What Is Purgeable Space on a Mac and How Do I Reclaim It?
Time Machine Local Snapshots and the Missing Gigabytes
Short answer: purgeable space on a Mac is data macOS can delete on its own when it needs room, and most of it consists of Time Machine's hourly local snapshots. List them in Terminal with tmutil listlocalsnapshots / and reclaim space with tmutil thinlocalsnapshots / 50000000000 4, where the number is the target in bytes. To stop new snapshots from being created, you have to turn off Time Machine's automatic backups.
The situation that confuses Mac users most is this: Finder reports 200 GB available, yet copying a file fails with "not enough space", or Disk Utility and Finder disagree completely. The reason is that APFS counts purgeable space as available even though it is not actually free yet. This guide explains where that space comes from, how to get it back, and what you give up in the process.
Key Takeaways
- Purgeable space is data macOS can delete when needed; most of it is Time Machine local snapshots.
- With Time Machine enabled, macOS takes hourly local snapshots and keeps them for roughly 24 hours — even when no external drive is connected.
- Finder and Disk Utility report different numbers because Disk Utility does not count purgeable space as free.
- tmutil listlocalsnapshots / lists every existing snapshot with its timestamp.
- tmutil thinlocalsnapshots / <bytes> 4 removes enough snapshots to reclaim your target amount at the most aggressive urgency level.
- Deleting local snapshots does not affect your external Time Machine backup; you only lose fast rollback to the last 24 hours.
What Exactly Is Purgeable Space?
On the APFS file system, macOS marks certain data on disk as expendable when necessary. That data consists of real files, but macOS is allowed to delete them without asking when you run short on room. Finder counts this as part of available space because, technically, you can have it back at any moment.
Three kinds of data fall into this category. The first and largest is Time Machine's local snapshots. The second is files downloaded from iCloud Drive that could simply be downloaded again. The third is application caches, Xcode derived data and system temporary files.
The problem is that this mechanism does not always kick in fast enough. When you try to copy a large file in one go, macOS attempts to free the space it needs, but the operation sometimes times out and you get a "not enough space" error — while Finder still shows 200 GB free.
Time Machine Local Snapshots: The Main Source
When you enable Time Machine to back up to an external drive, macOS also starts taking hourly snapshots on your internal disk. These are taken even when the external drive is not connected, so you can roll back the last few hours while away from your backup drive.
Snapshots are typically kept for about 24 hours and, thanks to the copy-on-write design of APFS, take almost no space at first. But as your files change, older versions are retained and the footprint grows quickly. For anyone working with large files — video editing, virtual machines, large datasets — local snapshots can easily exceed 100 GB.
macOS thins these snapshots automatically when free space runs low. The catch is that the mechanism waits until the critical moment; if you want the space back before then, you have to intervene manually.
Why Finder and Disk Utility Disagree
Seeing two different numbers is confusing, but the explanation is simple: the two tools answer the same question differently. Finder includes purgeable space in the available figure, answering "how much could you theoretically have". Disk Utility reports only what is genuinely free right now.
That is how Finder can say 200 GB available while Disk Utility says 40 GB free. The 160 GB difference is snapshots and caches that should be deleted but have not been yet. Asked which number is correct, the answer is both: one measures potential, the other measures actual.
Which should you trust in practice? Go by Disk Utility's number before doing anything demanding. If you are about to copy a large file, install an application or run a macOS update, genuinely free space is what determines whether it succeeds.
What You Lose by Deleting Snapshots
Deleting local snapshots does not affect the Time Machine backups on your external drive. They are two entirely separate things: the external backup is your long-term archive, while local snapshots are temporary copies kept for fast rollback over the last few hours.
The only thing you give up is the ability to "go back to two hours ago" while your external drive is disconnected. If you accidentally deleted a file, you could previously restore it from those snapshots; afterwards, you need the external backup for that.
For most users that is a reasonable trade, particularly when the disk runs permanently full. But if you are not taking regular external backups, local snapshots are your only safety net — in that case, run at least one Time Machine backup before clearing them.
Other Sources of Purgeable Space
Snapshots are not the only source. The "System Data" category on the storage screen (previously called "Other") covers application caches, log files, iOS device backups, Mail attachments and downloaded software updates, and can reach tens of gigabytes on its own.
If you write software, Xcode is a category in itself: derived data, device support files and simulator images together can exceed 50 GB. Browser caches, Docker images and the Homebrew cache are part of the same quiet accumulation.
The macOS build of Disk Mop gathers these categories on a single screen: Cache Cleaner lists more than ten macOS-specific categories including user caches, Homebrew and Xcode, while Disk Analysis draws the drive as a treemap so the genuinely bloated folder is obvious. Large Files uses the mdfind index on macOS, so scans return quickly without walking the entire disk.
Setting Up Something That Lasts
A one-off thinning lasts a few days; as long as Time Machine stays enabled, snapshots accumulate again. For a durable arrangement you have two options: turn off automatic backups and run them manually, or keep the disk 15-20% free at all times so that macOS's own thinning mechanism has room to breathe.
The second option is safer because you do not lose backup protection. Achieving it means dealing not with purgeable space but with real files: old downloads, duplicate photos, applications you no longer use and large video archives.
You can download Disk Mop free and try it with limited features; the Pro version is a one-time $19.90 payment for a lifetime license. It runs on Windows 10 and 11 (64-bit) and macOS 12 or later.
Frequently Asked Questions
What is purgeable space on a Mac?
It is data macOS can delete on its own when it needs room. Most of it consists of Time Machine's hourly local snapshots; the rest is files that could be re-downloaded from iCloud plus application caches. Finder includes this in available space while Disk Utility does not.
Finder says 200 GB free but I cannot copy a file — why?
Because most of that 200 GB is purgeable space that has not actually been released yet. macOS tries to free it during the copy, but the operation can time out. Releasing the space in advance with tmutil thinlocalsnapshots / 50000000000 4 in Terminal resolves it.
How do I delete local snapshots?
List them in Terminal with tmutil listlocalsnapshots /, then remove enough of them with tmutil thinlocalsnapshots / <bytes> 4 — for example 50000000000 to reclaim about 50 GB. To remove one specific snapshot, use tmutil deletelocalsnapshots with the timestamp from the listing.
Does deleting local snapshots break my Time Machine backup?
No. The Time Machine backups on your external drive are entirely separate and are unaffected. The only thing you lose is the ability to roll back to the last few hours while the external drive is disconnected. Your long-term archive stays intact.
Can I disable local snapshots entirely?
The old sudo tmutil disablelocal command was removed in macOS High Sierra. Today the only route is turning off Time Machine's automatic backups: go to System Settings > General > Time Machine (System Preferences > Time Machine on macOS 12), disable automatic backups, and start backups manually instead.
Why is the "System Data" category so large?
That category combines application caches, log files, iOS device backups, Mail attachments, downloaded software updates and local snapshots. If you develop software, Xcode derived data and simulator images are added on top. It is not one item but the sum of dozens of small sources, which is why a folder-level analysis is needed to find the one that is bloated.
Verdict
Purgeable space on a Mac is a design decision in APFS rather than a fault: macOS counts data it could delete as available. The only real problem is that the mechanism waits until the critical moment to act.
If you want the space back now, the route is tmutil: list snapshots with listlocalsnapshots and release what you need with thinlocalsnapshots. Doing so does not touch your external Time Machine backup; it only costs you fast rollback over the last few hours.
The durable answer is not managing snapshots but creating genuine free space on the disk. Keep 15-20% free and macOS's own thinning runs quietly in the background, so you never encounter this problem in the first place.
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