A0 is twice a A1, but the proportions are maintained between these two formats, as between all A sizes, since the ratio of Lichtenberg, taken by Walter Portsmann and German DIN (*Deutsches Institut für Normung, German Institute for standards*), and finally the international standard ISO 216 in 1975, is still respected: 1.4142 (a / b = 2b / a = √ 2).

The A0 has an area of 1 square meter (or 1.2 square yard, 10,76 square foot, 1550 square inches). It measures 841 x 1189 millimeters or 33.11 x 46.81 inches. Its printable surface is slightly lower, since observe the print margins must be observed: 801 x 1139 mm.

The A0 corresponds to ½ 2A0 or ¼ 4A0 to larger sizes, and 2 A1 pages, 4 A2 pages, 8 A3 pages, 16 A4 pages ... for smaller sizes.

A0 is the reference for calculating the weight of a sheet, because it measures 1 m² and paper weight is always expressed in g/m². Thus, one sheet of paper A0 weighs 80 grams for a paper of 80 g/m². You can also refer to A0 for smaller sizes: for example, 1 A1 sheet, which is two times smaller, weighs 80/2 = 40 grams. An A4 sheet weighs 80/16 = 5 grams in same conditions.

- Square yard: 1.2
- Square foot: 10.76
- Square inch: 1550

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