Default cluster size for NTFS, FAT, and exFAT

All file systems used by Windows organize your hard disk based on cluster (or allocation unit) size, which represents the smallest amount of disk space which can be allocated to hold a file. So when file sizes do not come out to an even multiple of the cluster size, extra space must be used to hold the file (up to the next multiple of the cluster size). On the typical partition, this means that on average (cluster size)/2 * (number of files) worth of space is lost in this way.

If no cluster size is specified during format, defaults are selected based on the size of the partition. These defaults have been selected to reduce the amount of space lost and to reduce the amount of fragmentation on the partition.

The following default values are used by Windows when a volume is formatted to NTFS, FAT, or exFAT by using one of the following methods:

• Using the FORMAT command from the command line without specifying a cluster size.
• Formatting a volume from Windows Explorer when the Allocation Unit box in the Format dialog box lists Default Allocation Size.

The minimum default cluster size for NTFS under Windows NT 4.0 and later is 4 kilobytes (KB) because NTFS file compression is not possible on drives with a larger cluster size. So format will never use larger than 4 KB clusters unless the user specifically overrides the defaults by using the /A: switch (for command line format) or by specifying a larger cluster size in the Format dialog box in Windows Explorer.

When you use the Convert.exe utility to convert a FAT partition to NTFS, Windows always uses the original FAT cluster size as the NTFS cluster size for cluster sizes up to 4 KB. If the FAT cluster size is greater than 4 KB, then the clusters are converted down to 4 KB in NTFS. This is because the FAT structures are aligned on cluster boundaries, so any larger cluster size would not allow the conversion to function. Note also when formatting a partition under Windows NT 3.5, 3.51, and 4.0 Setup, the partition is first formatted to FAT and then converted to NTFS, so the cluster size will also always be as described earlier when a partition is formatted in Setup.

Default cluster sizes for NTFS

The following table describes the default cluster sizes for NTFS.
Note * means not supported due to the limitations of the MBR.

Default cluster sizes for FAT16

The following table describes the default cluster sizes for FAT16.Note * means only available on media with a sector size greater than 512 bytes.

Default cluster sizes for FAT32

The following table describes the default cluster sizes for FAT32.

Default cluster sizes for exFAT

The following table describes the default cluster sizes for exFAT.