The Writing Desk Reference Section: Questions about paper
What are the "A" paper sizes?
The "A" system of paper sizes is defined in International Standard ISO 216. The largest size, A0, was set so that its area is 1 square metre and all sizes have an aspect ratio of width to length of 1 to the square root of 2 (1.414). Therefore, if an A0 sheet is folded in half across its width, the resultant size is A1 and so on.
The table below summarises the sizes: