Allocation unit (cluster size)
Also known as: cluster size, block size, allocation unit size, slack space
An allocation unit, or cluster, is the smallest chunk of disk space a file system will give to a file. Even a 1-byte file occupies a whole cluster, so many small files waste space in the unused remainder, called slack space. Larger clusters speed up big files but waste more on small ones.
- Smallest block a file system assigns to a file
- Often 4 KB by default on NTFS
- Unused remainder in a cluster is "slack space"
Why files round up to a cluster
A file system does not hand out space byte by byte — it allocates in fixed blocks called clusters or allocation units, often 4 KB by default on NTFS and many drives. A file always takes a whole number of clusters, so a 1 KB file still occupies a full cluster and the leftover space inside it, called slack space, sits unused.
This is why a folder of thousands of tiny files can report a "size on disk" noticeably larger than the sum of the file sizes. The gap is wasted slack, not missing data.
Choosing a cluster size
You set cluster size when you format a drive. Larger clusters reduce fragmentation and slightly speed up large files like video, but waste more space when storing many small files. Smaller clusters pack small files efficiently but can add overhead.
For most drives the default is the right choice. A custom cluster size only pays off for specialized cases — for example, very large media files on a dedicated drive.