For Americans et al. who don't know:

- A3 is (within physical tolerances, anyway) twice the size of A4

- A5 is half the size of A4

- A0 is the biggest size and and has an area of 1 square meter, although it is not a square but instead it has an aspect ratio of 1:sqrt(2) (technically all A-series papers have this aspect ratio because math says it is the aspect ratio that retains itself when halved)

- In practical sense, A5 is the smallest pre-packaged A-series paper that I have personally encountered, although I guess someone would chime that they handled A6 (half of A5) or smaller.

Never saw A6 printer paper, but it’s quite common for note cards / index cards.

> technically all A-series papers have this aspect ratio because math says it is the aspect ratio that retains itself when halved

I never knew this, but that's awesome. Thanks for the info!

Sure, I meant the imperial sizes in inches, never heard about that before. I assumed A4 was the global standard.