Microsoft KB Archive/38110
MS-DOS: Determining the FAT Size for a DISK ID Number: Q38110
3.20 3.21 3.30 3.30a 4.00 MS-DOS
The File Allocation Table (FAT) size for a volume (disk partition) is determined by the number of clusters contained in the volume. If the number of clusters contained in a volume is 4086 or greater, MS-DOS uses a 16-bit FAT. Otherwise, a 12-bit FAT is used.
Other methods will work with versions of MS-DOS prior to 3.3x; however, the method described above should be used for compatibility with present and future versions of MS-DOS.