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When 5.25″ floppies were introduced, in the mid-to-late seventies, they would have had less capacity than the older 8″ because, well, less area on which to store data.

On the other hand, being cheaper and more compact, it didn't take them long to surpass 8″ in terms of market size and therefore the effort spent on optimizing them, and they were in use until a later date. I remember 5.25″ drives up through the late eighties, going up to 1.2 megabytes capacity. I'm almost sure 8″ never reached that figure. So there must've been a point during that decade when they coexisted, when 5.25″ data storage capacity passed 8″.

When did this happen?

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    It really depends on how you wrote the data to the floppies. In those days, everyone had their own format - there was no universal format. When I was working body scanners in the mid 80s, some of the 8" floppies held 1 to 1.5MB in proprietary format (double sided double density). You had the advantage if you could read everyone else's format and they couldn't read yours.
    – cup
    Nov 6 at 12:49
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    Is the premise correct? An alternative is surely that 5.25" discs were not viable until they had capacity at least equal to 8" discs. Nov 6 at 12:54
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    @another-dave lower cost always makes something viable if it’s Good Enough for your task, compared to the alternative.
    – RonJohn
    Nov 6 at 13:43
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    Commodore and Sirius did something that no other manufacturer did - they varied the speed of the drive depending on which track it was on. This allowed the disk to have a different number of sectors on each track, which, in turn gave 1MiB storage. This was possible because they added a processor to their disk drives. None of the 8" floppy drive vendors ever thought of doing that.
    – cup
    Nov 6 at 15:43
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    @cup Just beacuse 8" drive system vendors didn't produce variable speed drives doesn't mean that they didn't think about doing it. As can be seen from my answer below, innovations that could have been applied to all types of floppy drive systems often were not applied (or at least not widely applied) to types with declining market share, presumably because they were felt less likely to pay back the investment. (Also, variable-speed drives were far from unique to CBM and Sirius.)
    – cjs
    Nov 7 at 13:11
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In practice, 5.25" drives equalled or exceeded the capacity of 8" drives when 5.25" floppy drives started using HD media.

The Evolution of 5.25" Drive Systems

There are only two different recording media that were ever commonly used in floppy drives: the original media (often called "DD") and the later HD media. Note that when it comes to floppy diskette drive systems the term "density" is somewhat confusing because it's used not only for media (i.e., the composition of the recording surface) but also radial track density (e.g. 48 or 96 tracks per inch (tpi): a property of the drive, not the media) and the encoding of bits into flux transitions (a property of the controller, not the drive or the media).

Over time, physical capacity was expanded in various ways. The following roughly traces the evolution 5.25" diskette systems. The sizes are for popular formats but could easily vary by 20% or more in either direction, depending on sector size and other factors. (These answers mention a variety of formatting changes used to put more data onto a floppy using the same physical density specifications.)

  1. The original single-sided single-density systems used original media, 40 tracks (48 tpi) and FM encoding for a capacity of around 90 KB/diskette. (Some early drives could address only 35 tracks.)
  2. Single-sided double-density (SSDD) systems used original media, 40 tracks and MFM encoding for a capacity of around 160 KB/diskette.
  3. Double-sided double-density (DSDD or sometimes "2D") systems obviously doubled the above to about 320 KB/diskette.
  4. The move to 80-track (96 tpi) drives, sometimes called "quad-density" or "2DD" doubled that again to 720 KB. But note that the double- to quad-density increase was via a completely different method than the single- to double-density increase, via a change to the drive head rather than a change to how the controller wrote data to the diskette.
  5. HD drives used a different media that approximately doubled the linear recording density, and they started out as 80-track drives, so they stored about 1.2 to 1.44 MB/diskette. For reasons I won't get into the most popular 5.25" format was 1.2 MB (less than double) even though 3.5" systems (which are essentially slightly miniaturised 5.25" systems) exactly doubled it to 1.44 MB.

8" vs. 5.25" Drive Systems

8" drive systems in common use followed the first three steps of this progression, but at the same recording density offered both more tracks (77 instead of 40) and more sectors per track (typically 26 × 256 byte sectors versus 16 × 256 byte sectors for 5.25"). Thus, in one common format a DSDD 8" diskette stored around 2×77×26×256/1024 = 1000 KB whereas a similarly formatted 5.25" diskette stored 2×40×16×256/1024 = 320 KB. The most common format soon changed slightly to increase the sizes to 1.2 MB for an 8" diskette and 360 KB for a 5.25" diskette.

5.25" drives started their progress towards catching up when drives with narrower heads running at 96 tpi were introduced, bringing their standard capacity to 640 or 720 KB. But this was still considerably less than 8" drives. It was when HD media was introduced that the standard 1.2 MB 2HD 5.25" format approximately equaled the capacity of the 8" DSDD format.¹²

Capacity was immediately extended beyond this for special cases such as Windows installation diskettes; a change in formatting allowed 1.5-1.8 MB on an HD diskette. But the same formatting tricks (mainly, larger sectors for less overhead) were possible on 8" drive systems, though I don't know if they were ever used.


¹ In the case of the NEC PC-9801 series, it exactly equaled the 8" format because NEC for whatever reason decided that they would use exactly the 8" format: 77 tracks (even though the drive supported 80), the same number and size of sectors and, strangely enough, even running the drive at the standard 8" rate of 360 RPM rather than the 300 RPM at which 5.25" drives normally run.

² The 3.5" HD diskettes stored 1.44 MB instead of 1.2 MB because they used 18 instead of 15 sectors per track. They were capable of this because, as part of the process of "shrinking" the 5.25" diskette to 3.5" whilst maintaining exact format compatibility they had to increase the coercivity to compensate for the smaller surface area. DD/HD coercivity for 5.25" media was 300/600 Oersteds; for 3.5" media it was 660/720 Oe.)

Further Development

8" drives were perfectly capable of improving in the same way as the 5.25" drives did, moving from 48 to 96 tpi and DD to HD media. Indeed, someone had obviously manufactured such drives because Maxell manufactured media for them. But by the time these improvements were available 8" drives had become relatively expensive and inconvenient compared to even 5.25" drives, not to mention 3.5" drives, and this is probably the reason why 96 tpi HD 8" drive systems were very rare. But if you don't take this rarity into account, 5.25" drives never caught up with 8" drives in storage capacity.

3.5" drive system capacity continued to increase, though HD media was the last system that received truly widespread acceptance. 3.5" ED drives ("Extended Density," with a typical formatted capacity of 2.88 MB) introduced new ED media and perpendicular recording and were fairly widely available, though they never achieved the widespread acceptance of HD drive systems. By this point 5.25" drive system development, like 8" drive systems before it, had fallen off, but even so there was a 5.25" 2.44 MB drive system (probably also using ED media) standardised enough to be available from at least two manufacturers.

Eventually laser-servo designs bumped 3.5" capacity up to 20 MB and beyond. The only format here that achieved any significant popularity were the LS-120 and LS-240 SuperDisk formats (120 and 240 MB, respectively). I don't know how difficult it would have been to transfer laser-servo technology to 5.25" or 8" systems.

Further Information

A good source of many fascinating details about many kinds of floppy drive systems is Herb Johnson's Tech information on floppy disks drives and media web pages, which provided many of the references above. (But I may be biased since I am, in a small way, a contributor to those pages.)

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    Well, it would be interesting to hear the reason why 5,25" drives didn't manage to achieve the linear recording density required to record 1.44MB on HD media, whereas 3,5" drives, which use shorter tracks and thus lower linear velocities did. Do you have a pointer to the reasons you "won't get into" in this answer? Nov 7 at 19:39
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    @MichaelKarcher 3.5" DD diskettes used higher coercivity media than 5.25" diskettes. I've reworked the answer to explain this.
    – cjs
    Nov 8 at 6:02
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    Good clarification! Comparing 8" and 5,25" media, I guess DD coating is the same in both, so the main point of your answer still applies. I misread your introduction sentence as if all DD media (including 3,5") are made of the same material, and all HD media are made of the same material, whereas in fact the 3,5" DD coating seems more closely related to 5,25" HD coating than to 5,25" DD coating, at least if you just look at the coercitivity. Nov 8 at 10:15
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    @Patrick Schlüter: (1.4+1.47)/2 ≈ 1.44 ;)
    – D. Kovács
    Nov 8 at 11:31
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    @PatrickSchlüter: The high-density 3.5" drives hold twice as much as a 720KB drive, and writing the figure as 1,440KB drives was a bit awkward (in the parlance of the era when such drives were developed, lowercase k was a prefix for 1000 and uppercase K was 1024, a distinction which worked fine until larger units came about). Given that drive storage is allocated in power-of-two chunk sizes, it made sense to describe file sizes and remaining storage space as multiples of 1024 bytes, what would most often be useful would be units representing a power-of-1000 multiples of 1024.
    – supercat
    Nov 8 at 20:19
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In 1977, the available 8" formats were SSSD at ~250 kB, DSSD at 500 and DSDD at 1 to 1.2 MB.

The first 1976 5.25" was 98 but quickly moved to 113 in a SSDD. The Apple II used these same mechanisms but increased storage to 140. Tandon introduced a DSDD format in 1978 at 360 kB. This ultimately topped out at 720 in later models.

So 8" was larger throughout its history, but market forces rendered that moot.

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    What about 1.2MB on 5.25”? 8” drives were still in use when that became available... Nov 6 at 15:22
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Essentially, standard 5¼" floppies never exceeded the capacity of 8" floppies. That is because they used the same controllers. 5¼" floppies ran at 300rpm and a data rate of 250kbps (at DD) whereas 8" floppies ran at 360rpm and a data rate of 500kbps. When 5¼" floppies introduced HD disks, they also ran at 360rpm and a data rate of 500kbps, pulling even with 8" disks at 1.2MB formatted capacity (standard formatting). It was 3½" floppies that eventually overtook 8" floppies by retaining the original 300rpm speed even for HD floppies. That resulted in a 20% higher capacity.

But at least the "main stream" formatted 5¼" floppies were at best matching the capacity of 8" floppies. While you could get out more with formatting shenanigans, that was quite the same with 8" drives.

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