − max(R, G, B) / 255 |
|---|---|
| Cyan formula | C = (1 − R/255 − K) / (1 − K) |
| Input range | R, G, B from 0 to 255 |
| Output range | C, M, Y, K from 0% to 100% |
When it misleads you
- This is a mathematical approximation, not real color separation — actual print shops use ICC profiles for a specific printer and paper, which produce different numbers.
- RGB is an additive model for screens, CMYK is subtractive for ink on paper; some saturated RGB colors (especially bright blues and greens) can't physically be reproduced in CMYK and will print duller.
- The formula assumes the simplest black generation (100% GCR), while professional prepress uses more sophisticated ink-mixing schemes.
- Treat the result as a design-time guideline, not final press values — verify them in dedicated software (Adobe, CorelDRAW, etc.) before printing a run.
How it is calculated
R, G and B values are normalized by dividing by 255, giving numbers from 0 to 1.
The black channel K is computed as one minus the maximum of the normalized R, G, B — the darker the source color, the higher K is.
Each of cyan, magenta and yellow is computed as (1 − channel − K) / (1 − K), which removes the black contribution from the color components.
The resulting C, M, Y and K fractions are multiplied by 100 and rounded to give print-ready percentages.
Questions and answers
Why does CMYK look different on screen than on paper?
A screen displays light (RGB), while paper shows reflected light after ink is applied (CMYK), so the final printed shade should always be checked with a proof.
Can I use these percentages directly in a print layout?
As a guideline, yes, but before printing a run it's worth confirming exact values against the specific print shop's color profile.
Why doesn't C + M + Y + K add up to 100%?
That's normal: each CMYK channel is an independent ink-coverage percentage, not a share of a 100% total.
Is my data sent anywhere?
No, the whole calculation runs in your browser.