I once took the time to figure out how much ink I used per page. 1.1 mL of ink from the CON-70 in my Pilot Custom Urushi (FM) translated to 11 pages of writing on A4 paper (~ 350 words per page). Because A5 pages are half the size of A4, I now knew both how many pages of either size notebook it would take me to finish a bottle of ink: ~ 500 A4 or 1000 A5 pages to one bottle of Iroshizuku Shin-Kai.
That's going to take me while, but, due to the paper sizing properties, I have an easy translation of the information.
Eh, the half-size relationship is hardly an innovation of the A-series of paper: the US uses letter, tabloid/ledger (double-letter-size), and half and quarter letter size. US printed matter and blank books mostly end up in that same set of sizes, because they're all made by cutting and folding the starting stock in half, modulo some trimming of the edges to square them up after binding.
The innovation of the A-series line was using the golden ratio, so that the half-sized paper has the same aspect ratio; half-sized paper always had half the area.
That is among the plausible definitions of "half-sized paper". LOL. But the A-series innovation makes it easier to imagine scaling content/resources because you don't have to "retypeset" in your brain.
But, mostly, my fascination with A-series paper as an American is rooted in my utter disappointment that we refuse to use the metric system (even though every single American does without knowing it).
