The 1MB╩and 2MB ratings are unformatted capacities. Regardless of what computer you initialize these disks with, you do not get a data capacity of the full 1MB or 2MB. This is due to the way information is stored on floppy disks. Depending on what operating system is used, more information than 800K or 1.44MB can be placed on a floppy disk. This is because they use additional sectors per disk.
Macintosh Disk Capacities
=========================
| Double Density High Density
| 1 MB Unformatted 2 MB╩Unformatted
| 800K Formatted 1.44MB Formatted
| ---------------- -----------------
| Bytes/Sector 512 512
| Sectors/Tracks 8-12 (variable) 18
| Track Density tpi 135 135
| Tracks/Side 80 80
| Sectors/Disk 1600 2880
| Bytes/Side 409,600 737,280
| Bytes/Disk 819,200 1,474,560
Typical Disk Sector
===================
A╩typical disk sector consists of 648 bytes of information. Here is how this information is encoded on the disk:
| <------------ Typical Sector 648 bytes ------------------> |
+------------------------------------------------------------+
| 1 | 2 | 3 | 4 |╩ 5 |╩ 6 | 7 |╩ 8 | 9 | 10 |
+------------------------------------------------------------+
1 - Sector Gap 12 bytes
2 - ID Mark 4 bytes
3 - Track and Sector Address 4 bytes
4 - CRC 2 bytes
5 - ID Gap 22 bytes
6 - Date Block Gap 12 bytes
7 - Data Mark 4 bytes
8 - Data Field ╩ 512 bytes
9 - CRC 2 bytes
10 - Data Block Gap 84 bytes
How Disk Space is Calculated
============================
When formatted disk space is calculated, most of the sector information is not included. The only information used is the data field. Here is the formula used to determine the amount of information on a floppy disk.
Sides x Tracks x Sectors x Bytes/Sector = amount of information a disk can store
For a High Density disk this would be:
2 x 80 x 18 x 512 = 1,474,560 bytes
The Macintosh double density disk gets a bit more difficult because it uses 5 different speeds. When the speed changes, so does the sectors in per track. Here is a table with the track information and our formula:
Tracks Sectors
------- -------
00 - 15 12
16 - 31 11
32 - 47 10
48 - 63 09
64 - 79 08
Formula
-------
Sides x Tracks x Sectors x Bytes/Sector = Information
Double Density Disk
-------------------
2 x 16 x 12 x 512 = 196,608
2 x 16 x 11 x 512 = 180,224
2 x 16 x 10 x 512 = 163,840
2 x 16 x 09 x 512 = 147,456
2 x 16 x 08 x 512 = 131,072
. -------
.Total disk space = 819,200 bytes