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.