What is the international paper size standard?

The international paper size standard is defined by the ISO 216 Standard. There are three series of sizes, which are A, B and C. Within each series are eight sizes denoted by a letter number combination such as A1, B3, C8.

The lower the number, the larger the size. Each successive size of the A series is half the size of the next biggest. This is defined by folding parallel to the shorter sides. An A1 sheet is therefore twice the size of an A2 sheet. The C series standard defines the size of envelope appropriate to fit the corresponding A series size unfolded. A1 letters fit into C1; A2 fit into C2; and so on. The B series categorizes sizes not covered by the A series.