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[ The PC Guide | Systems and Components Reference Guide | Floppy Disk Drives | Floppy Disk Formats and Logical Structures ]

Formatted and Unformatted Capacity

Floppy disks often have two capacity specifications; they are often quoted with both their unformatted capacity, and their formatted capacity. Since the disk is useless unless it is formatted (well, other than as a coaster for your coffee table) the unformatted capacity means basically nothing. The formatted capacity is the true maximum capacity of the disk. Usually, the formatted capacity is about three-quarters the unformatted capacity.

Even the formatted capacity, however, doesn't show the true amount of space available for user files, because a certain amount of overhead is taken up for FAT file structures. This is true of hard disks as well, of course, although as a percentage more of the floppy is taken up by this information than a hard disk is. The amount of space remaining after these structures are placed on the disk is the true usable capacity of the floppy.

Note also that the "decimal vs. binary" measurements problem is in play again with the terminology used to specify floppy capacity. In fact, the terms are not even consistent in and of themselves. For example, a 1.44 MB floppy disk takes its name from the fact that the disk has 2,880 sectors, and each sector is 0.5 KB; 0.5 times 2,880 is 1,440, so the 1.44 is a decimal measure. But, each sector is really 512 bytes, so the 0.5 KB is a binary measure. As a result the "1.44" is a mixed measurement; the true raw formatted capacity is either 1.41 MB (binary) or 1.47 MB (decimal), and not 1.44 MB at all!

Here are the capacity figures for the various disk types:

Capacity Parameter

360 KB 5.25"

1.2 MB 5.25"

720 KB 3.5"

1.44 MB 3.5"

2.88 MB 3.5"

Unformatted Capacity

~480 KB

~ 1.6 MB

~1 MB

~2 MB

~4 MB

Formatted Capacity (binary kilobytes)

360

1,200

720

1,440

2,880

Formatted Capacity (bytes)

368,640

1,228,800

737,280

1,474,560

2,949,120

File System Overhead (bytes)

6,144

14,848

7,168

16,896

17,408

Total Usable Capacity (bytes)

362,496

1,213,952

730,112

1,457,664

2,931,712

Total Usable Capacity (binary KB)

354

1,185.5

713

1,423.5

2,863

Total Usable Capacity (binary MB)

0.346

1.158

0.696

1.390

2.796

Next: Summary of Floppy Disk Types and Specifications


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