How this calculator works
Two conventions describe how heavy paper is. Grammage is the mass of one square metre, a property of the paper itself that does not depend on what size sheet you cut. Basis weight is the mass in pounds of a ream of 500 sheets of the grade basic size, and that size changes with the grade: 17 by 22 inches for bond, 25 by 38 inches for text and 20 by 26 inches for cover. This is why a 20 lb bond sheet and a 20 lb cover sheet feel completely different. Converting between the two needs both the mass and the basic sheet area, which the calculator takes from the grade you choose.
Formula
grammage = (basis weight in grams) / (500 x basic sheet area in square metres). Basis weight = grammage x 500 x basic sheet area / 1000, giving kilograms, which the display converts to pounds. Basic sheet area = width x height, with inches multiplied by 0.0254 to reach metres.
Worked example
A 20 lb bond ream has a basic sheet of 17 x 22 inches, an area of 0.2413 m2. The ream weighs 20 lb, or 9,071.8 g, across 500 sheets, and the 500 sheets cover 120.7 m2 in total, so the grammage is 9,071.8 / 120.7 = 75.2 g/m2, the familiar 75 g office paper.
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Frequently asked questions
Why does 20 lb cover feel heavier than 20 lb bond?
The ream is weighed on different basic sheet sizes. Bond uses a 17 by 22 inch sheet and cover uses 20 by 26 inches, so the cover sheet has about 40% more area for the same weight, and the paper itself is thicker and denser.
What does text weight mean?
Text is the grade used for book interiors, weighed on a 25 by 38 inch basic sheet. Its numbers look much higher than bond for the same paper, which is why publishers usually quote grammage instead.
Does grammage include the coating?
Yes. Grammage is the mass of the finished sheet, coating included. Two papers with the same grammage can have different thicknesses because coatings and calendering pack the fibre differently.
How many sheets are in a ream?
A standard ream is 500 sheets. Some printing grades are packed in 480 sheet or 1,000 sheet reams, which shifts the basis weight, so check the packing before comparing grades.