The minimum storage space on an MS-DOS disk is one allocation unit. Each
allocation unit contains one or more sectors, depending on the disk size
and capacity. Sectors can also vary in size but are usually 512 bytes.
Since files seldom fill a single allocation unit entirely, a number of
bytes are left unused. These bytes are unavailable to MS-DOS for any other
file storage. For example, if the allocation unit size is 1024 bytes, a
file of 1000 bytes occupies one allocation unit, leaving 24 bytes unused
and unavailable to MS-DOS for storage of other files.
Calculating Disk Space Requirements
PC utility programs like Norton Utilities and PCTools determine the number
of bytes in a particular directory as well as the number of bytes occupied
on the current disk. If you do not have one of these programs, you can
estimate disk space requirements.
To estimate the number of bytes occupied by the files in a particular
directory, you must determine the allocation unit size for the disk. CHKDSK
return the allocation units for a MS-DOS disk.
The following equation is based on the law of averages that dictates that
one-half of an allocation unit will be unused for each file stored on a
disk. The more files you have in a particular directory, the closer the
estimate will be. If you have just a few small files, this formula is not
disk space occupied = (number of files in directory *
allocation unit size in bytes * .5) +
file size total reported by DIR
Use the following method to estimate the disk space required to copy the
files in a directory to a disk. Use CHKDSK to determine the allocation unit
size of the destination disk. Use the same equation as above, substituting
the allocation unit size of the disk for the allocation unit size in bytes.
For more information on allocation unit sizes, query on the following words
here in the Microsoft Knowledge Base:
allocation and unit and fat and disk