APFS Space Sharing
Also known as: apfs container space, shared apfs volumes, apfs space sharing
APFS space sharing lets multiple volumes inside one APFS container draw from a single shared pool of free space, so each volume's free space reflects the whole container rather than a fixed partition size. This is why free space on Apple devices can look inconsistent.
- Volumes in one APFS container share a single free-space pool instead of fixed partition sizes.
- Per-volume free space reflects the whole container, so multiple volumes report similar free space.
- Purgeable space and snapshots are reported alongside shared free space, making totals look inconsistent.
How space sharing works
In APFS (Apple File System), a physical disk holds one or more containers, and each container holds one or more volumes. Unlike classic fixed partitions, volumes inside a container do not own a static slice of the disk. Instead they all share the container's free blocks dynamically, allocating and freeing space on demand.
On an iPhone or iPad, the system, the user data volume, and hidden system volumes typically live in the same container. Because they share one free-space pool, the Settings > General > iPhone Storage total and the per-volume free space all reference the same underlying capacity. Adding data to one volume reduces free space everywhere in that container.
Why free space looks confusing
Space sharing is the main reason reported free space can seem to fluctuate or disagree across tools. Two volumes in the same container will each report nearly the same free space, since both see the shared pool. Purgeable space (caches and snapshots the OS can reclaim on demand) further blurs the line between used and free.
When you delete photos or clear caches, freed blocks return to the shared container pool, not to a single fixed partition. A cleaner like Cleanor surfaces what is actually consuming the shared pool, so you can target large files and duplicates rather than guessing why the numbers do not add up.