What is actually using storage on your Mac

The macOS storage bar lives in System Settings, General, Storage. It is less literal than it looks. Several categories count something narrower than their name suggests, one is not measured at all but calculated from what is left over. The whole view reports space taken on disk rather than the sizes you see in Finder.

The usual instinct is to start deleting. The trouble is that the bar you are reading to decide what goes is not saying quite what it appears to say. People remove a photo library while the actual problem sits in a category they never opened.

Where the Mac storage bar lives now

On macOS Ventura and later, which covers Sonoma, Sequoia and Tahoe, it is in System Settings, then General, then Storage. On Monterey and earlier the only route was About This Mac. The old route still works as a shortcut. Apple menu, About This Mac, More Info, Storage Settings drops you in the same place. What has gone is the separate Manage window that older guides describe, so screenshots from that era will not match what you see.

The pane has three parts. They are not equally useful.

Store in iCloud moves Desktop and Documents to iCloud Drive and lets you download originals again on demand. Empty Trash Automatically clears anything older than thirty days. Apple also notes that on a partitioned disk these apply only to the partition holding your home folder.

The All Volumes button widens the bar to other disks but gives no detail for them, so anything beyond the boot disk is a Disk Utility job. The two volumes it shows for the startup disk are explained in why a Mac has two Macintosh HD volumes.

One Recommendation is named confusingly. Optimise Storage in this pane is not the same as Optimise Mac Storage in iCloud Drive settings. The one here removes TV content you have already watched. The other offloads files to iCloud. People switch on the first expecting the second.

What each macOS storage category actually counts

This is where the reading goes wrong. Category names imply a folder. What they represent is a rule about which files get counted. Several of those rules are narrower than the name.

The Photos rule catches people out more than any other. Anyone with an old library still on disk sees a small Photos figure and blames the storage bar. It is not. macOS counts the second library somewhere else.

One more rule applies across the whole view. Storage reports what a file occupies on disk, not its nominal size. A virtual machine created as a 100GB sparse file shows as 62GB, because 62GB is what it has actually written. Finder shows you the same file differently, which is the beginning of most disagreements about Mac storage.

Why this pane cannot explain System Data

Every other segment on the bar is measured. That one is left over, so the pane has nothing to show you and leaves it as the only category with no More Info button.

For diagnosis that means one thing. Note the number and stop there. Take the question to Disk Utility for snapshots and reclaimable space, then to a disk mapping tool for whatever remains. Nothing in Storage settings will take you further.

Which storage Info buttons do something

Several categories carry a More Info button and the quality varies wildly. Knowing which ones repay a click saves a lot of wandering.

Apple names seven as particularly useful. In practice two of those open nothing you can act on, so the list below is what the buttons actually do rather than what the documentation claims. The recommendations they lead to are unpicked in what Optimised Storage and Store in iCloud actually do.

Why your Mac storage numbers move while you watch

Open the pane, look away, look back and the figures have shifted. Nothing is broken.

macOS calculates these totals in the background rather than holding them ready, so the view fills in as it goes. At the same time the disk is genuinely changing underneath. Snapshots age out and release space. Caches get thinned. An update leaves files behind and collects them later.

Apple says the same thing in its own guidance, which is to wait until every category has finished calculating before trusting any of the values.

The practical response is to stop treating any single reading as the truth. Take a figure, do something else for ten minutes, take it again. A number that settles was mid calculation. A number that keeps climbing is worth investigating.

Why Finder, Storage settings and Disk Utility disagree

Three tools will give you three answers for the same disk. Each is answering a different question.

There is a deeper reason they cannot agree. On APFS a copy does not have to duplicate the data, so two files can share the same blocks on disk. Ask which folder those blocks belong to and there is no single honest answer. Files that live in the cloud with only a stub on disk complicate it further, because the stub is small and the file is not.

So the disagreement is not a bug in one of them. It is three tools applying three schemes to a disk where the question itself has more than one correct answer. The reason the question has several answers is the way the Mac file system allocates space.

How to work out what is taking up space on your Mac

Work down from the largest number rather than around the edges. The order matters, because the top of the list is where the big wins are and the bottom is where people waste an afternoon. If Mail is high on your list, why Apple Mail takes up so much disk space is the faster route.

1. Read the bar and write down the three biggest segments. Not the ones that look suspicious. The biggest.
2. Open Disk Utility and note snapshots and reclaimable space. If that figure is large, it explains most of what the System Data category is holding before you look anywhere else.
3. Click More Info on each of the three. Applications, Documents, Developer and iOS Files all open something usable.
4. Check the Photos assumption. Look for a second library before concluding anything about that number.
5. Map what is left. A disk mapping tool draws the residue as a picture, which beats guessing at folders. GrandPerspective and OmniDiskSweeper are free and do nothing but look.
6. Only now decide what goes. By this point you know what you are deleting and why.

When to stop looking and act

Diagnosis has a point of diminishing returns. It is earlier than most people think.

A System Data figure in the tens of gigabytes on a machine that is not short of space is not a problem to solve. It grows after an upgrade, after a backup and after any spell of heavy application use. It settles on its own. Chasing it costs you an evening and gains you nothing.

Two situations do deserve the time. One is a disk that is genuinely close to full, where the machine has started behaving oddly. The other is a category that keeps climbing week on week with no explanation, which points at something accumulating rather than something normal.

If you are out of space right now and cannot wait to diagnose. Empty the Trash, then restart. Both are safe and both are immediate. A restart also releases space macOS was holding but could not free while things were open. Diagnose afterwards, with room to work in.

Everything else is curiosity. Satisfying it is fine, as long as nobody deletes a photo library to scratch the itch.