As a European, I have no clue what's the dimensions of A4, A5, etc. We just say A4. I don't think that laymen, like me, know anything about their real size, and I never had a problem not knowing it.
US sizes don't work like that; the closest thing to A3 here doesn't have the same proportions as the closest thing to A4. Which absolutely sucks if you are designing a poster and need to make it bigger or smaller.
With international A sizes you can reuse the same design for any size. That's why I like them.
Agree, I had to look it up as well. I can memorize A4 and A3 easily, but A5 is already counter-intuitive. It's an aspect ratio that's kept, so that's why the numbers don't add up easily.
With the paper in front of me it's easier, fold, double, you can navigate across all levels of A(n) quickly. All it takes is seeing this single graphic for a split second and you know all the DIN A-sizes, but the US sizes not. I enjoy the US Letter format though as a size, it feels somehow better than A5 as it's more square.
https://en.wikipedia.org/wiki/Paper_size#/media/File:A_size_...
Write the sizes in binary and bit-shift them?