Not enough space to install a macOS update: what to do

Why an update needs so much room

An update is downloaded, unpacked and prepared before anything is installed, and for part of that time the old and new system files exist side by side. A full macOS upgrade is a download of more than 10 GB, and the installation needs working room on top of that. Smaller point updates need less, but the same principle applies.

macOS also takes a snapshot of the system before updating, so it can go back if the installation fails. All of this is temporary. Most of the space returns once the update has finished and the Mac has restarted.

Find out how much is missing

Open System Settings, choose General, then Software Update. If there is not enough room, the message states how much more is needed. Note that number.

Then open General, then Storage, to see how much is available now and which categories are largest. The command below shows the strict free figure for your data volume. It only reads.

df -h /System/Volumes/Data

Clear these first

These give back space quickly and cost you nothing you cannot get again.

• Empty the Trash, after a glance at what is in it.
• Old macOS installers. Look in the Applications folder for apps named Install macOS followed by a version name. Each is a complete installer from an earlier download. Move them to the Trash. They can be downloaded again from Apple.
• Disk images and installer packages in Downloads for apps you have already installed.
• iPhone and iPad update files. If ~/Library/iTunes contains an iPhone Software Updates or iPad Software Updates folder, the .ipsw files inside are installers that can be downloaded again.
• App caches. Quit your apps, open ~/Library/Caches and move the contents of the large folders to the Trash. Apps rebuild them.

If that is not enough

The next steps free more, but each has a cost, so read the note with each one.

• Move large files you want to keep, such as videos and disk images, to an external drive. Copy, check the copy opens, then delete the original.
• Remove apps you no longer use. In Storage, click the info button next to Applications and sort by size. You lose the app, and it can be installed again.
• Old iPhone and iPad backups. In Storage, click the info button next to iOS Files. Delete only backups of devices you no longer own, or devices that also back up to iCloud. A deleted backup cannot be recovered.
• Developer files. Xcode build data, simulators and package caches are recreated by the tools when needed. The cost is a slower next build.
• Downloaded music, films and podcasts that you can stream or download again.

When free space does not rise after deleting

If you delete a large amount and the available figure barely changes, a local Time Machine snapshot is holding the deleted files. macOS releases snapshots when it needs room, and the installer normally benefits from that, but not always straight away.

Restart the Mac and check Software Update again. If it still refuses, list the snapshots with the first command below. The second asks macOS to release up to 20 GB from local snapshots, oldest first. You lose the ability to restore files as they were at those moments from the Mac itself. Backups on your Time Machine drive are not affected.

tmutil listlocalsnapshots /
tmutil thinlocalsnapshots / 20000000000 4

Before you install

Make a backup. An update on a nearly full disk is the situation in which a backup matters most. Connect your Time Machine drive and let a backup finish, or confirm that your important files are in iCloud or another service.

Keep the Mac plugged in, and leave a margin above the amount the installer asked for. An installation that runs out of room part way is far more trouble than freeing a few extra GB first.

How much free space to keep from now on

Apple does not publish a single figure. In practice macOS needs working room every day for swap when memory fills up, for snapshots, for caches and for app updates, and it behaves badly when that room is gone: apps fail to save, and the Mac slows down.

A practical habit is to keep enough free for the next system update plus normal working room, which on most Macs means a few tens of GB. If you are regularly below that, the disk is too small for what you keep on it, and it is worth moving a large library such as Photos or video projects to an external drive.

Finding the space with Diskzy

Diskzy maps the whole disk, so the largest folders and files are visible without opening each one. Its Clean up tab lists caches, logs, Xcode leftovers, project build folders, package-manager caches and old installers, each ticked or unticked with a reason.

Everything it removes goes to the Trash, so remember to empty the Trash before you run the update again. Until you do, the space is not free.