everyday mac tools

How much free space should a Mac have? Working out your own number

· 6 min read

A Mac needs enough free space, without you clearing anything first, to install the next macOS update or finish the biggest job you routinely do (an iPhone backup, a video export) while still writing swap on your heaviest day. Take the larger of the update and that job, then add your swap and a margin for caches. That total is your floor, and it is a number of gigabytes rather than a percentage.

For most Macs the update alone puts the floor into the tens of gigabytes. The rest of this post is how to work out your own figure and how to read the free space number honestly, because the Mac shows you more than one.

Why a percentage rule does not fit

Rules of thumb usually quote a percentage of the disk. They are a convenient shorthand, but the things that actually need room have fixed sizes. A macOS upgrade asks for the same amount of space on a 256 GB Mac as on a 2 TB one. So does a 40 GB iPhone backup.

A percentage therefore leaves a big disk with far more headroom than it needs, and leaves a small one short at exactly the moment an update arrives. Working in gigabytes from what you actually do gives you a figure you can defend, and one that does not change when you buy a bigger Mac.

What the free space is actually for

Swap. When the Mac runs out of physical memory, it writes some of it to disk, as a row of files named swapfile0, swapfile1 and so on in /System/Volumes/VM (1 GB each on the Mac this post was checked on). The row grows on heavy days and through long stretches without a restart. Run sysctl vm.swapusage after a heavy day to see your own total. If there is nowhere to write swap, memory pressure turns into the warning that your system has run out of application memory. Swap files on a Mac covers why it grows.

macOS updates. An update needs considerably more room than its download, because the installer is stored, unpacked, and the old system is kept until the new one is in place. Full installers for recent major versions have run to well over 10 GB, and the free space they ask for is a good deal more. Software Update names its own figure when it refuses, so use that number when you have it.

Caches and temporary files. Browsers, Photos, Mail, developer tools and video apps all write caches, and many apps write a temporary copy while they export or save. These grow and shrink through the day; you need room for the high point, not the average.

Snapshots and iCloud copies. Time Machine keeps local snapshots on the internal drive, and Optimize Mac Storage keeps local copies of iCloud files. Both count as purgeable, and both are the first things macOS gives up when space runs short. That is the safety valve working, but the cost is real: your local restore points disappear, and files go back to being download-on-open.

Your own one-off jobs. An iPhone or iPad backup to the Mac needs room for the whole backup. Exporting a video needs room for the export. Downloading a large disk image and installing from it needs room for both at once.

Work out your floor

  1. Swap. After your heaviest normal day, run sysctl vm.swapusage and note the total.
  2. The update. Use the figure Software Update gave you last time, or allow for an installer of well over 10 GB plus working room on top.
  3. Your largest routine job. Look at your last iPhone backup or your largest export. System Settings, General, Storage lists device backups under iOS Files; the info button beside it shows each one.
  4. A margin for caches. A few gigabytes, more if you edit video or build software.

An illustration, with made-up figures: say your swap peaks at 6 GB, the last upgrade wanted 30 GB, and your iPhone backup is 40 GB. You do not back up the phone during an upgrade, so take the larger of those two, add the swap and a margin, and the floor is somewhere around 50 GB. Your own figures come from the steps above.

Treat that figure as the point where you start clearing, not a target to stay just above. If clearing never gets you back over it, the honest conclusion is capacity rather than housekeeping, and not enough space to update macOS covers what to clear first when the update is the thing that is stuck.

What happens as a Mac runs out

The signs arrive roughly in this order:

  1. macOS quietly reclaims purgeable space. Local snapshots are thinned and, if Optimize Mac Storage is on, iCloud files lose their local copies. Nothing tells you, except that a file now shows a cloud icon and will not open on the train.
  2. Finder still says there is room. The Available figure includes purgeable space, so it looks better than it is.
  3. Updates can refuse to start even when Finder shows enough, because purgeable space is not always released when the installer asks for it. Purgeable space on a Mac explains the gap.
  4. Copies and exports fail. Finder says the item “can’t be copied because there isn’t enough free space”; other apps word it their own way.
  5. The alert appears: “Your disk is almost full” on recent versions (older ones say “Your startup disk is almost full”).
  6. The Mac gets sluggish under load, because swap and temporary files have nowhere to go, and apps that cannot write their caches can misbehave in ways that never mention disk space.

Read the real free figure

The Mac reports free space in several places, and they disagree for good reasons.

Finder. Select your startup disk (usually Macintosh HD) in the Finder sidebar, or in Go, Computer, then choose File, Get Info. The Available line may read like “62 GB (24 GB purgeable)”. That first figure includes the purgeable part. The status bar at the bottom of a Finder window (View, Show Status Bar) shows an available figure too.

System Settings, General, Storage. Good for what is using the space, by category, after it finishes calculating. Less good for how much is free this minute; it recalculates in the background and can lag, and much of what matters is rolled into System Data.

Terminal. This is the number an installer cares about:

df -h /System/Volumes/Data

Read the Avail column. diskutil info / reports the same thing on its Container Free Space line. Neither includes purgeable space, so either can be well below what Finder shows.

Use the Terminal figure when you compare against your floor. Use Finder’s when you want to know how much room you could have if macOS reclaimed everything it is allowed to.

When the figure is below your floor and you do not know why

The usual answer is something Finder does not show: snapshots, swap, other user accounts, old device backups, or a System Data figure nobody can explain. Crumb runs a read-only audit of the whole Mac that breaks System Data into caches, logs, swap, temporary files and leftovers, and lists local snapshots, iOS backups and which account on a shared Mac is using the space. Reclaiming is by category, with the total shown before you commit, and nothing is deleted until you approve the plan. The audit and one full cleanup are free. It does not repair disks or recover deleted files, so check your backup is current before clearing anything you cannot regenerate.

Questions

Is it bad for an SSD to be nearly full? It does not damage it. Drives generally write more slowly when they are close to full, and macOS has less room for swap and temporary files, so the Mac feels slower under load. Free space comes back as soon as you clear some.

Does free space on an external drive count? Not for swap, caches or macOS updates, which all need the startup disk. An external drive helps by giving large files of your own somewhere else to live.

Why does my free space change when I have not done anything? Swap, caches, snapshots and iCloud downloads move it all day. A swing of a few gigabytes between morning and evening is normal.

Should a bigger Mac keep more free? The same number of gigabytes, if you do the same work. A bigger disk simply takes longer to reach your floor.