What Is System Data on Mac? (And How to Reduce It)

You open About This Mac → Storage, and there it is: "System Data" taking up 40 GB, 60 GB, sometimes over 100 GB. But when you click on it, there's nothing useful to delete. Here's what System Data actually includes - and what you can actually do about it.

What counts as System Data on Mac

Apple lumps several different categories under "System Data" in the storage overview. It's a catch-all that includes:

macOS system files and frameworks

The base macOS installation itself, frameworks, libraries, and system binaries. This is typically 10–15 GB on a fresh install and not something you can reduce.

Virtual memory and sleep image

macOS uses disk space as virtual memory (the swap file) when RAM is under pressure. On Apple Silicon Macs, sleep images can be 16–32 GB (equal to your RAM). These are managed automatically and regenerate - you can't permanently delete them.

Time Machine local snapshots

This is often the biggest hidden contributor. When Time Machine is configured, macOS creates local snapshots hourly and stores them on your drive until space is needed. These snapshots can consume 10–50 GB and macOS counts them as System Data.

App caches and temporary files

Logs, temporary application data, browser caches, and various app support files that don't fit into other categories.

iOS device backups

When you back up your iPhone or iPad to your Mac instead of iCloud, the backups are stored locally. A full iPhone backup can be 20–60 GB. These are counted in System Data.

Core ML and Siri data

macOS stores machine learning model data locally for features like Siri suggestions, Photos recognition, and Spotlight. This is typically a few GB and not removable.

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Why System Data seems to grow over time

System Data expands because:

1. Time Machine snapshots accumulate (the most common cause of sudden growth)
2. App caches build up - Slack, Chrome, Xcode, and other heavy apps cache locally
3. iOS backups get created every time you plug in a device with Finder backup enabled
4. Log files accumulate from apps and system processes

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How to actually reduce System Data

1. Delete Time Machine local snapshots

This is usually the biggest win. To check how much space snapshots are using:

Open Terminal and run:

tmutil listlocalsnapshots /

To delete all local snapshots:

sudo tmutil deletelocalsnapshots /

Or delete a specific snapshot:

sudo tmutil deletelocalsnapshots [date-string]

This can free 10–50 GB immediately.

2. Delete iOS/iPad backups

1. Connect your device, or open Finder and click on your device in the sidebar
2. Click Manage Backups
3. Delete old or duplicate backups

Or navigate directly to ~/Library/Application Support/MobileSync/Backup and delete backup folders for old devices.

3. Clear app caches

Open Finder → Go → Go to Folder → type ~/Library/Caches

Delete the contents (not the folders themselves) of caches for apps you recognize. The biggest offenders:

• Slack (com.tinyspeck.slackmacgap) - often 5–15 GB
• Chrome (com.google.Chrome) - 1–5 GB
• Spotify - 1–3 GB
• Xcode (com.apple.dt.Xcode) - can be 5–20 GB

4. Clear Xcode derived data

If you develop on your Mac, Xcode's derived data and simulator files can be enormous:

• Derived data: ~/Library/Developer/Xcode/DerivedData - safe to delete, rebuilds on next build
• iOS Simulators: Open Xcode → Window → Devices and Simulators → delete simulators you don't use
• Archives: ~/Library/Developer/Xcode/Archives - delete old ones you've already submitted

5. Manage iCloud Drive and Photos settings

If iCloud is set to download everything locally:

• Go to System Settings → Apple ID → iCloud → Photos → enable "Optimize Mac Storage"
• In iCloud Drive settings, you can choose not to keep files downloaded locally

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What you can't reduce in System Data

Some things in System Data are genuinely not removable:

• macOS core system files (~10–15 GB) - required for macOS to run
• Virtual memory swap files - managed by macOS, regenerate automatically
• Sleep image - equal to your RAM size on Apple Silicon; required for hibernate
• Core ML model data - a few GB, required for system features

If your System Data is 20–40 GB and you've already cleared snapshots, caches, and backups, the remainder is likely these non-removable items. That's normal.

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How to see exactly what's inside System Data

Apple's built-in storage overview doesn't let you drill into System Data at the file level. A disk analyzer does.

FreeUpDisk scans your entire drive - including folders in ~/Library and other hidden locations - and shows you every folder and file with its exact size. You can see your Xcode cache at 35 GB, your Slack cache at 12 GB, and your Time Machine snapshots at 30 GB, all at once, in a single visual view.

This makes it much easier to decide what to tackle, versus clicking around in System Settings and hoping for the best.

Get FreeUpDisk free →

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Summary

System Data source	Typical size	Removable?
Time Machine snapshots	10–50 GB	✅ Yes
iOS device backups	10–60 GB	✅ Yes
App caches (Slack, Chrome, Xcode)	5–30 GB	✅ Yes
Xcode DerivedData	5–40 GB	✅ Yes
macOS system files	10–15 GB	❌ No
Virtual memory / swap	= RAM size	❌ No
Core ML data	2–5 GB	❌ No

The cleanable portion - snapshots, caches, backups, derived data - can easily add up to 30–100 GB on a Mac that's been in use for a year or two.