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A file allocation unit, also called a cluster, is the smallest unit of disk space a filesystem allocates to store file data. Because allocation happens in whole units, “Size on disk” can differ from a file’s logical “Size.” The unit’s size depends on the filesystem and volume configuration.
What is a file allocation unit?
A file allocation unit is a filesystem’s unit of storage allocation. Microsoft describes cluster size, another name for allocation unit size, as the smallest amount of disk space that can be allocated to hold a file. The filesystem assigns space in these units rather than in arbitrary byte increments.
“Allocation unit” and “cluster” are often used interchangeably. “Block” is related terminology, but implementation details vary across filesystems. This filesystem concept is not the same as a drive’s physical sector size.
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Why are “Size” and “Size on disk” different?
“Size” usually refers to the file’s logical data size. “Size on disk” reflects the space allocated to store it. Since a filesystem allocates in whole clusters, the final allocated unit may have unused space; that slack space can make the on-disk figure larger than the logical size when compression is not used.
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This is one reason a folder or scan can appear to use much more disk space than the sum of its files’ logical sizes. The gap depends on the filesystem’s allocation behavior and the files involved; it does not, by itself, establish that files are duplicated or missing.
Does every small file use a whole cluster?
Not necessarily. The simple whole-unit explanation has an important NTFS caveat: Microsoft’s filesystem comparison describes tail packing, in which small files can be stored resident in the Master File Table (MFT) stream descriptor. So it would be inaccurate to assume every tiny NTFS file consumes a full separate data cluster.
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Compression can also change the relationship between a file’s logical size and its allocated space. The exact accounting depends on filesystem features and how the volume is configured.
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How large is an allocation unit?
There is no single size that applies to every filesystem. For Windows NTFS, Microsoft’s overview lists 4 KB as the default cluster size in its documented context. Microsoft’s exFAT specification permits cluster sizes up to 32 MB under the sector-size limits stated in that specification. These figures describe different filesystems and documentation contexts; the exFAT maximum is not a default, and neither figure is a universal setting.
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For the underlying details, see Microsoft’s NTFS overview and exFAT File System Specification.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does cluster size affect performance?
It can affect both space efficiency and performance. Larger allocation units can mean more unused space in the final unit of some files; different sizes can also offer different performance benefits depending on the deployment. There is no universally best cluster size without knowing the filesystem, workload, compatibility needs, and volume requirements.
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Microsoft’s guidance on cluster size recommendations for ReFS and NTFS discusses how the choice depends on use. The practical point is to treat allocation size as a filesystem and workload decision, not as a hardware sector setting.
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What to remember
- A file allocation unit, or cluster, is the smallest unit of disk space a filesystem allocates for file data.
- Because allocation uses whole units, logical file size and “Size on disk” can differ.
- Filesystem, configuration, compression, and features such as NTFS tail packing affect the actual accounting.
- Cluster size is not physical sector size, and a documented size for one filesystem should not be generalized to another.
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