In MS-DOS 5.0 and newer, PC-DOS 5.0 and newer, Windows 9x, Windows NT 3.1-2000, and OS/2 4.x, the FORMAT command (present in some form or another in literally every single version of Q/86-/MS-/PC-DOS, Windows, and OS/2 ever) offers the /F switch for choosing what capacity to format a floppy disk to.1

In DOS 5+ and Windows 9x, the description of the /F switch (brought up, along with much other information, by entering format /? at the command prompt) is as follows:

/F:size Specifies the size of the floppy disk to format (such as 160, 180, 320, 360, 720, 1.2, 1.44, 2.88).

The description of the Windows NT 3.1-4.0 /F switch is (apart from a slight change in wording and the addition of support for 20385-KiB floptical disks) almost identical:

/F:size Specifies the size of the floppy disk to format (160, 180, 320, 360, 720, 1.2, 1.44, 2.88, or 20.8).

Windows 2000’s /F switch has considerably greater functionality:

/F:size Specifies the size of the floppy disk to format (160, 180, 320, 360, 640, 720, 1.2, 1.23, 1.44, 2.88, or 20.8).

The OS/2 /F switch is the simplest of all:

/F:xxxx Specifies the size to which the diskette is to be formatted. For example: 360, 720, 1.2, 1.44, 2.88.

For most of these options, even though the only thing specified is the (approximate) formatted capacity,3 there is no ambiguity as to what size, density, and sideness of floppies they are for:

  • 160: 160-KiB (40 tracks per side, 8 sectors per track) single-sided double-density (SSDD) 5.25” floppy
  • 180: 180-KiB (40 tps, 9 spt) SSDD 5.25” floppy
  • 720: 720-KiB (80 tps, 9 spt) double-sided double-density (DSDD) 3.5” floppy
  • 1.2: 1200-KiB (80 tps, 15 spt) double-sided high-density (DSHD) 5.25” floppy
  • 1.23: 1232-KiB (77 tps, 8 spt) DSDD 8” floppy4
  • 1.44: 1440-KiB (80 tps, 18 spt) DSHD 3.5” floppy
  • 2.88: 2880-KiB (80 tps, 36 spt) double-sided extended-density (DSED) 3.5” floppy
  • 20.8: 20385-KiB (755 tps, 27 spt) double-sided floptical disk

However, for the remaining three, there are multiple floppy formats with a capacity that fits:

  • 320:
  • 320-KiB (40 tps, 8 spt) DSDD 5.25” floppy, or...
  • 320-KiB (80 tps, 8 spt) single-sided quad-density (SSQD) 5.25” floppy, or...
  • 320-KiB (80 tps, 8 spt) SSDD 3.5” floppy
  • 360:
  • 360-KiB (40 tps, 9 spt) DSDD 5.25” floppy, or...
  • 360-KiB (80 tps, 9 spt) SSDD 3.5” floppy
  • 640:
  • 640-KiB (80 tps, 8 spt) double-sided quad-density (DSQD) 5.25” floppy, or...
  • 640-KiB (80 tps, 8 spt) DSDD 3.5” floppy

For each of these three /F options, which of the two (or three, in 320’s case) possible formats is the option intended to produce? Does it change at all, depending on whether the drive holding the floppy disk to be formatted is a 5.25” or a 3.5” drive (distinguishing between DSDD, SSQD, and DSQD 5.25-inchers, on the one hand, and SSDD and DSDD 3.5-inchers, on the other), or maybe on whether the /1 switch (used to tell FORMAT to format only one side of the disk) is used (distinguishing, for 320 and 360,5 between DSDD 5.25-inchers, on the one hand, and SSQD 5.25-inchers and SSDD 3.5-inchers, on the other)?

1: The /F switch is technically still present in every Windows NT version from Windows XP right up through Windows 10, but has been neutered, with only the “1.44” (1440-KiB DSHD 3.5” floppy) option available.2

2: Floppies can still be formatted to other capacities on these systems, assuming that the floppy drive’s firmware supports non-1440-KiB-DSHD-3.5” formats (most internal floppy drives do; most USB floppy drives don’t), but this requires the user to manually enter the number of tracks per side and the number of sectors per track (using the /T and /N switches, respectively), and the support present in earlier NT versions for formatting single-sided disks is, so far as I can tell, completely gone.

3: In contrast, the /F switch for DR-DOS’s FORMAT command explicity states what sizes of disks its various options correspond to; unfortunately, as DR-DOS is a completely independent development (though an essentially-completely-binary-compatible one) from Microsoft and IBM’s offerings, one can in no way assume that the list of floppy formats supported by DR-DOS is exactly the same as that supported by [MS-/PC]-DOS/Windows/OS/2.

4: The capacity of a DSDD 8” floppy is 1232 KiB, rather than the 616 KiB one would expect from the per-disk sector count, because DD 8-inchers use 1024-byte sectors rather than the 512-byters standard on the smaller sizes of floppies. (Single-density [SD] 8-inchers, which are not supported by any FORMAT version of which I am aware, use 128-byte sectors.)

5: It wouldn’t help for 640, as both of the two disk formats with 640-KiB capacity are double-sided.

  • @TomasBy: To properly format the disk, the OS has to feed the drive the track/sector/side arrangement for the correct disk geometry - which can differ between two disks of the same capacity (for instance, a 360-KiB 40/9 DSDD 5.25" floppy versus a 360-KiB 80/9 SSDD 3.5" floppy). – Sean Apr 1 '20 at 20:51
  • I think 20.8 MB might have also been the experimental triple-density disk standard that never caught on. – fuz Apr 1 '20 at 21:15
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    @fuz, I'm not sure if it was considered 'experimental' or not, but the 20Meg disk was called a "floptical" which had magnetic media but used optical head alignment. A floptical disk had 1250 tracks per inch, instead of the standard 1.44Meg's 135tpi. It could be considered a precursor to the Iomega Zip drive/disk system. – zmerch Apr 10 '20 at 16:24
  • The 8" drives I used with a CP/M-80 system, used 512 bytes sectors. – Thorbjørn Ravn Andersen Dec 26 '20 at 9:44

In fact, you can use any of these formats (up to 1.44MB) on a 3.5" or 5.25" DSHD floppy disk. For the 40-track formats, an 80-track drive would need to double-step; for the single-sided formats, only one head of a double-sided drive would be activated. There are even extended formats which fit, say, 1.6MB in by using a still-higher number of sectors per track.

Filling in the details, 320K and 360K are just double-sided versions of the 40-track, single-sided 160K and 180K formats. Then 640K would be an 8-sector variant of the 9-sector 720K format, probably intended for 5.25" DSDD drives on which 8-sector formats were more commonly used. This also makes it a logical 80-track upgrade from the 320K format.

  • 1
    Particularly for 3.5" drives in the early '80s, single-sided 80 tracks/side drives were not uncommon. (The Mac initially used these, for example.) The normal formatting of these would also be 320K/360K using PC formatting standards, if PCs had ever used them. (I am guessing they did not.) – cjs Apr 2 '20 at 12:48
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    @cjs the ever-mysterious (online info about it is essentially zero) Epson QX-11 (aka “Abacus”) sold in Latin America in the 80s used 3½" floppies with 640KB capacity. Few PC-compatible machines could read those disks and I‘m sure it was due to an unusual usage of tracks (Norton’s DiskEdit could always see all of track 0, but fail beyond). I always assumed it was double-sided but skipping every other track. Now you make me wonder if it was a single-sided drive instead. Do you have links with info on those? – Euro Micelli Apr 2 '20 at 14:00
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    @EuroMicelli I have no particular information on those, but 640 KB would be exactly what you'd expecte for a double-sided 80 track/side 8×512 byte sector format, which was quite standard (though perhaps not in the PC world) for both 5.25" and 3.5" diskettes. (The controller used both identically.) A single-sided double-density disk would hold only half that much, so it definitely wasn't that. If you get hold of any technical docs, I will be happy to look them over. – cjs Apr 2 '20 at 14:18
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    @cjs, Sorry. I meant 360KB. It was presented to DOS as a conventional 40-track 9-sector/track Double-Sided drive. The obvious way to implement that is by using half of the tracks of the disk, but it’s unclear “which” half other than “track 0 is the same” (see DISKEDIT comment). I would do anything for any technical info on the machine, but it’s been impossible to find any. I would avidly study anything I could find. Thanks for the offer anyway. – Euro Micelli Apr 2 '20 at 14:37
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    @EuroMicelli Or another likely way to implement it, especially given the time period (1984 or '85?) is as an actual conventional double-sided 40-track/side 9×512-byte/sector drive, using all of the tracks of each disk. These did exist (the Fujitsu FM77 series used them from 1984 through 1986) though they were rare in the PC-compatible world. Here's a datasheet. – cjs Apr 2 '20 at 15:39

KB entry Q75131 provides the following answer for MS-DOS FORMAT:

  • 320: 320-KiB (40 tps, 8 spt) DSDD 5.25” floppy
  • 360: 360-KiB (40 tps, 9 spt) DSDD 5.25” floppy

I don’t know about the Windows-only 640 format.

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