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Basis weight (commonly expressed in grams per square meter, g/m²) denotes the mass of a one-square-meter sheet of paper under standard conditions. This metric is fundamental to paper specification, procurement, and print production planning.
Core Terminology: Sheet dimensions, page count, printing sheet (or "sheet" as a unit), and format ratio.
Paper supplied by mills is typically available in two primary forms:
1. Roll stock: Continuous paper wound onto large-diameter cores, often weighing several tons per roll. Roll stock is optimized for high-speed rotary printing processes (e.g., newspaper, commercial web offset, and tissue production). Its width is standardized across multiple industry specifications (e.g., 787 mm, 889 mm, or 1092 mm), while its length-though technically finite-is sufficiently extended to support uninterrupted press operation.
2. Cut-sheet stock: Individual flat sheets used primarily in sheetfed offset, digital, and small-format printing. Standard cut-sheet sizes include:
• Conventional size: 787 mm × 1092 mm (historically designated as "standard" following widespread adoption in book and publication printing);
• Large size: 889 mm × 1194 mm (the most widely accepted large-format benchmark; other variants such as 880 mm × 1230 mm or 890 mm × 1240 mm also fall within this category, provided they exceed the conventional dimensions).
"Format" refers to the number of printed pages obtainable from a single full-size sheet after imposition and cutting. A full-size sheet-also termed an "original sheet" or "parent sheet"-serves as the base substrate. When divided into *n* equal recto-verso page units, it yields an *n*-page format (e.g., 2-page, 4-page, 8-page, 16-page, etc.). Common format classifications include portrait (cut parallel to the shorter edge), landscape (cut parallel to the longer edge), and irregular (non-orthogonal or custom trimming).
The "printing sheet" is a critical unit for material estimation and cost calculation in offset printing. It represents one double-sided impression on a single physical sheet-i.e., one sheet = one recto + one verso. Consequently:
• One full-size sheet = two printing sheets (since it accommodates two independent double-sided impressions when laid out);
• An *n*-page format corresponds to 2/*n* printing sheets per full-size sheet. For example:
– 4-page format → 0.5 printing sheet per full-size sheet;
– 8-page format → 0.25 printing sheet;
– 16-page format → 0.125 printing sheet;
– 32-page format → 0.0625 printing sheet;
– 64-page format → 0.03125 printing sheet.
Thus, for a 100-page publication printed in 64-page format, the required printing sheet count per copy is:
100 pages ÷ 64 pages/format = 1.5625 formats × 2 pages/format ÷ 2 (to normalize to double-sided units) = 1.5625 × 0.03125 × 100? Correction: More rigorously:
Total printing sheets = (total pages) × (1 printing sheet / 2 pages) ÷ (pages per format) = 100 × 0.5 ÷ 64 = 0.78125 printing sheets per copy.
However, given industry convention treats "format" as the number of *pages* per full sheet, and each full sheet yields *n*/2 double-sided impressions, the correct relationship is:
Printing sheets per copy = (total pages) / (pages per format) × (1/2) = 100 / 64 × 0.5 = 0.78125.
For 5,000 copies: 5,000 × 0.78125 = 3,906.25 printing sheets. Accounting for a standard 2% waste factor: 3,906.25 × 1.02 = 3,984.375 printing sheets. To convert to full-size sheets: 3,984.375 ÷ 2 = 1,992.1875 full-size sheets.
The unit "ai" (also spelled "ream" in some regional contexts, though distinct from the ISO ream of 500 sheets) is a traditional East Asian paper quantity unit defined as *500 full-size sheets*. Crucially, "full-size" must correspond to the parent sheet dimension specified for the given paper grade and application-not merely any 500 sheets. For instance, self-adhesive label stock supplied at 765 mm × 535 mm does not constitute a full-size sheet in the context of standard offset printing; thus, 500 such sheets equate to only 0.5 ai. Applying this to the above example: total full-size sheets required = 1,992.1875; therefore, ai required = 1,992.1875 ÷ 500 = 3.984 ai (rounded to 4.0 ai with allowance for waste and handling).
"Grammage" (basis weight) quantifies paper mass per unit area (g/m²) and directly correlates with thickness, stiffness, opacity, and printability. Customers routinely specify grammage alongside paper type-for example: 60 g/m² uncoated woodfree paper for writing; 80 g/m² double-coated art paper for brochures; 200 g/m² coated paper for premium flyers; or 250 g/m² grey-back white board for packaging. Within each paper category, grammage variations significantly affect mechanical behavior, ink absorption, and finishing suitability-making precise grammage selection essential for functional and aesthetic performance.
