> 90 mm floppy disks. https://jdebp.uk/FGA/floppy-discs-are-90mm-not-3-and-a-half-...

That page is part right and part wrong.

It is right in claiming that "3.5-inch" floppies are actually 90 mm.

It is wrong in claiming that the earlier "5.25-inch" floppies weren't metric

"5.25-inch" floppies are actually 130 mm as standardised in ECMA-78 [0]

"8-inch" floppies are actually 200 mm as standardised in ECMA-69 [1]

Actually there's a few different ECMA standards for 130 and 200 mm floppies – the physical dimensions are the same, but using different recording mechanisms (FM vs MFM–those of a certain age may remember MFM as "double density", and those even older may remember FM as "single density"), and single-sided versus double-sided.

[0] ECMA-78: Data interchange on 130 mm flexible disk cartridges using MFM recording at 7 958 ftprad on 80 tracks on each side), June 1986: https://ecma-international.org/publications-and-standards/st...

[1] ECMA-69: Data interchange on 200 mm flexible disk cartridges using MFM recording at 13 262 ftprad on both sides, January 1981: https://ecma-international.org/publications-and-standards/st...