FatUsed

FatUsed is a term used in some FAT filesystem tools and libraries to quantify the portion of the File Allocation Table (FAT) that is currently in use. The FAT is a compact index that maps clusters to their next cluster in a file’s data chain. In FAT variants such as FAT12, FAT16, FAT32, and exFAT, a cluster’s FAT entry can indicate free, end-of-chain, bad, or a pointer to another cluster. Since only free entries indicate unused space, fatUsed can be defined as the number of non-free entries, or as the portion of the FAT occupied by valid chain pointers, depending on the implementation. Some tools report fatUsed as a count of used clusters, while others report it as a percentage of total clusters.

Measurement and scope: The value can reference either the primary FAT or all copies present on disk. On FAT-based formats, the FAT tables occupy a fixed portion of the filesystem metadata; as files are created, extended, or deleted, entries change between free and used, causing fatUsed to fluctuate. In FAT32, the FAT entries are 4 bytes; in FAT16 they are 2 bytes, and in FAT12 the representation is 12-bit, handled across storage bytes in a variant-dependent way. The practical meaning of fatUsed depends on the tool: it may reflect allocation overhead, fragmentation risk, or simply track the utilization of the allocation metadata.

Limitations: There is no universal definition for fatUsed across all software. Consult the specific project’s documentation to determine exactly what metric is being reported. See also: FAT filesystem, File Allocation Table, free clusters, end-of-chain.