> How confident are you in that being the case for floppy disks? You sound
like you might be old enough like me to have been there at the time.
You have me questioning my memory so I actually just flipped through 3
copies of Creative Computing magazine from 1981, 1983, and 1986 as well
as a Computer Shopper magazine from 1986 and while there are lots of
full page ads for BASF, 3M, Fuji, Sony, Verbatim and Elephant and none
of them mention byte capacity at all, just list the density and
sided-ness.

Yes, and if you knew the drives back then, you also knew the raw capacity. DS/DD disks were good for 1 MB raw if used with 80 tracks. Simple to calculate: DD meant 256 KBit/sec data rate. The drives were spinning with 300 rpm, makes 5 rotations per second. So you get 51200 bits onto a single track. Now multiply that with 80 tracks and 2 sides and you get 8192000 bits. Divide by 8 and you get 1024000 Bytes which is close enough to call it 1 MByte.

Of course that is raw, you still need some encoding to be able to read again what you wrote, either MFM or GCR. So usable capacity will be lower. The Amiga managed 880KB by leaving out the gaps between sectors since its controller didn't need them. It always read a whole track in one go and used the blitter to decode everything in RAM. The controller used in the PC (NEC uPD765) needed the sector gaps, so it couldn't read disks written in Amiga format.

I remember having read the 1 MB on the packages of early 3.5" disks. Later that went away.