A few weeks ago my sister told me of a proposal for some contract that she’d been working on, and it ended up being 1600 pages, mostly double-sided A4, and printed in quadruplicate, and I rapidly worked out in my head, purely from the memory that A0 is 1m², that each subsequent paper size halves the area, and that 80g/m² is a common weight: that that was going to be something like 16kg of dead tree that was probably just going to clog up filing cabinets somewhere and mostly never actually be referred to. If I did this more often I’d remember 5g per sheet, but the mere fact that all this stuff is simple ratios and unit calculations means you can calculate it all from first principles quite easily. It’s really handy.