FAT Type and Cluster Size Depends on Logical Drive Size (67321)

The information in this article applies to:
    Microsoft MS-DOS operating system 3.1
    Microsoft MS-DOS operating system 3.2
    Microsoft MS-DOS operating system 3.21
    Microsoft MS-DOS operating system 3.3
    Microsoft MS-DOS operating system 3.3a
    Microsoft MS-DOS operating system 4.0
    Microsoft MS-DOS operating system 4.01
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22
    Microsoft Windows 95

This article was previously published under Q67321


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-DOS disk, a 1-byte file is allocated 1 cluster of disk space, wasting the unused area of the cluster. A file that is 3.2 clusters large is given 4 clusters. Overall, a smaller cluster size means less waste.

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

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

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

NOTE: DoubleSpace-compressed drives appear to have 8K clusters, but internally vary the sectors-per-cluster as necessary (this information is recorded in the MDFAT). For example, a 10K file which compresses by 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 Allocation Table) 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

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB67321