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FAT Type and Cluster Size Depends on Logical Drive Size

This article was previously published under Q67321
This article has been archived. It is offered "as is" and will no longer be updated.
MS-DOS allocates disk space for files in units of one or more sectors;these units are called "clusters" or "allocation units." On any MS-DOSdisk, a 1-byte file is allocated 1 cluster of disk space, wasting theunused area of the cluster. A file that is 3.2 clusters large is given4 clusters. Overall, a smaller cluster size means less waste.

The cluster size for a drive is decided by FORMAT, depending on thesize of the logical drive (see table, below). "Logical drive" refersto an MS-DOS volume accessed by a drive letter (A:, B:, C:, D:, and soforth). Hard disk users may want to consider cluster size whenchoosing how to partition their drive(s).

The cluster size of a floppy drive cannot be changed. The cluster sizeof a hard drive can be changed only by changing the size of thelogical drive, which is done by repartitioning the hard drive.

CHKDSK displays the allocation unit size for a logical drive. FDISK'soption 4 displays the size(s) of logical drives on the hard drive(s).

NOTE: DoubleSpace-compressed drives appear to have 8K clusters, butinternally vary the sectors-per-cluster as necessary (this informationis recorded in the MDFAT). For example, a 10K file which compressesby a factor of 2:1 actually uses 5K, or 10 sectors, of drive space.
The following is a table of logical drive sizes, FAT (File AllocationTable) types, and cluster sizes:
                  Drive Size      FAT Type     Sectors     Cluster               (logical volume)              Per Cluster     Size               ----------------   --------   -----------   -------(Floppy Disks)      360K           12-bit         2         1K                    720K           12-bit         2         1K                   1.2 MB          12-bit         1       512 bytes                   1.44 MB         12-bit         1       512 bytes                   2.88 MB         12-bit         2         1K(Hard Disks)     0 MB - 15 MB      12-bit         8         4K                16 MB - 127 MB     16-bit         4         2K               128 MB - 255 MB     16-bit         8         4K               256 MB - 511 MB     16-bit        16         8K               512 MB - 1023 MB    16-bit        32        16K              1024 MB - 2047 MB    16-bit        64        32K				
  • Sectors are 512 bytes in size, except on some RAM drives.
  • In the past, some OEMs have modified their versions of MS-DOS to support other sector and/or cluster sizes. The Microsoft MS-DOS 5 Upgrade Setup will, if possible, convert the logical drive to MS-DOS 5.0 compatible. This entails converting the sector size to 512 bytes while retaining the nonstandard cluster size.
  • MS-DOS determines the FAT size based on the number of clusters. If there are 4086 or fewer clusters, a 12-bit FAT is used. If there are 4087 or more clusters, a 16-bit FAT is used.
For more information on this topic, query on the following words:
cluster and disk and FAT
dblspace 3.20 3.21 3.30 3.30a 4.00 4.01 5.00 5.00a partition allocation unit 6.00 6.20 cluster granularity 6.21 6.22 win95 msdos

Article ID: 67321 - Last Review: 12/04/2015 09:05:31 - Revision: 2.1

Microsoft MS-DOS 3.1, Microsoft MS-DOS 3.2 Standard Edition, Microsoft MS-DOS 3.21 Standard Edition, Microsoft MS-DOS 3.3 Standard Edition, Microsoft MS-DOS 3.3a, Microsoft MS-DOS 4.0 Standard Edition, Microsoft MS-DOS 4.01 Standard Edition, Microsoft MS-DOS 5.0 Standard Edition, Microsoft MS-DOS 5.0a, Microsoft MS-DOS 6.0 Standard Edition, Microsoft MS-DOS 6.2 Standard Edition, Microsoft MS-DOS 6.21 Standard Edition, Microsoft MS-DOS 6.22 Standard Edition, Microsoft Windows 95

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