If your Mac's available storage has quietly shrunk without any obvious cause, Time Machine's local snapshots are often the culprit. Knowing how to thin Time Machine local backups — reclaiming a specific amount of space rather than nuking everything — keeps your safety net intact while giving disk space back where you actually need it.
What Are Local Snapshots and Why Do They Accumulate?
Time Machine keeps two kinds of backups: remote backups written to your external drive or NAS, and local snapshots stored directly on your Mac's internal drive. Local snapshots let Time Machine protect recent changes even when the backup destination is disconnected — on a laptop this happens constantly.
macOS manages these snapshots automatically under /.MobileBackups (older macOS) or as APFS snapshot metadata embedded in the volume itself (macOS 11 Big Sur and later). The system is supposed to reclaim this space when you run low on disk room, but "low" by Apple's definition is often far tighter than what you'd consider acceptable. The result: snapshots can quietly hold tens of gigabytes that Finder reports as "System Data" or "Purgeable."
Thinning vs. Deleting — What's the Difference?
There are two distinct operations you can perform on local snapshots:
- Delete a specific snapshot — removes one named snapshot entirely (
tmutil deletelocalsnapshots <date>). Surgical, but requires identifying each snapshot by date. - Thin snapshots — tells macOS to reclaim at least a target number of bytes by pruning the oldest or least-important snapshot data first, while leaving newer snapshots intact if they fit. This is what
tmutil thinlocalsnapshotsdoes.
Thinning is the safer, more precise approach when you need to recover a known amount of space — say, 10 GB before installing Xcode — without wiping out all your local recovery points.
How macOS Decides What to Thin
macOS uses an urgency system. When you invoke thinlocalsnapshots you supply an urgency level from 1 (low priority, least aggressive) to 4 (highest urgency, most aggressive pruning). At urgency 4, the system will thin down to the target even if it means removing nearly all local snapshots. At urgency 1 it may refuse to remove anything it considers still "useful."
This urgency mirrors what macOS does internally under memory pressure: when free space truly runs out, it escalates urgency automatically and reclaims purgeable space without asking.
Step-by-Step: Thin Local Snapshots with tmutil
Step 1 — Check What Snapshots Exist
Open Terminal and run:
tmutil listlocalsnapshots /You'll see output like:
com.apple.TimeMachine.2025-11-14-083201.local
com.apple.TimeMachine.2025-11-15-093015.local
com.apple.TimeMachine.2025-11-16-120042.localEach line is a snapshot name containing its creation date. Note the oldest ones — thinning at lower urgency targets those first.
Step 2 — Check How Much Space Is Purgeable
df -h /The "Available" column in df already includes purgeable space that macOS will free on demand. For a cleaner picture of what Time Machine specifically holds, use:
du -sh /.MobileBackups 2>/dev/null || echo "No legacy MobileBackups folder (APFS volume — snapshots are embedded)"On APFS volumes (macOS 10.15+) you won't see a .MobileBackups folder; the snapshot data lives inside the APFS container itself. diskutil apfs list will show snapshot counts per volume.
Step 3 — Run thinlocalsnapshots
The full syntax is:
sudo tmutil thinlocalsnapshots / <bytes> <urgency>Replace <bytes> with the number of bytes you want to reclaim, and <urgency> with a value from 1–4.
Examples:
# Reclaim 10 GB at moderate urgency
sudo tmutil thinlocalsnapshots / 10737418240 2
# Reclaim 20 GB at high urgency (more aggressive)
sudo tmutil thinlocalsnapshots / 21474836480 3
# Reclaim 5 GB at maximum urgency
sudo tmutil thinlocalsnapshots / 5368709120 4Byte conversions for reference: 1 GB = 1,073,741,824 bytes. So 10 GB ≈ 10,737,418,240.
You'll see no output if the operation succeeds quietly, or an error if the volume path is wrong. Re-run tmutil listlocalsnapshots / afterward to confirm snapshots were removed.
Step 4 — Verify the Reclaimed Space
df -h /The Available figure should have increased. If it hasn't moved much, try a higher urgency level (up to 4), or accept that the system doesn't yet consider those snapshots thin-able at the urgency you chose.
Urgency Levels at a Glance
| Urgency | Aggressiveness | When to Use |
|---|---|---|
| 1 | Minimal — may do nothing | Routine housekeeping, plenty of free space |
| 2 | Moderate | Want to reclaim a specific amount, not urgent |
| 3 | High | Need space soon (e.g. big download or install) |
| 4 | Maximum — removes most local snapshots | Critically low disk, need every byte back |
Important Caveats — Read Before You Run
- Local snapshots are not your real backup. Thinning them does not affect backups already written to your external Time Machine drive or Time Capsule. Your remote backup history remains intact.
- Thinning is permanent. Once a snapshot is removed, you cannot restore individual files from that snapshot. If you rely on local snapshots for "undo" on a laptop that rarely connects to its backup drive, be conservative with urgency.
- macOS may recreate snapshots. Time Machine will start creating new local snapshots again at its next scheduled interval. This is normal; you are not permanently disabling the feature.
- Do not disable Time Machine entirely just to avoid snapshots. Turning off Time Machine with
sudo tmutil disableremoves your backup protection entirely — a poor trade-off for recovering a few gigabytes. - Requires sudo. You need administrator privileges. On a company-managed Mac with restricted sudo, contact your IT admin.
Doing This Without the Terminal
If the tmutil arguments feel fiddly, Crumb handles local snapshot thinning in one click. Its Clean tab identifies purgeable space — including Time Machine local snapshots — and reclaims a target amount without requiring you to calculate byte values or pick urgency levels manually. It also shows a breakdown of what was cleaned so you know exactly what was removed. If you'd rather skip the Terminal entirely, download Crumb and run a clean from the menu bar.
When Thinning Isn't Enough
If snapshots aren't the main culprit, the purgeable space may be coming from elsewhere — large caches in ~/Library/Caches, old iOS device backups in ~/Library/Application Support/MobileSync/Backup, or developer artifacts in ~/Library/Developer/Xcode/DerivedData. These require separate cleanup steps and are worth investigating before blaming Time Machine.
Conclusion
Thinning local Time Machine snapshots with tmutil thinlocalsnapshots is the right tool when you need to reclaim a specific amount of space precisely and safely — leaving recent snapshots intact and your remote backup completely untouched. Start at urgency 2, verify the result, and escalate only if needed. Your real backup history on the external drive is never at risk from this operation.