This article was previously published under Q67321
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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:
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.
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