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Quick answer: Logical size is the amount of file data; allocated size is the storage occupied under the filesystem’s allocation rules. Compression, sparse files and cluster rounding can produce differences.
By DiskAnalyzer editorial resource · Documentation review: 2026-10-28
Why is the allocated size different from file size?
Logical size is the amount of file data; allocated size is the storage occupied under the filesystem’s allocation rules. Compression, sparse files and cluster rounding can produce differences.
Step-by-step diagnosis on Windows
- Select a file whose reported size and allocated size differ.
- Check the volume filesystem and whether compression or sparsity is used.
- Compare the same measurement for any Windows Storage estimate.
- Do not sum both parent folders and child files, because that counts the same bytes twice.
Interpretation checklist
| Finding | What it may mean |
|---|---|
| Allocated larger than logical | Small files may occupy full allocation units. |
| Allocated smaller than logical | Compression or sparse allocation may explain the difference. |
| Hard links | Multiple directory names can reference the same underlying file data. |
Safety and limitations
Different tools may account for hard links and metadata in different ways; match the exact metric before comparing totals.
WizTree provides a file inventory; it does not establish file ownership, deletion safety or the health of storage hardware. Review Windows and the relevant application's documentation before changes.
How to verify the result
Record the scan date, Windows version, filesystem, drive and metric used. Make one approved change at a time, then rerun the same scan and compare Windows free space. A predicted saving is not a measured saving.
Related Windows storage guides
Sources and methodology
Based on public WizTree technical documentation and Microsoft Windows storage management guidance. No independent runtime or binary testing is claimed. Check the instructions for your installed version before changing system files.