How much free space does a Mac really need?: dux

The short answer

Keep at least 10 percent of your startup disk free, and never less than about 30 GB. If you install macOS updates as soon as they arrive, edit video, or build software, aim for 15 to 20 percent.

Apple does not publish a single target, and a Mac will keep working below these numbers. But this range leaves room for the things macOS does in the background, and it keeps you out of the zone where updates fail and apps start to complain.

Why a Mac needs headroom

Free space is not idle space. macOS borrows it all the time, often without you noticing.

macOS updates

A macOS update downloads first, then unpacks and prepares itself before it restarts. That needs much more space than the download size shown in Software Update, and a major upgrade needs more than a point update. If space runs out halfway, the update stops and asks you to free space before it can continue. A buffer of a few tens of gigabytes is a safe margin for most updates. For the details, see how to make space for a macOS update.

Virtual memory (swap)

When your apps need more memory than the Mac has, macOS compresses memory first, then moves the least used parts to swap files on the disk. Those files live in /System/Volumes/VM and can grow to many gigabytes on a busy day. You can see how much swap is in use:

~ % sysctl vm.swapusage
vm.swapusage: total = 6144.00M  used = 5120.25M  free = 1023.75M  (encrypted)

With very little free space, macOS cannot grow swap. Apps then slow down sharply, and you may see a message that your system has run out of application memory. A restart resets the swap files.

Snapshots, caches and temporary files

Time Machine keeps hourly local snapshots, and apps write caches and temporary files while they work. Apple notes that local snapshots are stored only on disks with plenty of free space, so a full disk also quietly reduces how far back you can restore without your backup disk. More in Time Machine local snapshots taking up space.

The SSD itself

The SSD in your Mac manages its own free blocks to spread wear and keep writes fast. A drive that is almost full has less room to do that, and large writes can become slower. You will not notice this at 70 percent full. You may notice it at 98 percent.

A practical target for your Mac

Use the larger of the two numbers in each row.

How you use your Mac	Keep free	And at least
Email, web, documents	10 percent	30 GB
Photos and music libraries	15 percent	50 GB
Video editing, virtual machines	20 percent	100 GB
Software development, Xcode	15 to 20 percent	60 GB

On a 256 GB Mac, that means aiming for roughly 30 to 40 GB free. On a 1 TB Mac, around 100 GB gives you comfortable headroom. These are working rules of thumb, not Apple requirements.

Which free space number to trust

Your Mac shows at least two different free space figures, and the difference matters here.

Available in Storage settings (System Settings, then General, then Storage) and in Finder includes purgeable space: data macOS can remove on its own, such as local snapshots and iCloud files that are already in the cloud.
Free in Terminal (df -h, the Avail column) is what is empty right now, with nothing to clear first.

~ % df -h /System/Volumes/Data
Filesystem    Size  Used  Avail Capacity iused ifree %iused  Mounted on
/dev/disk3s1  460Gi 401Gi  38Gi   92%    2.4M  398M    1%   /System/Volumes/Data

For the targets above, use the smaller, truly free number when you plan a big install, and the available number for everyday comfort. macOS does clear purgeable space when it needs to, but it does so on demand, and not every installer waits for it. See Purgeable space on macOS explained for why the two numbers drift apart.

How to get back to a healthy margin

If you are below your target, work from the cheapest step to the most involved:

Restart. It resets swap and clears many temporary files.
Empty the Trash. Files in the Trash still use space until it is emptied.
Look at Downloads. Old installers and disk images are often the easiest gigabytes on a Mac.
Find what is really large. Sort folders by size before you delete anything. A few big items usually matter more than hundreds of small ones.
Use the Storage recommendations. Store in iCloud, Optimize Storage and Empty Trash Automatically each help macOS keep space free over time.

If System Data is the largest part of your Storage bar, read What is System Data on a Mac before you remove anything.

Keep an eye on it

Free space rarely runs out overnight. It drifts down as libraries, caches and projects grow. Checking it once a month, or when you plan a macOS upgrade, is usually enough to stay well clear of the red zone.