The update needs more space than it says, and here is why
An update needs room to download, unpack and apply at once. What the real requirement is, and the fastest gigabytes when you are short.
The update says it needs 12 GB, you have 15 GB free, and it refuses. That is not a bug. An installer needs room for three things at the same time, and the number it shows you is only one of them.
What the space is for
- The download. The figure the update screen shows.
- The unpacked installer. Roughly the same again, since the download is compressed.
- The work of applying it. New system files are written before the old ones are released, so both exist for a while.
- A snapshot. macOS takes one before a major update so it can roll back if something fails.
That is why the practical requirement for a major upgrade is tens of gigabytes rather than the download size. For a small security update it is far less, but the same shape applies.
Check what you really have free
df -h /System/Volumes/DataThen compare that with what Finder claims. The two disagree because Finder counts purgeable space as available, and an installer often cannot use it fast enough. What purgeable space is explains the gap, and it is the single most common reason an update fails on a Mac that looks like it has room.
The fastest space, in order
- Empty the Trash. Including on any connected external drive, which has its own Trash.
- Old device backups. Frequently the single largest item on a Mac, covered in iPhone backups on a Mac.
- Downloads. Installers and disk images, none of which are the only copy of anything.
- Developer output. Build folders and caches, all rebuildable.
- Delete local snapshots. Only if you are still short after the above, since they are your rollback safety net.
tmutil listlocalsnapshots /
sudo tmutil deletelocalsnapshots <date>If the installer has already failed once
A failed update usually leaves the downloaded installer on the disk, which means you are now short by the size of a thing you cannot use. Look for an Install macOS application in Applications and remove it, then start again with the space it was holding.
Disk full after a macOS update covers the other half of this problem, which is a disk that fills up after the update rather than before it.
Common questions
How much free space does a macOS update need?
Considerably more than the download size, because the installer has to download, unpack and apply at once, and macOS takes a rollback snapshot first. For a major upgrade plan on tens of gigabytes free rather than the figure shown next to the update.
Why does my Mac say there is not enough space when there is?
Because Finder counts purgeable space, mostly local Time Machine snapshots, as available. The installer often cannot wait for macOS to release it. Running df against /System/Volumes/Data gives a more honest figure.
Can I delete local snapshots to make room for an update?
Yes, with tmutil deletelocalsnapshots, and it is usually effective. Do it after the easier options, since snapshots are what lets you roll back a failed update or recover a file deleted in the last day.
Where does the failed macOS installer go?
It stays in your Applications folder as an Install macOS application, several gigabytes in size. Removing it before retrying frees the space it is holding, and the update downloads a fresh copy.