Comment by robocat

Nitpick: you mean A4. As an aside to those who don’t get to use ISO A paper sizes, they are trés cool. The x/y edge sizes are the (edit) square root of two. A0 is 1 square metre in area. Each step (A1, A2, A3, ...) just chops the sheet in half.

Edit: https://www.cl.cam.ac.uk/~mgk25/iso-paper.html