--- title: RGB to CMYK Converter｜Mai Strategy ナレッジアカデミー lang: ja source: https://mindsprt.dev/ja/tools/rgb-cmyk-converter/ --- # RGB to CMYK Converter > Estimate how far a screen color will drift in four-color printing before you commit to ICC conversion and proofing There is no single correct RGB-to-CMYK conversion: an unmanaged formula only approximates, and production work needs an ICC profile plus a proof. Enter HEX, RGB or CMYK to convert, with Lab reference values, saturation warnings and an in-browser ICC mode. ## How to Use 1. Type or paste a color code into the HEX field, for example #RRGGBB 2. Or fill the four CMYK fields C, M, Y, K instead; both sides sync automatically 3. Read the RGB, CMYK and Lab values, plus the Original and Process approx. swatches 4. Check the Process-print risk label (low, medium, high) and Delta E00 to judge if a proof is needed ## Method & Assumptions A client sends a HEX code and asks what it will look like in print. There is no single answer: screens mix light additively in RGB, presses lay down CMYK ink subtractively, and the same RGB value maps to different CMYK builds depending on paper and press condition. Any RGB to CMYK conversion is only meaningful once you know which method produced it. The default conversion here is the textbook formula: K = 1 − max(R′,G′,B′), C = (1−R′−K)/(1−K), likewise for M and Y, with R′G′B′ normalized to 0–1. It is an unmanaged mathematical approximation tied to no real press condition, useful for quick estimates and cross-team discussion, not for production files. Lab reference values follow sRGB (D65) → linearization → CIEXYZ → CIELAB. Because Lab is device-independent, writing it into a spec sheet gives design, prepress and the pressroom one shared set of numbers, and it feeds directly into our Delta E calculator when you need a quantified difference. Colors turning darker and duller after conversion is physics, not a bug. Saturated teals, bright oranges and neon-like screen colors sit outside what four process inks can reproduce on paper, so lightness and chroma get compressed. The tool flags high-saturation inputs so you know which areas need attention before files go out. The ICC mode is where real conversion happens. Load the CMYK ICC profile your printer supplies (.icc/.icm) and Little CMS (2.19.1, compiled to WebAssembly) runs the conversion, soft proof and gamut check inside your browser; the file is never uploaded and never leaves your device. Rendering intent (perceptual or relative colorimetric) and black point compensation are selectable. We bundle no press-condition profiles, since redistribution of standards like ECI's PSO series is restricted by their rights holders; ask your printer, or download FOGRA39/FOGRA51 or GRACoL characterizations from official sources. In practice the division of labor is simple: math approximations for early checks and conversations, ICC conversion for production. Printers usually keep characterization profiles matched to their press and stock, and asking which profile fits your job beats guessing. For color-critical work, add a physical proof; most color disputes end there. ## Use Cases - Before a brand book is signed off: the designer converts the primary color #49BCBD, sees the saturation warning, and adds the spot-versus-process question to the client agenda - Social visual becoming a flyer: the bright orange #FF6600 converts to roughly C0 M60 Y100 K0; as a high-saturation color it gets flagged, so a digital proof is ordered before full-bleed use - Presentation blue to press blue: the deep blue #1A46E5 is a classic conversion trap; the approximation of about C89 M69 Y0 K10 plus the warning gives you numbers to bring to the printer - After the printer replies: they name a FOGRA51-class profile for coated stock, you load the .icc into ICC mode and soft-proof the layout for out-of-gamut areas - Business card check: body text was set in rich black, switched back to single-ink K100 so misregistration cannot blur the type - Prepress training: a supervisor has a new hire run the approximation first, then explains why production jobs still need the ICC mode and a proof ## FAQ ### Why do different apps give different CMYK values for the same RGB? Because most design software converts through ICC color management, and each app defaults to a different profile. This tool deliberately offers the unmanaged textbook formula as a neutral reference, so you can see where the differences between apps come from. ### Can I send these CMYK values straight to print? Not recommended. The math conversion knows nothing about paper, ink, dot gain or press conditions, so sending it to press is leaving color to chance. For production, have prepress convert with the correct ICC profile and confirm with a digital or physical proof. ### Why do colors turn darker and duller after RGB to CMYK conversion? Because the gamut of four process inks is smaller than a screen's. Saturated teals, bright oranges and neon-like colors simply cannot be printed, so lightness and chroma get compressed. That is physics; any software shows the same effect, differing only in how the compression is done. ### When is ICC conversion mandatory? Brand color reproduction, multi-vendor handoffs, color acceptance testing, and any job where a wrong color means reprinting. ICC conversion gives predictable results for a specific press condition (coated, uncoated, newsprint), which a math approximation cannot do. ### K=100 black versus rich black, what is the difference? K100 is single-ink black, best for text and fine lines. Rich black (e.g. C40 M30 Y30 K100) prints deeper, but the slightest misregistration across four plates blurs small type. In practice: K100 for small text and rules, rich black only for large dark areas. ### Where do I get an ICC profile, and why doesn't the site bundle one? The best source is your printer: the characterization of their press and stock is the real matching reference. Standard press-condition profiles (such as ECI's PSO series) carry license terms that forbid redistribution, so we bundle and host none. Download from ECI or Adobe under their terms, then load the file here; it is processed only inside your browser. ## Limitations & Disclaimer - Default results are mathematical approximations with no ICC profile applied; they do not represent printed output - ICC-mode results are only as good as the loaded profile's match to the actual press condition - On-screen previews vary by panel, brightness and ambient light - Printed color depends on paper whiteness, ink, dot gain and finishing - Confirm with a physical or digital proof before production Updated: 2026-08-12 · Method version: v1.1.0 --- > HTML version: https://mindsprt.dev/ja/tools/rgb-cmyk-converter/ > MINDS — 麥思印刷整合有限公司 · https://mindsprt.dev