There is no such thing as a 3.5 inch floppy disk

The term 3.5 inch floppy disc is in fact a misnomer. Whilst the specification for 5.25 inch floppy discs employs Imperial units, the later specification for the smaller floppy discs employs metric units.

The standards for these discs are […] all of which specify the measurements in metric, and only metric. These standards explicitly give the dimensions as 90.0mm by 94.0mm. It’s in clause 6 of all three.

↫ Jonathan de Boyne Pollard

Even the applicable standard in the US, ANSI X3.171-1989, specifies the size in metric. We could’ve been referring to these things using proper measurements instead of archaic ones based on the size of a monk’s left testicle at dawn at room temperature in 1375 or whatever nonsense imperial or customary used to be based on. I feel dirty for thinking I had to use “inches” for this.

 

The term 3.5 inch floppy disc is in fact a misnomer. Whilst the specification for 5.25 inch floppy discs employs Imperial units, the later specification for the smaller floppy discs employs metric units.

The standards for these discs are:

• ECMA-100 (Data Interchange on 90 mm Flexible Disk Cartridges Using MFM Recording at 7 958 ftprad on 80 Tracks on Each Side — ISO Type 301) a.k.a. ISO/IEC 8860-1:1987 (double density)

• ECMA-125 (Data Interchange on 90 mm Flexible Disk Cartridges using MFM Recording at 15 916 ftprad on 80 Tracks on Each Side — ISO Type 302) a.k.a. ISO/IEC 9529-1:1989 (high density)

• ECMA-147 (Data Interchange on 90 mm Flexible Disk Cartridges using MFM Recording at 31 831 ftprad on 80 Tracks on Each Side — ISO Type 303) a.k.a. ISO 10994-1:1992 (extra-high density)

… all of which specify the measurements in metric, and only metric. These standards explicitly give the dimensions as 90.0mm by 94.0mm. It's in clause 6 of all three.

In most countries, the national standard is simply derived from the ISO standard. In the U.S., however, the applicable standard is instead ANSI X3.171-1989. That, too, specifies the measurements in metric, though. It specifies the cartridge dimensions as 90.0mm by 94.0mm by 3.3mm and the diameter of the magnetic disc material itself as 85.80mm.

If you have a floppy disc cartridge that does not measure 90mm, then you have one manufactured by someone that is not careful about manufacturing tolerances. (And you should beware. If their product isn't up to specification in one area, it may not be so in others.) If the disc measures 3.5 inches, then it is the wrong size. 90.0mm is 3.5433 inches.

---

© Copyright 2003,2020 Jonathan de Boyne Pollard. "Moral" rights asserted.
Permission is hereby granted to copy and to distribute this web page in its original, unmodified form as long as its last modification datestamp is preserved.