Used vs Available Storage
Also known as: used vs free storage, available space meaning, used vs available storage
Used storage is the space already occupied by the OS, apps, and your files; available storage is what remains free for new data. The two add up to roughly the formatted capacity, which is always less than the advertised size.
- Used + Available is measured against formatted capacity, which is smaller than the advertised box size.
- On iOS, fluctuating System Data makes "used" change even when you add no new files.
- Filesystems hold back a small reserve, so not every byte of "available" is actually writable.
What the numbers actually mean
On a phone, used storage counts everything currently written to flash: the operating system, installed apps and their data, photos, videos, downloads, and caches. Available storage (often labeled "Free" or "Available") is the remaining writable space the system reports you can fill before hitting capacity.
Both figures are measured against the formatted capacity, not the box capacity. A "128 GB" phone exposes roughly 119 GiB because manufacturers count 1 GB as 1,000,000,000 bytes while the OS counts in powers of two (1 GiB = 1,073,741,824 bytes). The firmware and reserved partitions trim it further, so used + available rarely sums to a clean round number.
Why used and available drift on iOS and Android
On iOS, Settings > General > iPhone Storage shows a colored bar broken into Apps, Media, Photos, System Data, and free space. System Data is a catch-all bucket (caches, logs, indexes) that grows and shrinks unpredictably, so "used" can change without you adding files.
On Android, Settings > Storage (or Settings > Device care > Storage on Samsung) splits used space into Apps, Images, Videos, Audio, Documents, and System. The System slice covers the OS image and protected partitions you cannot delete. Available space also shifts as app cache and temporary files churn in the background.
Available storage is not always fully usable: filesystems keep a small reserve, and purgeable space on iOS may be flagged as free even though it still holds data the system can evict on demand.