How much space does a macOS update need? Several times its download size, and Apple doesn't publish a figure for most updates. The size Software Update shows is the download. The room the update needs while it unpacks, checks and installs itself is larger, and macOS never shows it to you.
Measured cases give a sense of scale. A macOS Ventura point update with a 2.53 GB download needed about 14 GB free before it would start.
A macOS Tahoe point update with a 3.68 GB download worked out that it needed about 16.9 GB to prepare. The last major upgrade Apple put a number on, macOS Big Sur, asked for 35.5 GB.
So for an update within your version, plan on 15 to 20 GB of truly free space. For a major upgrade, plan on 35 GB or more. Those are planning figures drawn from the cases below, not Apple's; Software Update checks the real requirement before it downloads, and its answer is the one that counts.
This post is about the sizing question: what the numbers mean, why they differ from Mac to Mac, and how to read them. If you already know you're short, making room for a macOS update goes through where to find the space.
How much space does a macOS update need, in short?
The answer depends on three things: whether it's an update or an upgrade, which version your Mac is coming from, and whether your Mac has Apple silicon. The table pulls together what Apple publishes and what has been measured.
| Kind of update | Example | Download | Free space it needed |
|---|---|---|---|
| Point update, measured | Ventura 13.0 to 13.1 | 2.53 GB | About 14 GB |
| Point update, measured | Tahoe 26.2 to 26.3 | 3.68 GB | About 16.9 GB to prepare |
| Full update for a Mac far behind | Tahoe 26.3, full version | 17.48 GB | Not measured |
| Major upgrade, Apple's figure | Sierra or later to Big Sur | About 13 GB | 35.5 GB |
| Major upgrade, Apple's figure | Before Sierra to Big Sur | About 13 GB | Up to 44.5 GB |
| Major upgrade today | Tahoe 26 to Golden Gate 27 | Shown in Software Update | Not published |
The measured cases come from Eclectic Light, which tested updates in virtual machines and read macOS's own logs. Big Sur's figures are from Apple's technical specifications for that version. Each case is described in full further down.
Is the download size the space you need?
No. The download is compressed, and the installer has to keep it, unpack it, check it and stage a new copy of the system before it restarts. Eclectic Light put it plainly: the download sizes reported to the user are far smaller than the free space needed to prepare and install the update.
In the measured cases, a point update needed four to six times its download. For Big Sur, the one major upgrade with both figures, the room Apple asked for was under three times the installer's size.
Why does Apple not give one number?
Apple's current pages don't give a storage figure for macOS 26 Tahoe or macOS 27 Golden Gate. Its guide to troubleshooting updates says only that if the message says your Mac doesn't have enough storage space, you should free up storage space.
The size really does vary. The same update has different downloads for different starting versions, for Apple silicon and Intel, and for Macs that have fallen behind. A single published figure would be wrong for most Macs.
What does Software Update do about it?
It checks before it starts. Eclectic Light found that macOS works out the space an update will need before any download begins, and cancels the update if there isn't enough. It checks again while it prepares the update's parts.
An Apple engineer put it in five words on Apple's developer forums: "Disk space is a hard limit." There's no setting that lets an update install without the room it asks for.
Download size and working space are two different numbers
Think of an update as a delivery. The download is the box that arrives. The working space is the floor you need to unpack it, check every part, assemble the new system beside the old one, and swap them over.
What happens to free space during an update?
Eclectic Light traced a macOS 26 update through its stages in macOS's own logs. Space is used at each of them, and some of it at the same time:
- Checking the size. Before downloading, macOS estimates the space the update needs and stops if it isn't there.
- Downloading. The update arrives as a compressed archive and is unpacked as it streams in, so the download and its unpacked contents are written at once.
- Preparing. macOS checks the update's parts and builds what it will install, including a new system volume and its supporting pieces.
- Applying. The Mac restarts, installs firmware, makes the new system snapshot, seals it and starts up into it.
- Cleaning up. After the restart, macOS removes what it no longer needs and checks for further updates.
In the Tahoe update Eclectic Light followed, downloading and preparing each took just under 40 percent of the progress bar. The space peaks during preparation, when the download and the new system both exist.
Why is the working room so much bigger than the download?
Since macOS Ventura, parts of the system arrive as cryptexes: sealed disk images that macOS mounts as it starts up. A Mac with Apple silicon gets them for both Apple silicon and Intel code, so it can run Intel apps through Rosetta 2. An Intel Mac needs only one.
In the 26.3 update Eclectic Light examined, macOS's own estimate added up the new system snapshot, the cryptexes, 4 GB of slack for virtual memory, Recovery and the Preboot volume, and smaller templates. The grand total came to 16,918 MB, for a download of 3.68 GB.
That estimate isn't padding. It's what macOS calculates it must have free at once, and macOS checks the volume's free space against it before going on.
Where does a downloaded update wait?
The update itself is written to /, according to Eclectic Light. macOS uses / for information about available updates and for some downloads.
Both are protected by System Integrity Protection, so only macOS changes them. Installing a pending update lets Software Update remove the files it no longer needs.
Does the space come back afterwards?
Mostly. The download and the files staged for the install go once the update finishes and macOS cleans up. What stays is the difference in size between the old and new system, which is usually small.
Eclectic Light found macOS Ventura 13.1 used about 0.5 GB more than 13.0. Apple's specifications for High Sierra said it needed about 1.5 GB more than Sierra once the upgrade was done. Major upgrades tend to grow the system more than point updates do.
One thing can hold space for a day or so. If you use Time Machine, Apple says macOS saves a local snapshot before installing any update, and the snapshot keeps the old system's files until it's removed.
Minor updates, major upgrades and full installers
Apple separates two kinds of change. A minor update stays within your version, such as macOS 26 to 26.4. A major upgrade moves to a new version, such as macOS 26 to macOS 27. Both arrive through System Settings › General › Software Update, but their sizes are very different.
Delta, combo and full: why the same update has different sizes
Eclectic Light describes three sizes of the same update, chosen by the version your Mac is coming from:
- Delta. From the last public release, such as 26.1 to 26.2. The smallest, because it carries only what changed.
- Combo. From an earlier point release of the same version, such as 26.0 to 26.2. Larger, and it grows with each later release.
- Full. From an earlier major version. It carries the whole system for both kinds of Mac.
For macOS 26.3, the short version in Apple's catalog was 3.68 GB and the full version 17.48 GB. A Mac that skips updates for months gets a larger download, and needs more working room, than one that updates every time.
| Update type | When your Mac gets it | Size compared |
|---|---|---|
| Delta | It's on the latest public release | Smallest |
| Combo | It's on an older release of the same version | Larger, and grows with each release |
| Full | It's on an earlier major version | Largest, close to a full installer |
| Full installer app | You download Install macOS yourself | A whole copy of macOS in Applications |
Why are updates larger on Macs with Apple silicon?
Because they carry Intel code too. Eclectic Light found that Apple silicon Macs receive Intel versions of the system's shared libraries for Rosetta 2, as well as their own, which accounts for much of the difference between the two kinds of Mac.
macOS 27 Golden Gate runs only on Macs with Apple silicon, so every Mac upgrading to it gets the larger kind. On an Intel-based Mac, Software Update offers macOS 26 Tahoe and its updates instead, if the Mac supports them.
What about Security Responses and system files?
Software Update's automatic settings have a separate switch for them: Install Security Responses and system files. Apple describes it as automatically installing system files and Rapid Security Responses. These arrive between full updates and are separate from the numbered releases.
Apple doesn't publish their size either. The sections below are about numbered updates and upgrades, which are where space runs short.
Is a full installer a better way to save space?
No. Apple says Software Update can use less storage space to download and install updates than the other methods. A full installer, such as Install macOS Tahoe, is a complete copy of macOS that sits in your Applications folder until you remove it.
You need one only to install that version again or to make a bootable installer. If you've downloaded one and don't need it, it takes many gigabytes for nothing; see macOS installers.
What Apple publishes, and what it doesn't
For some older versions, Apple printed a storage figure in the version's technical specifications. For recent versions, its compatibility pages list the Macs that can run them and say nothing about space.
Which versions had a published figure?
| macOS version | What Apple published | Where |
|---|---|---|
| High Sierra 10.13 | 14.3 GB available to upgrade | Technical specifications |
| Big Sur 11 | 35.5 GB from Sierra or later; up to 44.5 GB from earlier | Technical specifications |
| Tahoe 26 | No storage figure | Compatibility page |
| Golden Gate 27 | No storage figure | Compatibility page |
High Sierra's page also said that once upgraded, it needed about 1.5 GB more than Sierra 10.12.6. Big Sur's two figures show how the starting version changes the answer: a Mac on a much older version needed up to 9 GB more room.
What does Apple say today?
Three things, spread across its support pages. Software Update uses less space than other ways of installing. When an update says there isn't enough storage, free up storage space. And starting up in safe mode clears certain system caches, which can be enough for a small shortfall; more on that below.
Apple also recommends backing up before you install, and says your Mac may restart several times during the install. A backup to another drive takes no space on the startup disk.
Which Macs can run macOS 27?
Apple lists Macs with Apple silicon only: MacBook Neo, MacBook Air and MacBook Pro from 2020 and later, iMac from 2021, Mac mini from 2020, Mac Studio from 2022 and Mac Pro from 2023. If your Mac isn't on the list, Software Update doesn't offer macOS 27 at all.
How to read the size Software Update shows
Open System Settings, click General in the sidebar, then Software Update. On macOS 26 Tahoe the steps are the same. Software Update checks for updates by itself; click Check for Updates to check again.
What does the size mean?
Before you install, Software Update shows the expected size of the download, which comes from Apple's update servers before anything is downloaded. To see an update's details, click the information button next to Available Updates, then select the update.
It's the right number for one question: how long the download will take. It isn't the space the update needs. Eclectic Light notes that the total space needed to download, prepare and install is never displayed, though Software Update warns you if there isn't enough.
Why does the number change during the download?
Software Update starts with an estimate and replaces it with a more accurate figure once the download begins. Sometimes the two are far apart. For macOS 26.6, Eclectic Light saw an Apple silicon download first reported as 21 GB, of which about 4 GB was actually downloaded before the progress bar moved on to preparing.
A reader of the same site reported a similar pattern: updates under macOS 26 first promising anywhere from 4 to 24 GB, with real downloads near 5 GB. If the first figure looks alarming, wait for the download to start before you start removing things.
Why did your Mac get a bigger update than someone else's?
Usually because it started from an older version, so it was sent a combo or full update rather than a delta. For macOS 26.2, Eclectic Light expected a 3.78 GB download on Apple silicon, while some people reported over 10 GB.
One reader saw about 4 GB on one M4 MacBook and about 10 GB on an identical one. Eclectic Light's view is that two Macs of the same kind updating from the same version should get the same download, so a large difference usually points to different starting versions.
Does downloading automatically change the space needed?
It changes when the space is used. With Download new updates when available on, your Mac downloads updates in the background and keeps them ready. Apple notes that laptops must be connected to power to download updates automatically.
A downloaded update waits on the disk until it's installed or macOS clears it. Apple says macOS clears staged macOS and app updates when space is needed, so a waiting update may have to download again when you finally install it.
Does Update Tonight need less room?
No. Update Tonight installs downloaded updates overnight, and Apple says the Mac must stay on, connected to power and to the internet. It changes when the update installs, not how much room it needs.
If you free space for an overnight update, keep it free until the update is done. A large download or a video export in the evening can use up the room the install was counting on.
| What you see | What it means | What it doesn't mean |
|---|---|---|
| Size in the update's details | The expected download | The space needed to install |
| A different size once downloading | A more accurate estimate | That something went wrong |
| A bigger size than a friend's Mac | A combo or full update | A problem with your Mac |
| A message that there isn't enough storage | macOS's own estimate didn't fit | That Finder's available space is wrong |
Measured cases from cited sources
Apple doesn't publish figures, so the best evidence comes from people who measured. Each case below says what was tested, how, and what it found.
- Ventura 13.1: download2.5 GB
- Ventura 13.1: free space needed14 GB
- Tahoe 26.3: download3.7 GB
- Tahoe 26.3: space to prepare16.9 GB
- Tahoe 26.3: full update download17.5 GB
- Big Sur: Apple's figure to upgrade35.5 GB
Ventura and Tahoe figures from Eclectic Light's tests and macOS's own logs (2022 and 2026); Big Sur from Apple's technical specifications. Space needed for the full Tahoe update wasn't measured.
macOS Ventura 13.0 to 13.1, in virtual machines
In December 2022, Eclectic Light updated fresh Ventura 13.0 virtual machines with disks of different sizes. The update consistently said its download was 2.53 GB. A basic Ventura system used about 15 GB of the disk.
On a 30 GB disk with 11.19 GB free, the update stopped before downloading, saying it needed 12.97 GB. On a 32 GB disk with 13.32 GB free, it failed too; a second try asked for an extra 340.9 MB. On a 33 GB disk with 14.42 GB free, it worked.
The conclusion was that the update needed at least 14 GB free, about five and a half times its download. For comfort, the author suggested a disk of at least 40 GB with 20 GB free, and 50 GB with 25 GB free to keep updating over time.
macOS Tahoe 26.2 to 26.3, on a Mac mini with M4 Pro
In March 2026, Eclectic Light followed the 26.3 update through macOS's logs. The delta download was 3.68 GB, unpacking to 4.36 GB. The full version of the same update, for a Mac coming from an earlier major version, was 17.48 GB, unpacking to 18.14 GB.
macOS's own estimate of the space needed to prepare the update came to 16,918 MB: more than four and a half times the download. That's the kind of figure behind a message that there isn't enough storage, and it's well above anything Software Update displays.
macOS Tahoe 26.2 and 26.6: one update, many sizes
For 26.2, Eclectic Light expected a 3.78 GB download on Apple silicon, while some readers saw over 10 GB. Its explanation was the starting version: a delta from 26.1, a combo from 26.0 or 26.0.1, and a larger update still from macOS 15.
For 26.6, the first figure on an Apple silicon Mac was 21 GB, and about 4 GB was actually downloaded. Both cases show why the first number Software Update gives you is a poor guide to the space you'll need.
macOS Big Sur: a published figure, and an installer that didn't check
Apple's specifications asked for 35.5 GB of available storage to upgrade to Big Sur from Sierra or later, and up to 44.5 GB from earlier versions. 9to5Mac put the installer itself at about 13 GB.
In February 2021, Mr. Macintosh found that the Big Sur 11.2 installer didn't check free space properly. On a Mac without enough room, the upgrade could fail near the end and leave the Mac unable to start up. Apple then released installer 11.2.1, which stops before it begins when there isn't enough space.
That episode is the reason the check matters. The updates measured since then checked first and refused to start, which is frustrating when you're short, and far better than running out halfway.
What the cases have in common
- The free space needed was always several times the download.
- The first size shown was a poor guide; the starting version changed it most.
- macOS checked before it started, and refused rather than failing partway.
- Each figure applies to one update on one kind of Mac, so treat it as a scale, not a rule.
How much free space to have before you start
Here is a way to plan, built from the cases above. It's an estimate with a margin, not an Apple figure, and Software Update's own check always has the final word.
For an update within your version
Plan for 15 to 20 GB of truly free space. The measured point updates needed 14 GB and 16.9 GB, and a combo update for a Mac that has fallen behind can need more. Updating every time an update arrives keeps you on the smaller delta downloads.
For a major upgrade
Plan for 35 GB or more. That's the last figure Apple published for a major upgrade, and a full update for macOS 26 was already 17.48 GB to download before any working room. The further behind your Mac is, the larger the download.
Which free space figure should you compare?
The one in Disk Utility, not Finder's. Select your startup volume in Disk Utility and click Info: it lists used, purgeable and free space separately. Finder's available figure counts purgeable space, such as Time Machine's local snapshots, which macOS hasn't removed yet.
An installer that checks for room can find too little before macOS clears that space. The guide to which storage number to trust explains each figure in full.
| Situation | Free space to plan on | Why |
|---|---|---|
| Point update, Mac up to date | 15–20 GB | Measured cases needed 14 and 16.9 GB |
| Point update, Mac months behind | 20 GB or more | It gets a larger combo update |
| Major upgrade | 35 GB or more | Apple's last published figure |
| Major upgrade from an old version | 45 GB or more | Big Sur asked up to 44.5 GB from older versions |
| Any update, small disk | Enough to stay above 20 GB afterwards | Room for the next update and the snapshot |
A worked example
Say a Mac with a 256 GB disk is on macOS 26.5, and Software Update offers 26.6 with a download of about 4 GB. Disk Utility shows 12 GB free and 20 GB purgeable, so Finder says 32 GB is available.
The download fits easily, but the working room may not: the measured point updates needed 14 to 17 GB free. The purgeable space is the gap. If most of it is Time Machine's local snapshots, removing them turns it into free space, and the update has room with a margin.
If the same Mac were still on macOS 26.2, it would get a combo update, larger to download and to prepare. The same 32 GB might then be only just enough, and it would be worth freeing more before trying.
What about the days after the update?
Leave a margin. If you use Time Machine, the snapshot macOS took before the update holds the old system's files until it ages out, and Storage settings needs time to recalculate. Give it a day and a restart before you judge your free space.
If you use Apple Intelligence, its models take space too: Apple says up to 8 GB, or up to 14 GB on Macs with M3 or later and at least 12 GB of unified memory. Storage settings counts them under macOS.
Why is a full disk after an update a problem?
Because the next update needs the same room again, and so do backups, downloads and apps saving their own data. In 2022, Eclectic Light suggested keeping at least 25 GB free to go on updating. A Mac that only just fits this update won't fit the next one without another round of clearing.
On Apple's developer forums, someone who cleared their Mac to 21 GB free before installing a macOS Tahoe beta was left with 589 MB afterwards, and Mail couldn't save its data. An Apple engineer replied that beta releases may take additional disk space for testing and bug reporting.
When the space isn't there
If Software Update says there isn't enough storage, the fix is to free up truly free space on the startup disk, then try again.
The post on making room for a macOS update goes through where to find it, in the order worth trying. A few points belong here because they change the numbers above.
Do local snapshots count against an update?
Yes, until macOS removes them. Apple says your Mac counts the space used by snapshots as available, and Time Machine removes them as they age or when space is needed. An installer can ask for room before that happens. The guide to Time Machine local snapshots shows how to see them and remove them.
Does safe mode really help?
For a small shortfall, it can. Apple says starting up in safe mode clears certain system caches, which may give you enough space for a task that needs it only while it runs, such as a macOS update. The caches come back afterwards, which is fine: you need the room for an hour.
Should you delete the files Software Update downloaded?
No. The folders where updates wait are protected by System Integrity Protection, and only macOS changes them. Installing the pending update, or letting macOS clear it when space is needed, removes the files properly. Without them, Software Update has to download the update again.
Does an old installer app count?
Yes, it's just a large app. An Install macOS app in your Applications folder that you don't need is one of the quickest wins before an update. Apple lets you download installers again for the versions your Mac supports.
The post on deleting old macOS installers goes through which installers and update files are yours to remove, and which to leave to macOS.
How the app does it
Wombit is a native Mac app that shows what's using your storage, explains every item in plain language, and clears it safely: Trash first, undoable and logged. File names and paths never leave your Mac.
Its space meter shows the available space beside a bar that ends in what's truly free, and Why the numbers differ says how much of the difference is purgeable and how many local snapshots there are. That's the free figure an update's check cares about.
When macOS says an update is waiting, the menu bar item asks Apple's update server how big it is, once per update, the way System Settings does. Because Apple doesn't publish a figure for most updates, it estimates the room needed at twice the download, and says it's an estimate.
It alerts you only when there isn't room, and that alert comes whether or not you have Pro. Treat it as an early warning, not a guarantee: the measured cases above needed more than twice their download, and Software Update has the final say.
The places this post mentions are entries in the app's catalog, written and checked by hand with sources:
/
Library/ Updates Files Software Update downloads and keeps until an update is installed.
Managed by macOS · Explained only: never removed
/
System/ Library/ AssetsV2/ * Parts of macOS downloaded after installation, such as speech and language data, voices, fonts, dictionaries and on-device models.
Managed by macOS · Explained only: never removed
/
Applications/ Install {macOS,OS X} *.app Full macOS installer apps, such as Install macOS Tahoe, left in your Applications folder after a download.
Usually safe · Moves it to the Trash
Hourly snapshots of your startup disk that Time Machine keeps on the disk itself, so you can restore files even without your backup disk.
Managed by macOS · Runs tmutil for Time Machine's own snapshots, through the helper
Update downloads and macOS's downloaded assets are Managed by macOS: the app explains them and leaves them to Software Update and macOS. An old installer app is Usually safe, because you'd need to download it again to use it, so it never starts checked. Local snapshots are removed only through Time Machine's own mechanism.
The space meter, the menu bar item and the update check are free. Cleanup, the Apple Data actions for installers, and removing Time Machine snapshots are PRO: one payment of $4.99 with lifetime access. Every Pro tool's review is free to open, so you see what would go and why before you buy.
Frequently asked questions
How many GB is a macOS update?
It depends on the update and on the version your Mac is coming from. Recent point updates for macOS 26 Tahoe were about 3 to 4 GB for a Mac on the latest release, while the full version of the 26.3 update was 17.48 GB. Software Update shows the expected download for your Mac before you install.
How much space do I need to upgrade to macOS 27 Golden Gate?
Apple doesn't publish a figure for macOS 27. The last major upgrade Apple put a number on, Big Sur, asked for 35.5 GB, so plan on 35 GB or more of truly free space. Software Update checks the real requirement before it downloads and tells you if there isn't enough.
Why does my Mac need more free space than the update's size?
The size shown is the compressed download. The installer also has to unpack it, check it and build the new system before it restarts, and all of that needs room at once. In one measured macOS 26 update, a 3.68 GB download needed about 16.9 GB to prepare.
Can I update macOS with 10 GB free?
Usually not. The measured point updates needed about 14 GB and 16.9 GB of free space, and Software Update stops before downloading if its own estimate doesn't fit. Free up more room first, or try starting up in safe mode if you're only a little short.
Does macOS take up more space after an update?
A little, usually. Eclectic Light found Ventura 13.1 used about 0.5 GB more than 13.0, and Apple said High Sierra needed about 1.5 GB more than Sierra once installed. Most of the working room comes back after the update cleans up.
Is the full macOS installer smaller than updating through Software Update?
No. Apple says Software Update can use less storage space to download and install updates than other methods. A full installer is a complete copy of macOS that stays in your Applications folder until you remove it.
Where does macOS keep an update before it installs?
macOS writes the update's download inside its protected assets folder in the System Library, and uses the Library's Updates folder for update information and some downloads. Both are protected by System Integrity Protection, and macOS clears staged updates when space is needed.
Sources
- Update macOS on Mac · Apple
- How to download and install macOS · Apple
- macOS Big Sur - Technical Specifications · Apple
- How much free space does Ventura need to update? · Eclectic Light
- How macOS 26 Tahoe updates: 3 Catalogues and preparing to download · Eclectic Light
- Why do macOS update sizes vary so much? · Eclectic Light