Default Cluster Size for FAT and NTFS

ID: Q140365

The information in this article applies to:

SUMMARY

All file systems used by Windows NT organize your hard disk based upon 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 (cluster size)/2 * (number of files) worth of space is lost this way.

If no cluster size is specified during format, NTFS picks defaults based upon 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.


MORE INFORMATION

Windows NT uses the following default cluster sizes for NTFS (the value for number of sectors assumes a standard, 512 byte sector. On systems with sectors that are not 512 bytes, the number of sectors per cluster may change, but the cluster size is fixed): 


Drive Size              Cluster Size       Number of Sectors
---------------------   ----------------   -----------------
512MB  or less             512 bytes          1
513MB  to 1024MB(1GB)      1024 bytes (1KB)   2
1025MB to 2048MB(2GB)      2048 bytes (2KB)   4
2049MB to  4096MB(4GB)     4KB                8   NT V3.50 only **
4097MB to  8192MB(8GB)     8KB                16  NT V3.50 only
8193MB to  16384MB(16GB)   16KB               32  NT V3.50 only
16385MB to  32768MB (32GB) 32KB               64  NT V3.50 only
greater than 32768MB (32GB) 64KB              128 NT V3.50 only

** NOTE: NT V3.51 and NT V4.0 support file compression. File
         compression is not supported on cluster sizes above 4k
         so the default cluster size will never exceed 4k if a
         volume is formatted under Windows NT V3.51 or above.


These values are only used if an allocation unit size is not specified at format time, using the /A:<size> switch with the format command.

When you are using the Convert.exe utility to convert to NTFS, Windows NT by default uses a 512-byte cluster size to ensure that there is enough disk space to perform the conversion. The smaller the cluster size, the more efficient the storage of files and the more likely the Convert utility will have enough space to perform the conversion. To change the cluster size, use the "format /a:size" utility where you can specify the cluster size you want.

The FAT file system uses the following cluster sizes. These sizes the same under Microsoft Windows NT, Microsoft MS-DOS, Microsoft Windows 95 and any other operating system that supports FAT: 


   Drive Size          FAT Type   Sectors       Cluster
   (logical volume)               Per Cluster   Size
   -----------------   --------   -----------   -------
   0 MB - 15 MB        12-bit     8             4K
   16 MB - 127 MB      16-bit     4             2K
   128 MB - 255 MB     16-bit     8             4K
   256 MB - 511 MB     16-bit     16            8K
   512 MB - 1023 MB    16-bit     32            16K
   1024 MB - 2048 MB   16-bit     64            32K
   2048 MB - 4096 MB   16-bit     128           64K
   *4096 MB - 8192 MB   16-bit      256      128K  NT V4.0 only
   *8192 MB - 16384 MB  16-bit      512      256K  NT V4.0 only

   *NOTE: In order to support &gt; 4GB FAT partitions using 128k or
   256k clusters, the drives must use &gt; 512 byte sectors.


NOTE: On very small FAT partitions, a 12-bit FAT is used instead of a 16-bit FAT. The FAT files system only supports 512 byte sectors, so both the sectors per cluster and the cluster size is fixed.

Windows NT version 4.0 NTFS Compatibility with Windows NT Version 3.51

NTFS

The Windows NT File System (NTFS) file system supports clusters up to 64K. However, the file record size is always 1K regardless of the cluster size. The File Allocation Table (FAT) file system supports clusters up to 256K.

Due to the cluster size change, formatting a volume with 2K or larger clusters with NTFS can generate a volume that is not usable by Windows NT version 3.51. File record sizes that are smaller than the cluster size is not supported by Windows NT version 3.51. However, you can ensure backwards compatibility by running format from the command line and specifying the /a parameter. This parameter is used to override the default disk allocation unit size.

Default settings are strongly recommended for general use.

NTFS supports 512 bytes, 1K, 2K, 4K, 8K, 16K, 32K, 64K.
FAT supports 8K, 16K, 32K, 64K, 128K, 256K.

NTFS compression is not supported for allocation unit sizes above 4K.

Additional query words: prodnt 


Keywords          : kbother ntfilesys 
Version           : winnt:3.5,3.51,4.0
Platform          : winnt 
Issue type        :

Last Reviewed: May 11, 1999