Filesystem Overhead Calculator
Filesystems store data in fixed-size allocation units called clusters. Every file is rounded up to a whole number of clusters, so the space a file actually occupies on disk is usually larger than its content. The wasted remainder is called slack space. Enter your average file size, the cluster (allocation unit) size, and the number of files, and this calculator returns the on-disk size per file, the total allocated space, the total slack wasted, and the efficiency. The model rounds each file up to the next cluster boundary.
Filesystem overhead formula
Clusters per file = ceil(file size / cluster size)
On-disk per file = clusters per file * cluster size
Total allocated = on-disk per file * number of files
Slack per file = on-disk per file - file size
Efficiency = (file size / on-disk per file) * 100
Slack space is the unused tail of the last cluster. Smaller files relative to the cluster size waste a larger fraction.
Allocation unit context
- NTFS and many filesystems default to a 4 KB allocation unit on common volume sizes.
- Larger cluster sizes reduce fragmentation overhead but increase slack for small files.
- A file always occupies at least one full cluster, even a 1-byte file.
- Compression and sparse files can change actual on-disk usage; this model assumes neither.
- Verify your volume's exact cluster size before relying on a single default value.
Filesystem overhead calculator: frequently asked questions
What is filesystem slack space?
Slack space is the unused part of the last cluster a file occupies. Files are stored in fixed-size allocation units (clusters), so a file is always rounded up to a whole number of clusters. A 5 KB file on a 4 KB cluster filesystem uses 2 clusters (8 KB), wasting 3 KB of slack.
How is on-disk size calculated?
Round each file size up to the next whole multiple of the cluster size: on-disk size equals the cluster size times the ceiling of file size divided by cluster size. Multiply by the number of files to get total allocated space.
What is a typical cluster size?
The default allocation unit for NTFS and many other filesystems is 4 KB (4,096 bytes) for typical volume sizes. Larger volumes or specialized configurations may use 8 KB, 16 KB, 32 KB, or 64 KB clusters. Check your volume's format settings for the exact value.
Why do many small files waste more space?
Each file wastes up to one cluster minus one byte of slack. With many tiny files, that per-file waste adds up quickly. One thousand 1 KB files on a 4 KB cluster filesystem each use a full 4 KB cluster, so they occupy 4 MB to store 1 MB of data.
Does this include metadata overhead?
No. This calculator models cluster rounding (slack space) only. Filesystems also use space for directory entries, the master file table or inodes, and journals. Those are usually small relative to slack for large file counts but are not included here.
Official sources
- Microsoft Learn: NTFS overview and default cluster sizes.
- NIST: National Institute of Standards and Technology, storage units reference.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.