The ISO paper size system · why the 1:√2 ratio matters
The ISO 216 paper size standard is built on a single elegant mathematical principle: every size in the series has the same aspect ratio (width to height = 1:√2 ≈ 1:1.414), which means folding any sheet in half across its long dimension produces the next smaller size with the identical proportions. A0 folded in half is A1. A1 folded in half is A2. This property makes the ISO system uniquely efficient for printing and binding, content laid out for one size scales proportionally to any other size in the series.
A0 was specifically defined to have an area of exactly 1 m². This is not coincidental, it is the connection between paper sizes and paper weight. A single A0 sheet of 80 GSM paper weighs exactly 80 grams. A ream (500 sheets) of A4 paper at 80 GSM weighs 2.5 kg. The entire GSM system is anchored to the A0 = 1 m² definition. Understanding this connection makes paper weight calculation intuitive rather than arbitrary.
ISO A series · complete dimensions reference
| Size | Dimensions (mm) | Area (m²) | Common applications in India |
|---|---|---|---|
| A0 | 841 × 1189 | 1.000 | Large technical drawings, engineering plans, large format posters |
| A1 | 594 × 841 | 0.500 | Large posters, wall charts, architectural drawings |
| A2 | 420 × 594 | 0.250 | Large posters, flip chart pads, retail signage |
| A3 | 297 × 420 | 0.125 | Broadsheet brochures (folded to A4), menus, larger catalogues, presentation folders |
| A4 | 210 × 297 | 0.0625 | Standard document size, stationery, brochures, annual reports, product literature. The dominant commercial print format in India. |
| A5 | 148 × 210 | 0.031 | Small brochures, flyers, notepads, prescription pads |
| A6 | 105 × 148 | 0.016 | Postcards, greeting cards, compact brochures |
| A7 | 74 × 105 | 0.008 | Small cards, vouchers, compliment slips |
ISO B series
The B series is a supplementary range with sizes between consecutive A sizes. B4 (250×353mm) and B5 (176×250mm) are the most common B sizes used in Indian print for books, notebooks, and some brochure formats. B series dimensions are the geometric mean between adjacent A sizes.
DL (Dimension Lengthwise) · the envelope standard
DL (110×220mm) is the standard business envelope format in India, designed to accept an A4 sheet folded into thirds. DL is the correct format for business letterheads, invoices, and direct mail pieces intended to mail in a standard business envelope.
SRA and RA series · the press sheet sizes
Commercial printing presses do not typically print directly on A series sheets. They print on slightly larger "untrimmed" sheets, the SRA (Supplementary Raw format A) and RA (Raw format A) series, that include extra material around the finished sheet for grippers, colour bars, registration marks, and bleed. The printed sheets are trimmed after printing to produce the finished A series size.
| Press sheet | Dimensions | Fits finished size | Notes |
|---|---|---|---|
| SRA3 | 320 × 450mm | A3 (with bleed and grippers) | The most common press sheet size for B2 format sheetfed offset in India. Most Indian commercial press rooms run SRA3 or 320×450mm. |
| SRA2 | 450 × 640mm | A2 (with bleed and grippers) | Used on B2 format presses for larger format work, posters, double-page spreads. |
| SRA1 | 640 × 900mm | A1 | B1 format press sheet. Used on larger B1 presses common in large commercial and packaging operations. |
| SRA0 | 900 × 1280mm | A0 | Very large format, used for large packaging work, impositions of many A4 pages per sheet. |
| RA3 | 305 × 430mm | A3 | Slightly smaller than SRA3. Less common in India, SRA3 is the standard. |
Understanding the relationship between SRA3 (the press sheet) and A4 (the finished document) is the foundation of imposition planning, how many A4 pages fit on one SRA3 sheet, how they are arranged, and how many press passes are needed for a given publication. An SRA3 sheet can hold 4 A4 pages per side (8 pages total, 4 front and 4 back), making it the correct sheet for an 8-page A4 brochure in a single press pass per side.
Screen ruling · how halftone screens create the illusion of continuous tone
Offset printing can only print one ink density in any given location, either full ink (100%) or no ink (0%). It cannot print a "30% density" ink film. The halftone screen solves this problem by converting continuous tones (photographs, gradients) into a pattern of dots, varying the size of the dots creates the visual impression of varying tone density. Large dots in a grid = appears dark. Small dots in a grid = appears light. This is how all conventional printing reproduces the appearance of continuous tonal variation.
Screen ruling (also called screen frequency or screen ruling) is the number of rows of halftone dots per inch (LPI, lines per inch) or per centimetre (L/cm). Higher LPI = more rows of dots = finer dots = smoother tonal transitions = better apparent image quality. Lower LPI = fewer rows = larger dots = more visible dot structure = coarser apparent quality.
| Screen ruling | Dot size at 50% | Substrate | Applications |
|---|---|---|---|
| 85 LPI | ~0.30mm | Newsprint, very absorbent uncoated paper, corrugated post-print | Newspaper printing, economy corrugated case printing. Visible dot structure is expected. |
| 100–120 LPI | ~0.22mm | Uncoated offset paper, standard flexo on corrugated | Uncoated commercial work, corrugated pre-print flexo, general economy packaging. Visible dot structure on close inspection. |
| 133–150 LPI | ~0.17mm | Coated matte paper, standard coated art paper | Standard quality commercial print, brochures, catalogues, corporate literature. Good image quality for most applications. |
| 175 LPI | ~0.14mm | Gloss coated art paper, SBS board | Premium commercial print, high-quality photography reproduction, premium packaging cartons, colour-critical work. The standard for premium Indian offset production. |
| 200+ LPI (FM/stochastic screening) | Variable micro-dots | Gloss coated, high-quality packaging board | Ultra-premium work, eliminating moiré patterns, maximum photographic quality. Requires press and plate capability beyond standard. |
Selecting the correct screen ruling · the paper-process-quality triangle
Screen ruling is not a free choice, it must be matched to the paper (or substrate) and the print process. A screen ruling that is too fine for the paper causes dot gain problems: the small dots spread when they absorb into the paper surface, merging together in midtones and making images appear dark and muddy. A screen ruling that is too coarse for the paper and process wastes the quality potential of premium coated stock.
The standard guideline: screen ruling in LPI = approximately 1.5–2× the paper's surface smoothness in PPS (Parker Print Surf) units. For practical purposes: coated gloss paper with PPS below 1.5 μm supports 175 LPI. Coated matte paper with PPS 2–4 μm supports 150 LPI. Uncoated paper with PPS 5–10 μm supports 120–133 LPI. Very rough paper (PPS above 15 μm) should not go above 100 LPI.
The image resolution to screen ruling relationship
Image resolution (PPI) and screen ruling (LPI) are related, the image needs sufficient resolution to provide the information that the screen ruling can reproduce. The standard rule: image resolution = 1.5–2× the screen ruling. For 175 LPI printing, images should be at 263–350 PPI, in practice, 300 PPI is the standard target that meets the 1.5–2× requirement for 175 LPI work with a comfortable margin.
Print register · alignment between colour separations
Register is the alignment of one printed colour relative to another. In process colour printing, the four CMYK separations must be printed in exact alignment, if cyan, magenta, yellow, and black dots are not in register, the halftone pattern produces colour fringes on edges, soft and unclear detail, and visible colour contamination in what should be clean tones. Register is measured as the displacement (in mm) of one colour from its target position relative to another colour.
Types of register
- Front-to-back register: alignment between the printing on the front (face) and back (reverse) of a sheet. Critical for duplex printing where text or graphics must align through the sheet, for example, a table that must appear aligned when the sheet is held up to light.
- Colour-to-colour (side-lay) register: alignment between different colour separations printed in sequence on a multi-unit press. This is the register that determines halftone dot overlay quality and the sharpness of colour boundaries.
- Multi-pass register: alignment between the first and second printing passes on a perfector press or when printing both sides in sequence. More variable than single-pass register because the sheet must be repositioned.
ISO 12647-2 register tolerance
ISO 12647-2 specifies a maximum register deviation of ±0.1mm (100 microns) for sheet-fed offset on coated paper. This is the benchmark for commercial quality offset. In practice, well-maintained modern sheet-fed presses achieve ±0.05mm register consistently. Register worse than ±0.2mm is visible as colour fringing on fine text and detail, and is unacceptable for most commercial applications.
Register marks
Register marks are precise geometric targets (typically a circle with cross-hairs, or a hairline cross) placed outside the trim area on each colour separation. The press operator checks register by examining whether the marks from all separations overlay exactly. Digital presses and CTP systems use scanner-verified register marks for automatic closed-loop register correction. Manual register marks must be measured with a loupe at 10× magnification, visual inspection alone cannot detect ±0.1mm register variation.
Misregister · causes and correction
| Cause | Appearance | Correction |
|---|---|---|
| Plate mounting error | Consistent misregister in the same direction on all sheets, one colour consistently displaced from the others. | Re-mount the plate using the press's punch registration system. Verify plate position with register marks before starting the run. |
| Paper stretch under impression | Progressive misregister increasing toward the trailing edge of the sheet, tight at the gripper end, loose at the tail. | Reduce impression pressure. Check paper moisture content, wet paper stretches more. Consider a shorter sheet or adjust impression pressure across the cylinder width. |
| Gripper variation | Inconsistent misregister from sheet to sheet, some sheets in register, others not. Pattern may relate to gripper bar sequence. | Check and adjust gripper pressure on all gripper bars. Replace worn gripper pads. Verify gripper bar timing. |
| Paper moisture variation | Misregister that changes as the run progresses, paper at the centre of the ream may have different moisture content than paper at the edges. | Condition paper to press room humidity for 24 hours before printing. Fan the paper before loading to allow moisture equalisation. |
| Cylinder packing error | Consistent misregister pattern related to a specific unit, other units are in register but one colour is consistently off. | Check plate and impression cylinder packing heights. Incorrect packing alters the effective cylinder diameter and shifts the image position on the sheet. |
GSM and caliper · weight, thickness, and the difference between them
GSM (grams per square metre) is the weight specification for paper and board, how much a one-square-metre sheet of the material weighs in grams. Caliper (or thickness) is the physical thickness of the sheet, measured in millimetres or microns. These two measurements are related but not directly proportional, two papers with the same GSM can have meaningfully different calipers depending on their density and bulk.
Why GSM and caliper can differ
Paper bulk describes how thick a paper is relative to its weight. High-bulk paper has more air trapped between the fibres, it feels thick and rigid but does not weigh more. Low-bulk (dense) paper has fibres packed tightly together, it is thinner but the same weight. Offset printing papers are often bulked to increase perceived quality (a thicker feel) without increasing weight and therefore postage or paper cost. Packaging boards are bulked to increase stiffness without increasing material weight.
The practical implication: specifying a carton as "300 GSM SBS" does not fully define the board, because 300 GSM SBS from different manufacturers may have calipers ranging from 0.33mm to 0.42mm. For carton production, the caliper must also be specified, because the die-cutting crease rules are set for a specific caliper, not a specific GSM.
GSM to caliper reference table · paper and board grades
| Material grade | GSM | Typical caliper (mm) | Bulk (cm³/g) | Applications |
|---|---|---|---|---|
| Coated art paper (gloss) | 90 | 0.080–0.090 | 0.88 | Premium brochures, quality catalogues |
| Coated art paper (gloss) | 115 | 0.095–0.110 | 0.90 | Brochures, product literature |
| Coated art paper (gloss) | 130 | 0.110–0.130 | 0.92 | Brochures, annual reports |
| Coated art paper (gloss) | 170 | 0.145–0.170 | 0.92 | Heavy brochures, covers |
| Coated art paper (gloss) | 250 | 0.215–0.250 | 0.93 | Business cards, light cover board |
| Uncoated offset | 80 | 0.090–0.110 | 1.20 | Letterhead, office printing, high bulk for feel |
| Uncoated offset | 100 | 0.110–0.135 | 1.20 | Internal documents, reports, notepads |
| SBS carton board | 250 | 0.30–0.34 | 1.25 | Pharmaceutical cartons, light packaging |
| SBS carton board | 300 | 0.36–0.42 | 1.30 | Standard folding carton, FMCG, pharma |
| SBS carton board | 350 | 0.42–0.50 | 1.30 | Premium folding carton, luxury packaging |
| FBB carton board | 300 | 0.42–0.52 | 1.55 | FMCG packaging, FBB has higher bulk than SBS at same GSM |
| FBB carton board | 350 | 0.50–0.62 | 1.60 | Premium FMCG and beverage packaging |
FBB (Folding Box Board) has a mechanical pulp middle layer with significantly higher bulk than SBS's all-chemical-pulp construction. At the same GSM, FBB typically produces a caliper 20–30% greater than SBS, which means stiffer, more rigid cartons at the same material weight. This is the primary commercial advantage of FBB for FMCG packaging: equivalent or better structural performance at lower cost per square metre. When comparing SBS and FBB carton specifications, always compare caliper rather than GSM to get a fair structural comparison.