Floppy Disk Media Specifications:

8" Floppy Disks

Both hard and soft sector disks have an "index hole" punched in the disk to indicate where each track on the disk begins. This hole is near the hub (the large center hole of the disk). You will see a small, quarter-inch hole punched in the jacket. If you rotate the cookie (the magnetic media), you will find one hole in the cookie for soft sector disks, and multiple holes for hard sector disks. Most disks are soft sector; Wang is the largest user of hard sector 8" disks.

The disks can be one or two sided; the index hole in the jacket is punched in different locations for one and two sided disks, so the drive can detect the type of disk. Disks are certified for single or double density, but the magnetic coating is the same for both. A disk certified for "Double Density" can be recorded at Single Density, but disks certified for "Single Density" have not been tested for Double Density and should not normally be recorded in Double Density, as they have not been certified for Double Density.

Soft sector disks can be recorded with any sector size, but the common sizes are 128, 256, 512, and 1024 bytes of data per sector. Single density, also known as FM (frequency modulation) recording, is usually recorded at 128 bytes per sector, and double density, also known as MFM (modified FM), is usually recorded at 256 to 1024 bytes per sector, but this is not mandatory.

All 8" disks have 77 cylinders recorded at 48 TPI (tracks per inch), and rotate at 360 RPM.  Each sector on the disk contains a header which, among other things, contains the "address" of the sector. The address is the side, cylinder, and sector number.  This uniquely identifies that sector.  Sector, track, and side numbering schemes vary and are determined by the operating system device driver. Capacities range from about 256 KB for single sided - single density, to 1.2 MB for double sided - double density.

There are several thousand schemes used to write data to these disks. Some of the best known are the IBM "Exchange" formats, which come in single and double sided, single and double density formats with both blocked and unblocked records and spanned or unspanned sectors.
 

5.25" Floppy Disks

Three types of disks are available (six if you count single and double sided separately). They are:

    300 Oe, 40 cylinder, 48 TPI, termed "double density"
    300 Oe, 80 cylinder, 96 TPI, termed "Quad density"
    600 Oe, 80 cylinder, 96 TPI, termed "High density"

The first two types can be recorded as either FM or MFM (single or double density). The high density is recorded in MFM, using a different recording head for the 600 Oe media. Counting sides and density (but not sector size or hard sector disks) there are nine recording combinations.

Rotational speed is 300 RPM for the 300 Oe media and 360 RPM for the high density 600 Oe media. Capacities range from about 70 KB to 1.2 MB.

As with 8" disks, there are thousands of recording schemes in use.  MSDOS 360K and 1.2 MB are the most common 5.25" disk formats.
 

3.5" Floppy Disks

There are both single and double sided and density, as well as high density and ED "extended density", but only the  high density are commonly found today.  Single and double density disks are 600 Oe, high density are 750 Oe, and extended density are 900 Oe. Most disks are recorded with 80 tracks at 135 TPI, but a few early drives had 40 tracks at 67.5 TPI. Most drives are now double sided.

Most drives rotate at 300 RPM, but some, Sony/HP in particular, rotate at 600 RPM. Either speed records the same thing on the disk. Although any sector size can be recorded, 512 byte sectors with MFM recording are almost universal, with the notable exception of the Macintosh 400K and 800K formats which vary the speed of the disk as the track radius changes, and record in a GCR (Group Code Recording) format.  Macintosh 1.4 Meg disks use MFM recording.

Capacity ranges from about 90K (for the 40 track single side, single density) to 2.88 MB for the ED disk, but the MAC and the MSDOS 1.44 MB are the vast majority of disks in use today.

Additional Information

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