Guide

Mac storage full: what to do first

macOS tells you the startup disk is nearly full and then leaves you to guess which folder is to blame. Guessing is how people end up deleting a Photos library. Measure first, cut once.

Why the warning arrives before the disk is really full

macOS needs free space to work, not just to store things: swap files, app and system updates, Time Machine local snapshots, and scratch space for anything that renders or exports. That is why the warning shows up with gigabytes still left. Below roughly five per cent free, apps start failing to save and updates refuse to install, so treat the notice as a deadline rather than an opinion.

Clear it in five steps, in this order

01
Empty the Trash Nothing you deleted this month has actually left the disk until the Trash is empty. Do this first so every number you look at afterwards means something.
02
Measure the whole volume before deleting anything About This Mac shows coloured bars, not folders. You need a per-folder measurement of the entire volume, sorted largest first, so the answer is a row on a list instead of a hunch.
03
Take the largest cut first One 40 GB folder beats four hundred small files. On most Macs the biggest safe cut is old installers and disk images, exported video, or developer caches. Removing few large things is faster and much easier to undo.
04
Then remove exact duplicates Copies pile up from AirDrop, downloads and imports. Matched byte for byte, a duplicate is the one delete you can make with no judgement call at all, because an identical copy remains.
05
Restart, then check the number again Local snapshots and purgeable space are released on macOS terms, not yours. A restart plus a few minutes idle often frees several gigabytes more, and it stops you from deleting real files chasing a number that was about to drop anyway.

What not to delete when you are in a hurry

Do not open Photos, Music or Mail libraries and delete files inside them — they are packages with an index, and pulling files out of one corrupts it while barely saving space. Leave the sealed system volume and other user accounts alone; macOS will refuse anyway. Do not delete a Time Machine backup volume to make room on your Mac: snapshots and hard links make its sizes lie, and you may break the restore chain you are about to need. And be careful with anything in iCloud Drive that shows a cloud icon — it is already off your disk, so removing it deletes the only copy.

Where Sift fits

Sift measures every folder on the volume in one pass, byte by byte, and puts the largest first — including the System Data macOS refuses to itemise. Nothing is pre-selected, deletes go to the Trash, and ⌘Z puts the last move back. Scanning and browsing are free forever, so you can find out where the space went before you decide to pay for anything.

Measure your disk with Sift — scanning is free

Common questions

Why does my Mac say storage is full when I deleted files?

Two usual reasons. The files are still in the Trash, which keeps them on disk, or the space became purgeable rather than free — local snapshots and cached iCloud files that macOS releases when it decides to. Empty the Trash, restart, then measure again.

How much free space should a Mac have?

Aim for at least ten per cent of the volume, or 20 GB, whichever is larger. That leaves room for swap, updates and Time Machine snapshots. Below five per cent, macOS starts refusing to install updates and apps can fail to save.

Does deleting apps free much space?

Rarely. The app bundle is usually small compared with what it left in caches, support folders and project files. Xcode is the classic example: the app is a few gigabytes, its DerivedData and simulators can be ten times that.