Allocated vs Logical Size on Mac
In DiskTreeSize, Logical measures file length and Allocated uses allocated blocks reported by the file system. These measurements answer different questions. Their difference is not automatically an error or an amount of space you can clean up.
What each measurement means
| Toolbar option | Meaning | Useful question |
|---|---|---|
| Logical | Reported file length, aggregated across scanned items for folders | How large is the file by its content length? |
| Allocated | Reported allocated block size, aggregated for the scanned folder | Which items deserve attention based on allocated storage? |
DiskTreeSize calculates allocated size from the file system's block count multiplied by 512 bytes. That is file-system information, not a test of how much free space deletion will produce.
Compare the same item in one scan
- Click Choose Location, select a folder, and start Analyze.
- Select Allocated in the toolbar and follow the size-sorted tree to an item you want to inspect.
- Switch to Logical and compare that item, noting any changes in order and parent percentage.
Keep the selected location the same. A project's logical total and a disk-wide used-space figure from System Settings measure different scopes.
For help with the tree and sorting controls, see the folder size guide.
Why allocated size can be smaller
A sparse file can contain blank regions without allocating blocks for them. Some disk images or database files use this approach, so a large file length need not represent the same amount of allocated local storage.
Apple's APFS supports sparse files and clones that share unchanged data. See Apple's APFS explanation for these storage features. DiskTreeSize does not treat the difference between its two measurements as unwanted data.
Why a folder total can differ from a disk total
The app measures scanned items within the selected, accessible location. System Settings measures the disk and applies its own categories. The objects being measured can therefore differ before any calculation begins.
Files can also share underlying data. Adding per-item measurements is useful for comparing folders, but does not guarantee a total of unique physical space across the disk. DiskTreeSize's size columns do not manage APFS snapshots or recreate system storage categories.
Use the numbers to guide inspection
Start with allocated size to find large folders, then expand them to individual files. Switching to logical size helps explain content length. For a substantial difference, identify the file type, owning application, and whether it is still in use.
Do not remove an unfamiliar system folder or app database just because its allocated size is large. The difference between logical and allocated size is not a cleanup budget. Continue with the large file guide for practical inspection steps.
Check available storage after taking action
Trashed files remain on disk, and shared data may still be used by other files. A reduction in the tree does not guarantee an equal increase in available capacity.
After handling content you have identified as unnecessary, check the system storage overview. DiskTreeSize helps locate and explain disk usage; the final change is the disk's actual available space.
Frequently asked questions
Does a difference between allocated and logical size mean corruption?
Not necessarily. They describe file length and allocated space reported by the file system. Storage features such as sparse files can make them differ; the difference alone does not establish corruption.
Will deleting a file recover its displayed allocated size?
That is not guaranteed. Trash and shared APFS data can affect available space. Allocated size is a measurement for analysis, not a promise of the capacity recovered after removal.
Which size should I use to find space-consuming items?
Start by sorting allocated size, then switch to logical size to compare the same item. For a large difference, identify the file type and purpose. With either measure, inspect the content before taking action.
