How to Choose Paper Stock by GSM, Coating, and Colour Reaction for Print

The same CMYK file will not produce the same colour on coated and uncoated paper. Uncoated stock absorbs ink into its fibres, causing dot gain. Ink spreads beyond the halftone dot boundary. The printed result is darker, less saturated, and shifts toward grey. Coated paper holds ink on its surface, keeping dots sharp and colour bright. The difference is not subtle: a rich black formula that looks deep on coated stock prints as a muddy grey on uncoated. The fix is not guessing at the screen. Choose the stock first and build the file to match its Fogra characterisation data.

GSM means grams per square metre. It is the standard metric for basis weight and the number you need when ordering any commercial print. Text stocks for uncoated paper run from 60 to 100 gsm. Coated text stocks start higher, from 80 to 130 gsm, because the coating adds weight. Cover stock, for folders, postcards, or booklet covers, ranges from 170 to 400 gsm. Business cards use the same range, 250 to 400 gsm, depending on whether you want a card that flexes or one that snaps.

GSM does not directly tell you thickness. Caliper, the physical thickness in millimetres, varies by paper type. Standard copy paper measures 0.09 to 0.11 mm. Premium cover stock runs 0.25 to 0.40 mm. A 120 gsm coated sheet can be thinner than a 100 gsm uncoated sheet because the coating compresses and smooths the fibre base. Always request a dummy or swatch book before committing to a stock for a print run. The feel in hand, not the number, is what your client will judge.

How the Surface Changes the Ink

Coated paper receives a sealant, clay, latex, or similar compound, that creates a smooth, non-absorbent surface. Uncoated paper has no sealant; its fibres are exposed. Ink lands on coated stock and dries on top. Ink lands on uncoated stock and is pulled into the fibre network. That absorption changes how the ink looks.

The colour difference is measurable and predictable. On coated stock, the halftone dot holds its shape. On uncoated stock, the dot spreads by 20 to 24 percent at the 50 percent tonal value, per ISO 12647-2 for uncoated paper (PT4). The equivalent dot gain target for coated offset (PT1 and PT2) is 15 to 18 percent. That extra 5 to 9 points of dot gain shifts midtones darker, reduces saturation, and crushes shadow detail. A file that looks correct under soft-proof with a FOGRA51 (coated) profile will appear wrong when printed on uncoated stock because the correct profile is FOGRA52 (uncoated).

Brightness and Whiteness

Brightness and whiteness also differ. Economy uncoated stock has a brightness of 80 to 92 on the TAPPI T452 scale. Premium coated stock runs 92 to 100 or higher. Archival-grade paper can reach a whiteness of 120 to 160 on the CIE D65/10° scale. The brighter the paper, the more contrast the ink has against it. A colour that pops on bright coated stock will look dull on a low-brightness uncoated sheet, even if the dot gain is identical.

Gloss, Silk, and Matte

Finish, gloss, silk, matte, or uncoated, changes how light interacts with the ink film. Gloss finish reflects light directionally, making colours appear more saturated and contrasty because less light scatters from the surface. Matte finish scatters light, reducing perceived saturation. Silk or satin finish sits between the two, offering a compromise that suits photographic work where you want some lustre without the glare of gloss.

The effect is most visible in dark areas. A gloss-coated sheet can make a 60C 50M 50Y 100K rich black appear deep and neutral. The same build on matte coated stock loses depth because the surface scatter lifts the shadows. On uncoated stock, that same build prints as a dark, muddy grey. The reason is not just dot gain but also the absence of a reflective surface to hold the ink film intact.

Uncoated Textures and Fine Type

Uncoated finishes come in their own grades: smooth, vellum, laid, wove, and felt. Each has a different surface texture that affects ink transfer. A smooth uncoated sheet accepts ink more evenly than a felt-marked one. If your design includes fine reversed text (white type out of a dark background), use a smooth uncoated or any coated finish. Reversed text below 6 pt on a textured uncoated sheet will fill in and become illegible.

Total ink coverage (TIC) is the sum of the CMYK percentages at any point on the page. Coated stock can handle a TIC of 300 to 340 percent, depending on the press and the Fogra profile in use. Uncoated stock cannot. Its limit is 240 to 280 percent. Exceed it and the ink cannot dry properly, causing set-off (ink transferring to the back of the next sheet), smearing, or cracking along folds.

These limits come from Fogra process-control standards, specifically the characterisation data for ISO 12647-2 printing conditions. Fogra 39 (coated) and Fogra 47 (uncoated) define the ink limits and dot gain targets that commercial printers use to calibrate their presses. If you send a file with a TIC of 320 percent to a printer running a Fogra 47 uncoated condition, the press will either reject the job in preflight or produce a smeared result that fails inspection.

The practical fix is to set your working space to the correct ICC profile before you start. For coated stock, use FOGRA51 (PSO Coated v3) or GRACoL2013_CRPC6.icc. For uncoated, use FOGRA52 (PSO Uncoated v3) or GRACoL2013UNC_CRPC3.icc. Soft-proof in InDesign or Photoshop under the ISO 3664 D50 viewing condition, 2000 ± 500 lux with a neutral grey surround. What you see on screen under D50 is what you get on paper, provided your monitor is calibrated.

Worked Example: Rich Black on Coated vs Uncoated

A standard rich black for coated stock is C60 M40 Y40 K100. TIC: 240 percent. On coated stock, this prints as a deep, neutral black. On uncoated stock, the same build prints as a dark, muddy grey because the ink spread fills the halftone dots and the excess ink cannot dry cleanly.

The corrected rich black for uncoated stock uses less colour: C40 M30 Y30 K100. TIC: 200 percent. For uncoated, an even safer option is a single-colour black: 0C 0M 0Y 100K with overprint set to on. Overprinting the black prevents the knockout halo that occurs when registration shifts, and it keeps the TIC at 100 percent. This is the industry default for black text below 12 pt on any stock. Above 12 pt, set black to knockout to avoid the slight darkening that overprint causes on large areas.

Dot Gain and Ink Spread

Dot gain is not a defect; it is a characteristic of the press-paper combination. The ISO 12647-2 standard specifies the target dot gain for each paper type. For coated offset (PT1 and PT2), the target is 15 to 18 percent at the 50 percent tint. For uncoated offset (PT4), the target is 20 to 24 percent. A press that meets these targets is in control. A file that does not compensate for the higher dot gain on uncoated stock will look heavier than intended.

The compensation happens in the separation, not in the artwork. Use the correct ICC profile during CMYK conversion. Do not try to lighten the file manually by reducing all colour values; that shifts the hue. The profile handles the dot gain compensation automatically if you assign it before you place the first image.