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.

Default cluster sizes for NTFS

The following table describes the default cluster sizes for NTFS.

Volume size

Windows NT 3.51

Windows NT 4.0

Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP, Windows 2000

7 MB–512 MB

512 bytes

4 KB

4 KB

512 MB–1 GB

1 KB

4 KB

4 KB

1 GB–2 GB

2 KB

4 KB

4 KB

2 GB–2 TB

4 KB

4 KB

4 KB

2 TB–16 TB

Not Supported*

Not Supported*

4 KB

16TB–32 TB

Not Supported*

Not Supported*

8 KB

32TB–64 TB

Not Supported*

Not Supported*

16 KB

64TB–128 TB

Not Supported*

Not Supported*

32 KB

128TB–256 TB

Not Supported*

Not Supported*

64 KB

> 256 TB

Not Supported

Not Supported

Not Supported


Note The asterisk (*) means that it is not supported because of the limitations of the master boot record (MBR).

Default cluster sizes for FAT16

The following table describes the default cluster sizes for FAT16.

Volume size

Windows NT 3.51

Windows NT 4.0

Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP, Windows 2000

7 MB–8 MB

Not supported

Not supported

Not supported

8 MB–32 MB

512 bytes

512 bytes

512 bytes

32 MB–64 MB

1 KB

1 KB

1 KB

64 MB–128 MB

2 KB

2 KB

2 KB

128 MB–256 MB

4 KB

4 KB

4 KB

256 MB–512 MB

8 KB

8 KB

8 KB

512 MB–1 GB

16 KB

16 KB

16 KB

1 GB–2 GB

32 KB

32 KB

32 KB

2 GB–4 GB

64 KB

64 KB

64 KB

4 GB–8 GB

Not supported

128 KB*

Not supported

8 GB–16 GB

Not supported

256 KB*

Not supported

> 16 GB

Not supported

Not supported

Not supported

Note The asterisk (*) means that it is available only 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.

Volume size

Windows NT 3.51

Windows NT 4.0

Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP, Windows 2000

7 MB–16MB

Not supported

Not supported

Not supported

16 MB–32 MB

512 bytes

512 bytes

Not supported

32 MB–64 MB

512 bytes

512 bytes

512 bytes

64 MB–128 MB

1 KB

1 KB

1 KB

128 MB–256 MB

2 KB

2 KB

2 KB

256 MB–8GB

4 KB

4 KB

4 KB

8GB–16GB

8 KB

8 KB

8 KB

16GB–32GB

16 KB

16 KB

16 KB

32GB–2TB

32 KB

Not supported

Not supported

> 2TB

Not supported

Not supported

Not supported

Default cluster sizes for exFAT

The following table describes the default cluster sizes for exFAT.

Volume size

Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP

7 MB–256 MB

4 KB

256 MB–32 GB

32 KB

32 GB–256 TB

128 KB

> 256 TB

Not supported

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