What Cluster Size Should Be Used When Making Large FAT32 Partitions?

By David Dunning

A storage device, such as a hard disk drive, can be divided into a series of logical partitions, each with its own directory and file allocation table. Under the FAT32 file system, each partition is divided in clusters, each identified by a 32-bit number, or a series of 32 binary digits, or "bits." Each cluster consists of one or more smaller units, known as sectors, depending on the size of the partition.

Recommended Cluster Size

According to Hitachi, the recommended cluster size for a 16GB FAT32 partition is 4KB, the recommended cluster size for a 64GB FAT32 partition is 16KB, the recommended cluster size for a 128GB FAT32 partition is 32KB and the recommended cluster size for a 256GB partition is 64KB. Note that some versions of Windows prior to Windows XP, including Windows 95 and Windows 98, don’t support cluster sizes above 32KB.

Wasting Space

The optimum cluster size for a large FAT32 partition depends on the number of clusters required to fill the entire partition. By definition, larger FAT32 partitions can handle larger cluster sizes, but note that the larger the cluster size the more space is wasted. If your FAT32 partition has a cluster size of 32KB, a file with a logical size of just 1 byte will still have a physical size of 32KB, effectively wasting 31KB of storage space. Similarly, a file with a logical size of 33KB -- 1 byte more than the cluster size -- will have a physical size of 64KB.

Maximum and Minimum Clusters

Microsoft states that a partition must contain at least 65,527 clusters to use the FAT32 file system. Furthermore, you cannot increase the cluster size beyond 64KB because this would cause certain programs to calculate disk space incorrectly. Conversely, you cannot decrease the cluster size so low that the FAT -- the table that keeps track of where the various fragments of files reside on a partition -- becomes smaller than 15.36MB in size.

Maximum Partition Size

Microsoft reports that a FAT32 partition can contain, at most, 268,435,445 clusters. Assuming a cluster size of 32KB, this suggests that -- at least in theory -- the FAT32 file system is a capable of supporting partition sizes up to 268,436,445-by-32 -by-1024 bytes or 8.8 terabytes. However, in practice, Windows XP Professional limits the size of FAT32 partitions to 32GB and reports a “Volume size too big” error if you attempt to format a FAT32 partition larger than this.