Article ID: 140365 - View products that this article applies to.
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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.
Collapse this tableExpand this table
Volume size Windows NT 3.51Windows NT 4.0Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP, Windows 2000
7 MB–512 MB 512 bytes4 KB4 KB
512 MB–1 GB 1 KB4 KB4 KB
1 GB–2 GB 2 KB4 KB4 KB
2 GB–2 TB 4 KB4 KB4 KB
2 TB–16 TB Not Supported*Not Supported*4 KB
16TB–32 TB Not Supported*Not Supported*8 KB
32TB–64 TBNot Supported*Not Supported*16 KB
64TB–128 TBNot Supported*Not Supported*32 KB
128TB–256 TBNot Supported*Not Supported*64 KB
> 256 TBNot SupportedNot SupportedNot 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.
Collapse this tableExpand this table
Volume size Windows NT 3.51Windows NT 4.0Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP, Windows 2000 
7 MB–8 MB Not supported Not supportedNot supported
8 MB–32 MB 512 bytes512 bytes512 bytes
32 MB–64 MB 1 KB 1 KB 1 KB
64 MB–128 MB 2 KB2 KB2 KB
128 MB–256 MB4 KB4 KB4 KB
256 MB–512 MB8 KB8 KB8 KB
512 MB–1 GB 16 KB 16 KB 16 KB
1 GB–2 GB 32 KB32 KB32 KB
2 GB–4 GB 64 KB64 KB64 KB
4 GB–8 GB Not supported 128 KB*Not supported
8 GB–16 GB Not supported 256 KB*Not supported
> 16 GBNot supported Not supportedNot 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.
Collapse this tableExpand this table
Volume sizeWindows NT 3.51Windows NT 4.0Windows 7, Windows Server 2008 R2, Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP, Windows 2000 
7 MB–16MB Not supported Not supportedNot supported
16 MB–32 MB 512 bytes512 bytesNot supported
32 MB–64 MB 512 bytes512 bytes512 bytes
64 MB–128 MB 1 KB1 KB1 KB
128 MB–256 MB2 KB2 KB2 KB
256 MB–8GB4 KB4 KB4 KB
8GB–16GB 8 KB8 KB8 KB
16GB–32GB 16 KB16 KB16 KB
32GB–2TB 32 KBNot supported Not supported
> 2TBNot supported Not supportedNot supported

Default cluster sizes for exFAT

The following table describes the default cluster sizes for exFAT.
Collapse this tableExpand this table
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

Properties

Article ID: 140365 - Last Review: July 12, 2013 - Revision: 9.1
Applies to
  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows Server 2003, Datacenter Edition (32-bit x86)
  • Microsoft Windows Server 2003, Enterprise Edition (32-bit x86)
  • Microsoft Windows Server 2003, Standard Edition (32-bit x86)
  • Microsoft Windows Server 2003, Web Edition
  • Windows Server 2008 Datacenter without Hyper-V
  • Windows Server 2008 Enterprise without Hyper-V
  • Windows Server 2008 for Itanium-Based Systems
  • Windows Server 2008 Standard without Hyper-V
  • Windows Server 2008 Datacenter
  • Windows Server 2008 Enterprise
  • Windows Server 2008 Standard
  • Windows Vista Enterprise 64-bit Edition
  • Windows Vista Home Basic 64-bit Edition
  • Windows Vista Home Premium 64-bit Edition
  • Windows Vista Ultimate 64-bit Edition
  • Windows Vista Business 64-bit Edition
  • Windows Vista Enterprise
  • Windows Vista Home Basic
  • Windows Vista Home Basic Beta
  • Windows Vista Home Premium
  • Windows Vista Starter
  • Windows Vista Ultimate
  • Microsoft Windows XP Home Edition
  • Microsoft Windows XP Professional
  • Microsoft Windows XP Starter Edition
  • Windows Server 2008 R2 Datacenter
  • Windows Server 2008 R2 Datacenter without Hyper-V
  • Windows Server 2008 R2 Enterprise
  • Windows Server 2008 R2 Enterprise without Hyper-V
  • Windows Server 2008 R2 for Itanium-Based Systems
  • Windows Server 2008 R2 Foundation
  • Windows Server 2008 R2 Standard
  • Windows Server 2008 R2 Standard without Hyper-V
  • Windows 7 Enterprise
  • Windows 7 Home Basic
  • Windows 7 Home Premium
  • Windows 7 Professional
  • Windows 7 Starter
  • Windows 7 Ultimate
  • Windows 8
  • Windows 8 Enterprise
  • Windows 8 Pro
  • Windows Server 2012 Datacenter
  • Windows Server 2012 Essentials
  • Windows Server 2012 Foundation
  • Windows Server 2012 Standard
Keywords: 
kbinfo kbother kbsmbportal KB140365

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