The A3 is ⅛ an A0. The latter is the reference, because it is the largest A paper standardized by ISO 216. This norm dates from 1975, and comes from an ancient German standard (DIN476 - 1922), that took its own origin in a written source dated 1786 from Lichtenberg. This professor invented the ratio which now allows to use A size paper.
Between all formats, the same proportion is observed: width x √2 = length (or a / b = 2b / a = √2, or 1.4142). This proportion keeps the contents of a sheet when we reduce or enlarge its proportions (when moving from A4 to A3 or vice versa for example).
The width of an A3 corresponds to the length of an A4 while its length corresponds to the width of a A2. A3 measures 420 x 297 mm or 16.54 x 11.69 inches. Its printing surface, including margins, is 394 x 257 millimeters. Its area is 0.125 square meters, or 0.15 square yards,1.35 square foot, 193.75 square inches. A3 is obtained by folding an A2 sheet in the width direction. An A3 folded in the same way is an A4.
We can calculate the weight of an A3 sheet knowing it is ⅛ an A0 sheet (measuring 1 m²). For a paper of 80 g / m², an A3 sheet weighs 80/8 = 10 grams.
or 11 11/16 x 16 9/16 inches
or 297 x 420 millimeters
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