file allocation table (FAT)

Definition of file allocation table (FAT) in The Network Encyclopedia.

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What is File Allocation Table (FAT)?

Specifically, a table maintained on a hard disk by MS-DOS and Microsoft Windows operating systems that acts as a table of contents, showing where directories and files are stored on the disk. By extension, the acronym FAT is also used to refer to the file system itself for MS-DOS and Windows platforms.

In other words, when we refer to the FAT file system, we simply call it the FAT. The FAT is widely supported by all Windows platforms and can be installed on partitions of up to 2 GB in size on Windows 95 and Windows 98, and on partitions of up to 4 GB on Windows NT and Windows 2000.

The FAT is often used in dual-boot scenarios, or when the security and reliability of the NTFS file system is not required.

How it works

The FAT file system is based on the FAT, a structure that maps the locations of the clusters in which files and folders are stored on the disk. The FAT records the location of each cluster that makes up a given file and the sequence in which it is stored. This is necessary because files are usually not stored in a contiguous location on a hard disk because of the presence of disk fragmentation caused by the creation and deletion of files on the disk.

For each file on a FAT volume, the FAT contains the entry point for the allocation unit in which the first segment of the file is stored, followed by a series of links called the allocation chain. The allocation chain indicates where succeeding segments of the file are located and is then terminated by an end-of-file (EOF) marker.

Two copies of the FAT are kept in fixed locations on the disk to provide redundancy. A disk formatted with the FAT file system is said to be a FAT volume. The sizes of the individual clusters in which file information is stored on a FAT volume depend on the size of the partition or logical drive formatted using FAT, as shown in the following table.

For compatibility reasons, these cluster sizes are the same whether the FAT volume is on an MS-DOS or Windows platform. In the table, you’ll see that on small FAT partitions (under 15 MB in size) a special 12-bit FAT file system is used instead of the usual 16-bit FAT.

FAT Information for Different Volume Sizes
Drive Size FAT Type Sectors/Cluster Cluster Size
0 MB–15 MB
4 K
16 MB–127 MB
2 K
128 MB–255 MB
4 K
256 MB–511 MB
8 K
512 MB–1023 MB
16 K
1024 MB–2047 MB
32 K
2048 MB–4095 MB
64 K

Different versions of Windows support different file systems. The original release of Windows 95 supports only FAT, while Windows 95 OSR2 and Windows 98 support FAT and FAT32. FAT32 is a newer 32-bit version of FAT that was first included with the OSR2 release of Windows 95. The original version of FAT is 16-bit and is sometimes referred to as FAT16. Windows NT supports both FAT and NTFS, but not FAT32. Windows 2000 supports FAT, FAT32, and NTFS. Possible advantages of using FAT volumes with Windows NT and Windows 2000 include the following:

  • Multiboot capability:
    If you need to dual boot between Windows 95 or Windows 98 and Windows NT, use FAT instead of NTFS because Windows 95 and Windows 98 cannot read or recognize NTFS volumes.


  • Efficiency on small partitions:
    FAT requires less overhead than NTFS and is more efficient on smaller volumes (those under 400 MB in size).



The root directory on a FAT volume has a fixed size and can contain only a limited number of entries.