Short answer
CMYK describes a printed colour by how much cyan, magenta, yellow and black ink to lay on paper. Inks subtract light that the paper would otherwise reflect, so the model is subtractive and depends entirely on the particular inks, paper and press. A CMYK value is therefore a recipe for one printing condition, and one colour can be printed with many different CMYK recipes — which is why an RGB-to-CMYK conversion needs a profile and involves choices, not a formula.
Licensing: Colourwise computes only a naive, uncalibrated CMYK for reference.
Convert any colour into this system with the colour converter
Cyan ink absorbs mostly red light, magenta absorbs green and yellow absorbs blue. Overprinting two gives the additive primaries (cyan plus yellow reflects green; magenta plus yellow reflects red), and all three in theory give black. In practice real inks are impure, so full cyan, magenta and yellow make a muddy dark brown, use a lot of ink, dry slowly and depend on perfect registration. Black ink — the K, for 'key' plate — solves all of these: it gives a neutral, dense black, sharp text from a single plate, and lets a printer replace part of the grey component of any colour with black.
Because black can stand in for equal-ish amounts of cyan, magenta and yellow, a dark green could be printed with a lot of CMY and little K, or with less CMY and more K. Both can match visually. Which one a conversion chooses depends on the separation settings — how much black to generate, total ink limits, how grey balance is handled — built into the ICC profile or chosen by the operator. The naive formula shown in the table on this page, which simply inverts RGB, always puts the grey component into K and ignores ink limits, dot gain and paper colour, so it is a teaching illustration, not a print specification.
The same CMYK percentages printed on coated and uncoated paper, or on two presses with different inks, produce visibly different colours: uncoated paper absorbs ink and spreads dots, lowering saturation and darkening mid-tones. Standards such as the ISO 12647 series define process-control aims for printing conditions, and characterisation data for those conditions is packaged into ICC output profiles. A CMYK file is meaningful only alongside the condition it was separated for; sending the same numbers to a different condition is exactly how 'the print does not match' happens.
Process CMYK cannot reach the brightest saturated screen colours, particularly vivid blues, bright greens and oranges, while it can print some cyans and yellows that sRGB screens cannot quite show. The shapes of the two gamuts differ, so conversion both clips some colours and leaves others unused. This is why spot inks exist for colours outside process CMYK, and why designers who need a vivid brand colour in print specify it separately rather than trusting a conversion.
| Colour | C / M / Y / K (%) | Total ink |
|---|---|---|
| Bright mid blue (#1e90ff) | 88 / 44 / 0 / 0 | 132% |
| Brick red (#c0392b) | 0 / 70 / 78 / 25 | 173% |
| Navy (#000080) | 100 / 100 / 0 / 50 | 250% |
| Sea green (#2e8b57) | 67 / 0 / 37 / 45 | 150% |
| Near-black (#1a1a1a) | 0 / 0 / 0 / 90 | 90% |
Why: CMYK values with very high total ink from a naive or unsuitable conversion.
Fix: Convert with the printer's ICC profile, which applies its ink limit.
Why: The screen blue lies outside the process CMYK gamut.
Fix: Specify a CMYK value approved on a printed proof, or a spot ink.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
ICC output profiles characterise a specific printing condition so that colour-managed software can convert device-independent colour to that condition's CMYK values.
Source: International Color Consortium: specifications and information
FactModerate evidence
The ISO 12647 series specifies process-control parameters for printing processes, which is why CMYK data is exchanged together with a stated printing condition.
Caveat: Colourwise relies only on ISO's catalogue records; the standards' contents are not reproduced.
Source: International Organization for Standardization (ISO) catalogue
Colourwise interpretationModerate evidence
A naive arithmetic RGB-to-CMYK conversion puts all of a colour's grey component into black and ignores ink limits, dot gain and paper, so it cannot predict press output.
Based on: Follows from the formula Colourwise uses (K = 1 − max(R, G, B), with CMY scaled by the remainder), set against what ICC separations account for.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.