How to free up disk space on a Mac or PC, safely

First rules for any computer

• Back up before a big clean. Time Machine on a Mac, File History or a system image on Windows, or any backup you trust. A clean should never be the moment you find out your backup is broken.
• Keep 10 to 15% of the disk free. Both systems need room for updates, virtual memory, caches and snapshots. A disk that is 98% full gets slow, and updates can fail.
• Know what you delete. Size alone says nothing. A 20 GB folder can be a cache that rebuilds in a minute, or the only copy of your project. If you can't tell what something is, don't delete it.
• Don't "clean" the system. System folders, language files inside apps, the Windows Installer folder and WinSxS are not junk. Removing them by hand breaks apps and updates.
• Restart now and then. A restart clears temporary files, finishes updates and releases space that running apps still hold on to.

Keep your Mac healthy

What is taking up space on my Mac? Start with Storage settings

Open System Settings → General → Storage. The bar shows what uses the space, and the list below it gives Apple's own recommendations, such as Store in iCloud, Optimise Storage and Empty Trash Automatically. Click the ⓘ next to a category, such as Applications or Documents, to see and remove large files.

A big System Data slice is normal. It includes caches, logs, Time Machine local snapshots, virtual memory files and sometimes developer files. Much of it is managed by macOS and shrinks on its own when space runs low. Don't try to delete it from the Finder.

Empty the Trash, tame Downloads

• Files in the Trash still use space. In Finder → Settings → Advanced, turn on Remove items from the Trash after 30 days.
• Delete the .dmg installer once the app is in Applications. Eject the disk image first.
• Sort Downloads by size now and then. Old videos, zip files and installers are the usual culprits.

Tidy Tank lists big, old downloads, installers and duplicates under Downloads & Trash when the Downloads and apps box is on. They go to the Trash, never straight to deletion, and are never ticked for you.

Xcode: DerivedData, simulators and device support

For developers, Xcode is often the biggest single space user on the Mac. What you can delete:

• DerivedData (~/Library/Developer/Xcode/DerivedData): build output and indexes. It is safe to delete when Xcode is closed. Xcode rebuilds it, and the next build of each project is slower. You can also use Product → Clean Build Folder per project.
• Simulators: xcrun simctl delete unavailable removes simulators for runtimes you no longer have. In Xcode, Window → Devices and Simulators lets you delete single devices.
• Simulator runtimes: each iOS or watchOS runtime takes several GB. List them with xcrun simctl runtime list, and remove old ones in Xcode → Settings → Components (called Platforms in older Xcode).
• iOS DeviceSupport (~/Library/Developer/Xcode/iOS DeviceSupport): symbol files for each iOS version you connected a device with. Old versions are safe to delete. Xcode copies them again when you connect a device with that version.
• Archives (~/Library/Developer/Xcode/Archives): only delete archives you will never need again. They hold the dSYM files used to read crash reports for shipped builds.

Tidy Tank finds all of these. DerivedData is labelled Safe but stays locked while Xcode is running. Simulators are removed through Xcode's own simctl tool, never by deleting files. Because Xcode's test-device copies are APFS clones, Tidy Tank measures what really comes back. On one Mac they looked like 243.3 GB and would really free 28.0 GB. Why the two numbers differ.

Other developer leftovers

• Old node_modules and build folders in projects you no longer work on. They come back with npm install or a build. Leave the ones you use every day.
• Package caches: brew cleanup removes old Homebrew downloads and versions. npm cache verify tidies the npm cache. pip cache purge empties pip's cache.
• Docker: docker system df shows what images, containers, volumes and build cache use. docker system prune removes stopped containers, unused networks, dangling images and build cache. Add -a for all unused images, and --volumes only if you are sure no volume holds data you need. Docker Desktop stores everything in one disk image. You can set its size limit in Docker Desktop's settings.

Time Machine local snapshots

When Time Machine is on, macOS keeps local snapshots on your startup disk, usually for about 24 hours. They let you restore files even when the backup disk isn't connected. A snapshot keeps deleted files' space until it is removed, which is why space sometimes doesn't come back right after a clean.

• See them: tmutil listlocalsnapshots /
• macOS removes them on its own as they age and when space runs low. You rarely need to do anything.
• If you must free space now: tmutil deletelocalsnapshots <date> removes one snapshot.

Tidy Tank lists local snapshots under System & snapshots as Check first, and removes them with Apple's tmutil if you tick them.

iPhone and iPad backups

Local device backups can take tens of GB each. Manage them in Finder: select your device, then Manage Backups. Never delete folders inside ~/Library/Application Support/MobileSync by hand. Tidy Tank shows their size as Look only and points you to Manage Backups.

iCloud Drive, OneDrive and other cloud folders

Deleting a file inside a sync folder deletes it in the cloud and on your other devices. To free space but keep the file, right-click it in Finder and choose Remove Download (iCloud) or Free Up Space (OneDrive, Google Drive, Dropbox). Turning on Optimise Mac Storage in iCloud settings lets macOS do this for you.

What not to delete on a Mac

• Anything in /System, /usr or /private/var (macOS protects most of it anyway).
• Language files (.lproj) inside apps. "Removing languages" saves little and breaks app signatures and updates.
• The Mail, Messages and Photos libraries. Remove items inside the apps instead.
• Virtual memory and sleep image files. macOS manages them.
• Browser caches, as a habit. They speed up the sites you visit and fill again within days. Clear them only to fix a problem.

Disk health on a Mac

• Mac SSDs never need defragmenting, and macOS runs TRIM on Apple SSDs automatically.
• Check the disk's status with diskutil info disk0 | grep SMART, or in Disk Utility, where First Aid checks and repairs the file system.
• Keep macOS up to date. Updates often fix storage bugs too.

Keep your Windows PC healthy

Turn on Storage Sense

Open Settings → System → Storage and turn on Storage Sense. It deletes temporary files, empties the Recycle Bin after a time you choose, can clean Downloads (off by default), and can turn unused OneDrive files into online-only ones. Under Cleanup recommendations Windows suggests large files, unused apps and files already synced to the cloud.

Temporary files and Windows Update leftovers

In Settings → System → Storage → Temporary files you can remove, among others:

• Windows Update Cleanup: old update files, often several GB.
• Delivery Optimization Files: update pieces shared with other PCs.
• Previous Windows installation(s): kept after a big upgrade so you can go back, and removed automatically after 10 days. Delete it sooner only if you are sure you won't roll back.
• Temporary files in %TEMP%. Files still in use are skipped.

The classic Disk Cleanup tool (cleanmgr) still works. Click Clean up system files to see the same system items.

WinSxS and the component store

The C:\Windows\WinSxS folder looks huge, but much of its size is hard links to files that also live elsewhere in Windows. Never delete anything in it by hand. To shrink it safely, run this in an administrator terminal:

DISM /Online /Cleanup-Image /StartComponentCleanup

Windows also runs this cleanup on a schedule.

Hibernation and page files

• hiberfil.sys can take several GB. If you never hibernate, powercfg /h off (in an administrator terminal) removes it. This also turns off Fast Startup. powercfg /h /type reduced keeps Fast Startup with a smaller file.
• pagefile.sys is virtual memory. Don't delete it or turn it off. Let Windows manage its size.

What not to delete on Windows

• C:\Windows\Installer: apps need these files to repair, update and uninstall. Deleting them breaks that.
• WinSxS, System32 and anything else in C:\Windows by hand.
• The Windows registry, through "registry cleaners". They don't free meaningful space and can break things.
• Files inside a OneDrive folder, if you want to keep them. Right-click and choose Free up space instead.

Developer leftovers on Windows

• The same rule applies to node_modules, target, bin/obj and other build folders of old projects: they rebuild.
• Package caches: npm cache verify, pip cache purge, and the NuGet cache with dotnet nuget locals all --clear.
• WSL: each Linux distro lives in a virtual disk that grows but doesn't shrink on its own. On recent WSL versions, wsl --manage <distro> --set-sparse true lets it give space back.
• Docker Desktop: docker system prune as on the Mac, or Troubleshoot → Clean / Purge data in Docker Desktop.

System Restore

Restore points are useful, but their space can grow. Under System Properties → System Protection → Configure you can set a maximum, often 5 to 10% of the disk.

Disk health on Windows

• Settings → System → Storage → Advanced storage settings → Disks & volumes shows the health and temperature of NVMe SSDs.
• Optimize Drives runs TRIM (retrim) on SSDs and defragments hard drives on a schedule. Leave it on. Don't run third-party defrag tools on an SSD.
• With Fast Startup on, Shut down doesn't fully restart Windows. Use Restart when updates or a stuck app need a clean start.

Tidy Tank for Windows uses the same engine as the Mac app. It follows the same rules: Windows system folders and C:\Windows\Installer are never offered, and nothing risky is ticked for you.

A monthly five-minute checklist

1. Look at the storage bar. Is free space under 15%?
2. Empty the Trash or Recycle Bin.
3. Sort Downloads by size. Delete installers and anything you no longer need.
4. Developers: clear DerivedData and old simulators, prune Docker, and delete build folders of finished projects.
5. Check that your backup ran.
6. Install pending system updates and restart.

Or let Tidy Tank do the looking: open it, read what it found, tick what you want gone and clean the tank. The FAQ explains exactly how it decides what is safe.