Default cluster size for NTFS, FAT, and exFAT

Summary

All file systems that are used by Windows organize your hard disk based on cluster size (also known as allocation unit size). Cluster size represents the smallest amount of disk space that can be used to hold a file. When file sizes do not come out to an even multiple of the cluster size, additional space must be used to hold the file (up to the next multiple of the cluster size). On the typical hard disk partition, the average amount of space that is lost in this manner can be calculated by using the equation (cluster size)/2 * (number of files).

If no cluster size is specified when you format a partition, defaults are selected based on the size of the partition. These defaults are selected to reduce the space that is lost and to reduce the fragmentation that occurs on the partition. 

More Information

A hard disk partition (also known as a volume) can be formatted to NTFS, FAT, or exFAT. The default values are used by Windows when one of the following methods is used to format the partition:

• 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.

By default, the maximum cluster size for NTFS under Windows NT 4.0 and later versions of Windows is 4 kilobytes (KB). This is because NTFS file compression is not possible on drives that have a larger cluster size. The format command won't use clusters larger than 4 KB unless the user specifically overrides the default settings. You can do this by using the /A: switch together with the Format command 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. Therefore, any larger cluster size would not allow for 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.