Paper Weight Converter

Grammage is mass per unit area, expressed in grams per square meter and defined by ISO 536, which requires the sample to be conditioned at 23 degrees C and 50 percent relative humidity before weighing. Because it normalizes for sheet size, one number describes any format: sheet weight is simply grammage times area. An A4 sheet of 0.210 by 0.297 meters has an area of 0.06237 square meters, so at 80 g/m2 it weighs 4.99 g and a 500-sheet ream weighs about 2.49 kg, or 5.5 lb. North American basis weight works differently and is the source of most confusion. It is the weight in pounds of 500 sheets cut to a grade-specific basis size: 17 by 22 inches for bond, 25 by 38 for text and book, 20 by 26 for cover, and 25.5 by 30.5 for index. Because the basis areas differ, 20 lb bond and 20 lb cover are not the same paper at all, roughly 75 g/m2 against roughly 54 g/m2, and the conversion multipliers used here (0.266 for bond, 0.6757 for text, 0.3702 for cover) simply encode those areas. The important limitation is that grammage says nothing about thickness, stiffness or opacity: a coated sheet is denser and noticeably thinner than an uncoated sheet of identical weight, so specifying weight alone does not pin down how a job will feel, fold or feed.

The ream is the older unit. European papermakers inherited it from the Arabic rizmah, meaning a bundle, and for centuries a ream was 480 sheets, twenty quires of twenty-four; the 500-sheet ream became standard for machine-made printing paper during the nineteenth century. American mills of that era sold by the weight of a ream, and because each grade was customarily cut to its own sheet size, bond, book, cover and index each ended up with a different basis size. The system was internally consistent for a mill selling one grade and unusable for anyone comparing across grades. The metric alternative arrived with the rationalization of paper sizes. Walter Porstmann's DIN 476, published in Germany in 1922, defined the A series on a constant aspect ratio with A0 fixed at exactly one square meter, which made mass per square meter the natural way to state weight, since the grammage of a stock and the weight of one A0 sheet became the same number. The A series was adopted internationally as ISO 216 in 1975 and grammage was codified in ISO 536, so most of the world now quotes a single figure. The United States has retained basis weight in trade practice, which is why paper merchants still publish dual-scale conversion charts after a century of coexistence.