Default Cluster Size for FAT and NTFS

• Microsoft Windows NT Workstation versions 3.5 and 3.51
• Microsoft Windows NT Server versions 3.5 and 3.51
• Microsoft Windows NT Workstation versions 4.0
• Microsoft Windows NT Server versions 4.0

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
   4097MB  to  8192MB(8GB)    8KB                16
   8193MB  to  16384MB(16GB)  16KB               32
   16385MB to  32768MB (32GB) 32KB               64
   >      to  32768MB (32GB)  64KB               128

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

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.