How much free space a Mac actually needs

Keep 25GB free on a 256GB Mac, 40GB on a 512GB and 60GB on a 1TB. Those are not percentages, because what needs the space does not scale with the drive. Swap and update headroom are close to fixed costs. Only the flash headroom grows.

Ask this anywhere and you get a percentage. Ten per cent, fifteen, twenty. Sometimes a flat twenty gigabytes. Most of those answers arrive attributed to Apple, which has never published any of them.

The working figures

Notice the third column falling as the drive grows. That is the part every percentage answer gets wrong.

Why a percentage is the wrong shape of answer

Four things want that free space. Three of them cost roughly the same whatever drive you own.

Swap does not care how big your disk is. It scales with how much memory you have and how hard you work the machine. An update stages itself at a size set by the release, not by your drive. Local snapshots track how much you change day to day, which is a habit rather than a capacity.

Only the fourth grows with the drive. That one is the flash headroom the controller needs.

So ten per cent of 256GB is about right and ten per cent of 4TB is 400GB of deliberately wasted disk. A flat number under-reserves on a small drive and over-reserves on a large one. The table above is built the other way round, from what the space is for.

Swap

macOS writes memory it is not using right now to disk. It creates those files as it needs them rather than reserving a fixed block in advance.

On a machine with plenty of memory and a couple of applications open, that stays small. On one with 8GB running a browser with forty tabs, a video call and an editor, it does not. Swap can reach several gigabytes without anything unusual happening.

The failure here is quiet. Memory pressure climbs. macOS compresses harder to avoid writing. Everything slows down before anything warns you.

Snapshots

If Time Machine is on, macOS keeps local snapshots on the startup disk and holds each for about a day.

Their cost is not the size of your files. It is the size of what you changed. A quiet week costs almost nothing. A day spent re-encoding video costs a lot. The space comes back on its own schedule rather than yours.

Finder counts most of that as available, which is why the number you are looking at is already softer than it appears. That accounting has an answer of its own.

Update headroom

A macOS update needs considerably more room than the installer size suggests, because it stages a full copy before applying anything and takes a snapshot on the way in.

This is the one that turns a slow Mac into a stuck Mac. It arrives once or twice a year on a schedule you do not set. The gap between the installer size and the real requirement is covered in detail there.

Budget for it in advance rather than clearing space in a hurry on the morning it appears.

Flash headroom

The last one is not macOS at all. It belongs to the drive.

Flash cannot overwrite a page. It erases in whole blocks, so the controller keeps a pool of already erased blocks to write into. It refills that pool in the background by consolidating half used blocks and erasing what is left.

When free space runs low, that pool runs low with it. The controller ends up shuffling live data around to clear a block for every incoming write, which is why writes to a nearly full drive get slower rather than just later. Measured degradation becomes noticeable past roughly 85% full and severe in the last few per cent.

This part is real and it is not macOS. The same behaviour shows up on any SSD in any machine. It is the reason the reserve should grow with the drive even though nothing else does.

The question this is not

Two things get confused with each other here.

How much space a Mac needs free is a headroom question with one answer per drive size. How much space you happen to have used is a different question with a different answer, changing every day.

The table is a floor to stay above rather than a target to hit. Nothing improves by leaving 200GB empty on a 512GB drive. Once the four things above have what they need, further empty space does nothing at all.

That is worth saying because the percentage answers imply otherwise. Read as advice, fifteen per cent on a 4TB drive tells you to keep 600GB permanently unused. No part of macOS or the drive would notice.

The warning is not the line

macOS shows a startup disk almost full alert. Apple does not publish the threshold that triggers it. Reports put it very low, in the region of a gigabyte or two remaining.

Whatever the exact figure, it lands long after the trouble starts. By the time it appears, swap is already constrained, an update would already fail and the drive has already been writing slowly for months.

Treat it as a fire alarm rather than a fuel gauge. The numbers in the table are the fuel gauge.

What a full disk looks like before it breaks

The symptoms arrive in a fairly reliable order.

Saves and exports take longer than they used to. Applications beachball on operations that used to be instant. A software update refuses to install, then starts and stops halfway. Photos and Mail stop syncing new items without saying why. Then things start failing to save.

None of those announces itself as a storage problem, which is why the disk is rarely the first thing anybody suspects.

Checking where you stand

The figure in Storage settings is the one to read, with the caveat that it already counts purgeable space as available.

If the number is below the row for your drive, the next question is what is taking the room, which is a different job from this one. Reading the storage bar properly is where that starts.

One habit is worth more than any number here. Check when things are fine rather than when something has already failed, because every option is cheaper with 40GB free than with two.