You updated macOS and lost 20 GB — here's where it went
You ran the macOS update, everything went smoothly, and then you checked storage. Somehow your Mac has less free space than before the update — sometimes 10–20 GB less. You didn't download anything new. Nothing in the Storage panel explains it. What happened?
The short answer: a major macOS update temporarily needs a lot of extra room. Some of that space comes back on its own within a day or two. Some of it you need to reclaim manually. Here's the full picture.
Why updates eat so much space
A macOS update involves several things that all take up disk space at once:
| What | Typical size | Goes away on its own? |
|---|---|---|
| The update installer itself | ~12 GB | Sometimes |
| Pre-update APFS snapshot (rollback safety net) | 5–15 GB | Yes, after a few days |
| Rebuilt system caches and indexes | 2–8 GB | Yes, but takes hours |
| Expanded APFS container metadata | 1–3 GB | No (normal growth) |
| Leftover update staging files | 1–5 GB | Usually |
Added together, that's easily 20 GB or more of space that seems to vanish overnight. The good news: most of it is temporary.
What comes back on its own (just wait)
After a major update, macOS runs a wave of background tasks that can take 24–48 hours to finish:
- Spotlight reindexing. Your entire drive gets re-indexed. While this runs, you may notice the disk working hard and "System Data" looking unusually large. This is normal and finishes on its own — usually within a few hours on an SSD.
- APFS defragmentation and optimization. The file system reorganizes itself in the background. This can temporarily inflate reported disk usage.
- Old snapshots being released. macOS keeps a pre-update snapshot so it can roll back if something goes wrong. After it confirms the update is stable (typically a few days), it releases this snapshot and the space returns.
What you need to clean manually
1. Delete the macOS installer
After a major update, the installer sometimes stays in your Applications folder — look for an app called something like "Install macOS" followed by the version name. It's around 12 GB. If the update is done and working, you don't need it anymore. Just drag it to Trash and empty Trash.
To check if it's there, open Finder, go to Applications, and sort by size. The installer will be one of the largest items.
2. Check for leftover update files
Update staging files sometimes linger in /Library/Updates or as downloaded content in System Settings → General → Software Update. If there's no pending update waiting to install, you can safely clear these.
3. Clear old Time Machine snapshots
If macOS created pre-update snapshots and hasn't released them yet, you can check manually. Open Terminal and run:
tmutil listlocalsnapshots /
If you see snapshots from before the update date and you have a separate backup, you can remove them:
tmutil deletelocalsnapshots 2026-08-20-100000
(Replace the date string with the actual snapshot name from the list.)
4. Flush rebuilt caches
After an update, macOS rebuilds various caches — fonts, dyld shared cache, kernel extensions. These are normal and necessary, but the old versions of these caches sometimes aren't cleaned up immediately. A restart usually takes care of this. If your System Data category still looks bloated a few days after the update, clearing ~/Library/Caches is a safe next step.
When to worry (and when not to)
Don't worry if: it's been less than 48 hours since the update, your Mac is still warm/fans spinning, or Spotlight shows a progress bar in System Settings. These are all signs background work is still running.
Do investigate if: it's been more than a week, you've restarted at least twice, and you're still missing 10+ GB compared to before the update. At that point, something specific is holding the space — old snapshots, the installer, or leftover files — and the steps above will help you find it.
If you've cleaned up after the update but your Mac is still tight on space, the problem is likely older than the update itself. Check our guide on what to do when your startup disk is full for the broader picture.
Post-update cleanup, explained in plain English
Gigatidy scans your Mac after an update, shows you exactly what's temporary and what needs manual cleanup, and handles the safe stuff for you. Coming soon to the Mac App Store.
Meet Gigatidy →FAQ
Is it normal to lose storage after a macOS update?
Yes. A major update can temporarily use 10–20 GB for the installer, rollback snapshots, and cache rebuilds. Most of this space returns within 24–48 hours as background tasks finish. If it doesn't come back after a few days, you may need to delete the installer or old snapshots manually.
Can I delete the macOS installer after updating?
Yes. Once the update is installed and running normally, the installer in your Applications folder is no longer needed. It's typically around 12 GB. Drag it to Trash and empty Trash to reclaim the space.
Why does System Data grow after an update?
macOS rebuilds caches, re-indexes Spotlight, and keeps rollback snapshots — all of which temporarily inflate the System Data category. Most of this shrinks on its own within a day or two. If it stays large after a week, clearing user caches and old snapshots usually brings it back down.