FAT File Systems. FAT32, FAT16, FAT12

The File Allocation Table (FAT) file system is a simple file system originally designed for small disks and simple folder structures.

The FAT file system is named for its method of organization, the file allocation table, which resides at the beginning of the volume. To protect the volume, two copies of the table are kept, in case one becomes damaged.

In addition, the file allocation tables and the root folder must be stored in a fixed location so that the files needed to start the system can be correctly located.

A volume formatted with the FAT file system is allocated in clusters. The default cluster size is determined by the size of the volume. For the FAT file system, the cluster number must fit in 16 bits and must be a power of two.

Structure of a FAT Volume

The figure below illustrates how the FAT file system organizes a volume.

fat32, fat12, fat16

Differences between the FAT systems

System Bytes Per Cluster Within File Allocation Table Cluster limit
FAT12 1.5 Fewer than 4087 clusters.
FAT16 2 Between 4087 and 65526 clusters, inclusive.
FAT32 4 Between 65526 and 268,435,456 clusters, inclusive.