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Deleted gigabytes, but free space barely moved

Usually one of three things: the files shared their blocks through APFS clones, a local Time Machine snapshot still holds them, or macOS counts the space as purgeable.

By Ivan Stepanok Updated Tested on macOS 27.0 and Xcode 27.0
Two frosted glass blocks overlapping, the shared part glowing brighter blue
Short answerUsually one of three things: the files shared blocks with others through APFS clones, a local Time Machine snapshot still holds them, or macOS counts the space as purgeable. Each has a quick check.

APFS clones: shared blocks

When you install a build on several simulators, or Xcode clones a simulator for parallel testing, APFS doesn't copy the files. The copies share the same blocks and take "no additional space on disk" until one of them changes (Apple). Finder and du still count each copy in full.

So deleting one of three simulators with the same app frees only what that simulator had to itself. The shared blocks go when the last copy goes. In one report, deleting 57 GiB of test clones freed 3.3 GiB.

Local Time Machine snapshots

If Time Machine backs up your Mac, it keeps local snapshots on the disk, and data you delete can stay in them for a while (Apple Support). One developer saw the space from deleted runtimes come back only after the second backup. To see the snapshots:

tmutil listlocalsnapshots /

macOS removes them on its own as they age or when it needs the space.

Purgeable space and System Data

The "available" figure in Finder and System Settings "includes free storage space and purgeable disk caches" (Apple Support), space macOS can reclaim when it needs to. Two tools reading the disk at the same moment can disagree by that much.

System Data is everything that doesn't fit another category (Apple Support): local snapshots and runtime images in system storage count there. It goes down when those do, not always the moment you delete something.

Runtimes that came back

In Xcode 27.0, some simulator runtimes weren't completely deleted and reappeared after a restart; Xcode 27.2 beta 2 fixes it. The runtimes guide has the details.

Measure what you actually freed

Note the free space before and after, rather than adding up folder sizes:

df -h /

Expect the number to keep moving for a few minutes as macOS finishes deleting and updates its counts.

With SimCleaner

SimCleaner shows sizes as a range when blocks are shared: the least a deletion frees and the most. Its "Freed about" total counts only what is gone from disk. Each item adds the bytes it held alone where APFS can tell, and its size on disk where it can't; a runtime adds the size Xcode reports. So it's an estimate, not a reading of the disk. Your free space lags behind it while a snapshot still holds the deleted files, and runs a little ahead of it when shared blocks are freed together.

SimCleaner's storage bar: simulators 16.72 GB, leftover folders 272.6 MB, runtimes 8.07 GB, DerivedData 7.82 GB, device support 4.63 GB, agent workspaces 692.1 MB, and 121 GB available
The storage bar at the bottom of every SimCleaner page. Sample projects.

Questions

Why does Finder show a different size than SimCleaner?

Finder adds up every file in full, so each clone counts as if it were a copy. SimCleaner shows that figure too, and next to it how much of it nothing else shares.

Can I force macOS to free purgeable space?

There's no supported switch. macOS frees it when an app needs the space.