If your Mac storage keeps filling up even though you haven't saved anything new, something on the Mac is saving for you. Apps keep caches, Time Machine takes a snapshot every hour, Messages and Photos bring down what your iPhone sends to iCloud, and macOS downloads updates and parts of itself in the background. None of it asks first.
Most of this growth is normal, and much of it gives space back by itself when the disk gets tight. The trouble starts when something grows faster than macOS clears it, or grows somewhere macOS never clears: an iPhone backup, a folder of AI models, a developer tool's cache.
To find the cause, compare rather than guess. Note the largest categories in Storage settings today, look again in a few days, and the category that moved tells you where to look. Then limit the source with the setting that controls it, instead of deleting the same files again every month.
The sections below go through each thing that grows by itself, how to tell which one it is on your Mac, and the setting that keeps it in check.
Why Mac storage keeps filling up on its own
A Mac writes to its disk all day. Most of it is small and short-lived, but a few kinds of data build up: they're added often and removed rarely, or only when space runs short.
What grows without you saving anything?
Here are the usual sources, where Storage settings counts each one, and whether it goes away by itself.
| What grows | What makes it grow | Storage settings category | Clears itself? |
|---|---|---|---|
| Time Machine local snapshots | Files that change while a snapshot exists | System Data | Yes, as they age or when space is needed |
| App caches | Using apps, browsing, streaming | System Data | Partly, when space runs low |
| Swap files | Apps needing more memory than the Mac has | System Data | Yes, when you restart |
| Messages attachments | Every photo and video in your chats | Messages | Only if Keep messages isn't Forever |
| The Photos library | New photos from your other devices | Photos | Only with Optimize Mac Storage |
| iPhone and iPad backups | Each backup made on this Mac | iOS Files | No |
| Updates and parts of macOS | Background downloads | System Data or macOS | Staged updates, when space runs low |
| Developer caches and AI models | Builds, installs and model downloads | Developer or System Data | Rarely |
Is it normal for free space to drop a little every day?
Yes. Caches fill as you browse and use apps, logs gain lines, and a snapshot taken every hour holds on to whatever changed in that hour. A few hundred megabytes a day that come and go are part of a Mac working normally.
Free space also comes back by itself. Snapshots expire, a restart clears swap, and macOS clears caches, logs and staged updates when space runs low. A figure that wobbles from day to day isn't a sign of a problem.
When is it worth looking into?
Look when the trend only goes one way for weeks, or when it jumps. Tens of gigabytes gone overnight usually means one large thing: photos syncing, a backup, an update downloading, a model you pulled, or a log or index that has run away.
The other sign is a low-space warning that keeps coming back after you've cleared space. Something is refilling the disk, and clearing it again won't fix that. Finding the source will.
How to find what grew
macOS has no single screen that shows what changed. Storage settings, Finder, Activity Monitor and Disk Utility each show part of the answer, and together they're usually enough.
Start with Storage settings, and write the numbers down
Choose Apple menu › System Settings, click General, then click Storage. Wait for the bar to finish counting, then write down the size of the five or six largest categories, or take a screenshot.
Storage settings shows today's figures only; it keeps no history. Look again in a few days, at about the same time of day. A category that grew by several gigabytes is where to look next. One that moved up and down is probably snapshots or swap.
The path is the same in macOS 26 Tahoe and macOS 27. Before macOS Ventura, the same figures were in Apple menu › About This Mac, under Storage.
Which category grew, and what does it point to?
| Category that grew | Likely causes | Where to look next |
|---|---|---|
| System Data | Snapshots, swap, caches, developer tools, AI models | Disk Utility, then your Library folder |
| Messages | Attachments in your conversations | Messages, conversation by conversation |
| Photos | Originals arriving through iCloud Photos | Photos settings, iCloud tab |
| iOS Files | A new backup or device software file | The iOS Files list in Storage settings |
| Documents | Downloads, recordings, exports | The Documents list, sorted by size |
| macOS | An update or Apple Intelligence's models | Software Update |
| Developer | Xcode's build files and simulators | Xcode's settings |
System Data is the category that most often grows by itself, because everything without a category of its own lands there. The post on what System Data is takes it apart piece by piece.
Compare folders in Finder
For System Data and Documents, Finder can show which folder grew. Open a Finder window and choose View › as List. Then choose View › Show View Options and select Calculate all sizes. Folder sizes appear in the Size column, and clicking its heading sorts by size.
Start in your home folder, then look in its Library folder: in Finder, choose Go › Go to Folder, type ~/ and press Return. Note the largest folders, and check them again in a few days. The Date Modified column shows which folders changed recently.
Finder can't see everything. It doesn't show snapshots, and it can't open some folders macOS protects, so System Data rarely adds up in Finder. A folder that grew is still a strong clue.
See what's writing to the disk right now
If free space drops while you watch, open Activity Monitor from the Utilities folder in Applications and click Disk. Choose View › All Processes, then click the Bytes Written column heading to put the processes that have written the most at the top.
The name at the top is a clue, not an answer. A cloud drive downloading files, a backup running, Software Update fetching an update and an app writing an endless log each lead to a different fix.
To keep an eye on it, choose View › Dock Icon › Show Disk Activity. Activity Monitor's icon in the Dock then becomes a small graph of reads and writes.
Check for snapshots in Disk Utility
Open Disk Utility from the Utilities folder and choose View › Show APFS Snapshots. Select your startup disk's volume in the sidebar, and its snapshots appear in a table at the bottom of the window, with the date each was created.
A set of hourly snapshots from the last day is normal when Time Machine backs up automatically. Snapshots made by another backup app follow that app's own rules, so if you find old ones, check the app's settings.
A worked example
Say Storage settings showed System Data at 38 GB on Monday and 61 GB on Wednesday, with nothing else changed. Finder shows the Library folder grew by about 2 GB, mostly in Caches, which doesn't explain it.
Disk Utility lists a day's worth of hourly snapshots. On Tuesday you copied a 20 GB video project to the Mac, worked on it, then moved it to an external drive. The snapshots still hold it. By Thursday they've aged out, System Data drops back, and nothing needed deleting.
A second Mac shows Messages up by 9 GB in a month, with System Data steady. The month began with signing in to Messages on a new Mac with Messages in iCloud on. That's years of attachments arriving, and the fix is the Keep messages setting, not Finder.
Snapshots, swap and caches: space macOS lends and takes back
Three of the fastest-moving parts of System Data belong to macOS itself. They grow and shrink on their own, and they're why the figure can change by gigabytes between two looks.
Why do Time Machine snapshots take space?
When Time Machine backs up automatically, it saves a snapshot of your startup disk about every hour and keeps each one for 24 hours. It also keeps one of your last successful backup until space is needed, and macOS saves one before each update.
A snapshot holds on to files that existed when it was taken. If you change or delete large files, the snapshots keep the old versions until they expire, so space you just freed can stay used for a day.
Do snapshots pile up when the backup disk isn't connected?
They keep being made, because their purpose is to let you restore files when the backup disk isn't there. Each one still expires, so they don't build up for weeks. Apple says Time Machine stores snapshots only on disks with plenty of free space, and deletes them as they age or as space is needed.
Your Mac also counts their space as available. Downloads, copies and installs can use it, and Time Machine makes room as they do, so snapshots rarely stop you working.
How do you stop snapshots taking space?
Usually there's nothing to do: they leave by themselves. If you need the room right away, open System Settings › General › Time Machine, click Options, set Backup Frequency to Manually, wait a few minutes, then set it back.
Don't turn Time Machine off to save space. Snapshots are the safety net between backups, and backups are what lets you undo a mistake. The local snapshots guide has the details.
Why does System Data shrink after a restart?
Swap files. When apps need more memory than your Mac has, macOS moves some of it to swap files on the disk, and restarting clears them. A Mac that runs for weeks with many apps and browser tabs open can carry several gigabytes of swap.
Quitting apps you're not using keeps swap smaller. Never delete swap files yourself. The post on swap files and the sleep image explains how much is normal and how to check it in Activity Monitor.
Do caches grow forever?
No, but they grow as you use your Mac. Apps keep caches so they can load things faster, and they fill as you browse, stream and work. macOS clears caches and logs it knows are safe to delete when space runs low, and many apps trim their own.
What stays are the caches of apps you've removed, and caches of apps that never trim them. Those are worth clearing once. Clearing the caches of apps you use only buys a few days, because they fill again.
What about temporary files?
They mostly look after themselves. macOS gives each user private temporary folders and removes temporary items that haven't been used for three days. The shared temporary folder is cleared the same way, every day.
The private folders also hold caches, and those aren't cleared on a schedule, so they can grow. macOS and apps recreate what they need, but removing files there while apps use them can lose work, so leave them to macOS.
- App caches2–30 GB · Safe
- Logs and crash reports0.1–5 GB · Safe
- Swap and sleep image1–20 GB · Managed by macOS
- Messages attachments5–100 GB · Review first
- iPhone and iPad backups10–200 GB · Review first
- Xcode DerivedData5–100 GB · Safe
- Local AI models20–150 GB · Review first
Typical ranges on Macs that have these items at all, from the research behind the app's catalog. Many Macs have far less, and some have none.
Messages, Photos and iCloud: what syncs to your Mac
Some of the largest growth comes from your other devices. Every photo you take on your iPhone and every video someone sends you can land on your Mac too, depending on a few settings.
Why does Messages keep getting bigger?
Messages keeps a copy of every attachment in your conversations: photos, videos, audio messages and files. Nothing is removed until you delete it in Messages, or until Messages removes old conversations for you.
With Messages in iCloud on, the messages stored in iCloud become available on your Mac too. A Mac signed in for the first time can gain years of conversations as they sync. That's expected, and it's where many people's Messages category comes from.
To let Messages trim itself, choose Messages › Settings, click General, and change Keep messages from Forever. Conversations older than the period you choose are removed with their attachments. With Messages in iCloud, they're removed on your other devices too.
Why does the Photos library grow when you haven't imported anything?
With iCloud Photos on, photos and videos you take on your iPhone are added to the library on your Mac. If Download Originals to this Mac is selected, the Mac keeps a full-size copy of every one, so the library grows each time you take a photo anywhere.
To keep smaller versions instead, open Photos, choose Photos › Settings, click iCloud, and select Optimize Mac Storage. When space is limited, Photos keeps smaller versions on the Mac and the full-size originals in iCloud, and downloads an original when you open it.
Photos you delete stay in Recently Deleted for 30 days, still taking space. Never delete files inside the Photos library in Finder; change the library only in Photos.
Why does iCloud Drive take space on the Mac?
Every iCloud Drive file you open is downloaded and stays on the Mac until macOS or you remove the download. With Desktop & Documents Folders turned on, those two folders move into iCloud Drive, and the same applies to them.
Open System Settings, click your name, click iCloud, then Drive, and turn on Optimize Mac Storage. Your Mac can then store documents only in iCloud when more space is needed. If space isn't needed, everything stays on the Mac.
To free space now, Control-click files in iCloud Drive and choose Remove Download. Don't delete them to save space: deleting a file in iCloud Drive deletes it from iCloud and from your other devices.
Dropbox, OneDrive and Google Drive
Other cloud storage apps work the same way under their own names. Files you open, or mark to keep offline, take space on the Mac; online-only files don't.
Use the app's own option rather than deleting files in its folder. In Finder, Control-click and choose Make online-only for Dropbox or Free up space for OneDrive. In Google Drive, turn off Available offline for folders you don't need here.
Mail, Podcasts and the TV app
Mail keeps its own copy of the messages and attachments it downloads, and the folder grows with every one. To keep fewer attachments on the Mac, set Download Attachments to Recent or None in Mail › Settings › Accounts › Account Information.
Podcasts can download new episodes of every show you follow, and its settings can remove played downloads. In Storage settings, Optimize Storage removes Apple TV movies and shows you've already watched, and keeps only recent mail attachments when space is needed.
Updates and downloads macOS and your apps make
macOS and many apps download what they need before you ask. Most of these downloads are temporary, but a few stay until something removes them.
Does Software Update download updates by itself?
It can. Open System Settings › General › Software Update and click the information button next to Automatic Updates. With Download new updates when available turned on, your Mac downloads updates in the background and keeps them until they're installed.
That's useful, because an update is ready when you are. When space runs low, macOS clears staged macOS and app updates, and installing a waiting update lets Software Update remove files it no longer needs. The options are the same in macOS 26 Tahoe and macOS 27.
Why does space drop after a macOS update?
Several things happen at once. macOS saves a snapshot before installing, the new version may download parts of itself, and features you use fetch what they need. Some of it is temporary: Time Machine removes that snapshot as it ages or when the space is needed.
The post on storage after a macOS update goes through what an update changes and what's safe to do.
A full installer you downloaded but didn't run, such as Install macOS Tahoe, stays in Applications and takes many gigabytes. You only need it to install that version again or make a bootable installer, and Apple lets you download it again for the versions your Mac supports.
What does macOS download in the background?
macOS downloads many parts of itself separately: voices, speech and language data, fonts, dictionaries and the models Apple Intelligence uses. It keeps them in a protected folder and adds or removes them as features need them. Storage settings counts them under macOS.
Apple says Apple Intelligence needs up to 8 GB of storage, or up to 14 GB on Macs with M3 or later and at least 12 GB of unified memory. On a Mac that supports it, that space can go in one step, with no download of your own.
App updates waiting to install
Many apps from outside the App Store update themselves and keep the download until you quit or restart them. Apps that use Sparkle keep updates inside their cache folder and remove items older than 10 days on their own.
Many apps built with Electron stage updates of several hundred megabytes in a cache folder named after the app, ending in ShipIt or updater. Quitting the app lets its updater install the waiting update. A folder left by an app you've since removed stays until you remove it.
iPhone and iPad backups and software files
Each time you back up a device to this Mac in Finder, its backup is updated. Backups of devices you no longer have, and backups you archived, stay until you delete them, and each one can take tens of gigabytes.
Updating or restoring a device from Finder also leaves its software file, several gigabytes, on the Mac. Both appear under iOS Files in Storage settings, where you can delete them. A backup may be the only copy of a device's data, so check the device backs up to iCloud first.
Developer tools and AI models
On a Mac used for development, the fastest growth usually comes from tools. They download, build and cache as part of normal work, and many of them never clean up after themselves.
Why does Xcode keep growing?
Xcode keeps build files for every project you build in DerivedData. It adds a set of device support files each time a device you develop on runs a new OS version, and older sets stay. Simulator runtimes and simulators stay after you update Xcode.
DerivedData is safe to clear with Xcode closed, at the cost of a slower next build. Remove runtimes in Xcode › Settings › Components, not in Finder. The Xcode guide goes through each folder.
Package managers that never clean up
Package managers keep a copy of every package they download, so installs are faster next time. npm never removes anything from its cache by itself, so it grows with every new dependency in every project. Homebrew is tidier: its regular cleanup removes outdated downloads and any older than 120 days.
Projects add their own copies. A JavaScript project keeps its dependencies in a node_modules folder, often hundreds of megabytes, and it stays in every project you've finished. Each can be installed again from the project's own files, so finished projects are the place to start.
Docker and virtual machines
Docker Desktop keeps every image, container and volume in one disk image file, which grows as you pull and build images. Remove what you don't need in Docker Desktop, and Docker gives the space back to your Mac by itself, sometimes after a few minutes.
Never delete that disk image file: it takes every volume's data with it. A virtual machine from UTM, Parallels or VirtualBox is also a whole computer's disk, and it grows as the system inside writes. Manage it in the app that made it.
AI models you pulled, and models apps download for you
A local language model takes a few gigabytes when it's small and tens of gigabytes when it's large, and every model you try stays until you remove it. Remove models in the tool that downloaded them, such as Ollama or LM Studio, never by hand in their folders.
Some apps download models of their own. On eligible Macs, Chrome downloads its Gemini Nano model in the background, updates it, and removes it when free space runs low. In Chrome's settings, turning off On-device AI under AI Innovations deletes the model and stops it coming back.
When one thing runs away
Sometimes the growth isn't a slow build-up but one item that has gone wrong. These are rarer, but they explain most of the stories of a disk that fills by tens of gigabytes a day.
A log that never stops
Apps and services write logs to record what they do, and some keep old logs until they're removed. An app stuck in a loop can write the same error over and over, and its log can reach many gigabytes.
Look in the Logs folder of your Library folder in Finder, with Calculate all sizes on. A single log far larger than the rest points to the app that's misbehaving. Quit or update that app first, then move the log to the Trash; removing the log alone just starts a new one.
Crash reports from an app that keeps crashing
When an app crashes or hangs, macOS writes a report. One report is small, but an app or background helper that crashes again and again writes a new one each time. The Console app lists them under Crash Reports, and they're safe to remove once you've dealt with the app.
A Spotlight index that keeps growing
Spotlight keeps an index of each disk so it can find files quickly. It's normally a small part of the disk, but some people have seen it grow to well over 100 GB when indexing goes wrong. Storage settings counts it in System Data, and Finder doesn't show it.
Rebuilding the index replaces it with a fresh one. Open System Settings, click Spotlight, and click Search Privacy at the bottom. Add your startup disk to the list, wait a few seconds, select it and click the remove button. Searches are incomplete until indexing finishes.
A download or sync that started over
A cloud drive that signs in again, a Photos library that resyncs or a large download that restarts can each pull down tens of gigabytes. Activity Monitor's Disk tab shows the process doing the writing. Let it finish, then use the app's settings to keep only what you need on this Mac.
How to stop each one from coming back
Clearing space fixes today. A setting fixes next month. For most sources, one setting does most of the work. A short monthly look catches the rest.
| Source | Setting that limits it | Where it is |
|---|---|---|
| Messages attachments | Keep messages | Messages › Settings › General |
| The Photos library | Optimize Mac Storage | Photos › Settings › iCloud |
| iCloud Drive files | Optimize Mac Storage | System Settings › your name › iCloud › Drive |
| Mail attachments | Download Attachments | Mail › Settings › Accounts |
| Watched Apple TV purchases | Optimize Storage | System Settings › General › Storage |
| The Trash | Empty Trash automatically | System Settings › General › Storage |
| Update downloads | Download new updates when available | System Settings › General › Software Update |
| Chrome's on-device model | On-device AI | Chrome › Settings › AI Innovations |
Which settings are worth turning on?
Storage settings' recommendations are the easiest start: Store in iCloud, Optimize Storage and Empty Trash automatically. Each hands part of the job to macOS, so space keeps coming back after today.
Read the cost of each first. Store in iCloud counts against your iCloud plan, and opening older files needs a connection. An item emptied from the Trash after 30 days is gone for good. If your iCloud storage is already full, the iCloud options can't help until you make room there.
What to check after a big change
Growth often starts with a change made weeks earlier: a new iPhone, a new Mac signed in to Messages, iCloud Photos turned on, a new development tool. When a category starts climbing, ask what changed around that time.
A new iPhone is a common one. Its first backup to the Mac, the software file from setting it up and a burst of new photos can all arrive in the same week. Delete the old device's backup once the new one is backed up.
What should you leave alone?
Leave Time Machine on, even though it makes snapshots, and leave swap and the sleep image to macOS. Leave the caches of apps you use: they come back within days, and clearing them on a schedule only costs time.
App preferences and saved window state aren't growth either. They're tiny, and removing them only resets your apps.
How much free space should you keep?
Apple doesn't publish one figure. Keep enough for the largest thing you do: a macOS update, a video export or a big download each needs room to work, and can stop when it isn't there.
There's a quieter reason too. Time Machine keeps snapshots only on disks with plenty of free space, so a disk kept nearly full also loses that hour-by-hour safety net.
Mistakes that make it worse
Clearing caches every week
Caches of apps you use come back, often within a day, and the apps are a little slower while they rebuild. Clearing them frees space briefly and doesn't make a Mac faster. Clear the caches of apps you've removed, and any that have grown far beyond the rest, then leave the others.
Deleting cloud files to free local space
Deleting a file in iCloud Drive, Dropbox or OneDrive deletes it everywhere. To free space on this Mac only, remove the download or make the file online-only, and it stays in the cloud and on your other devices.
Turning off features you rely on
Turning off Time Machine, iCloud Photos or Messages in iCloud stops some growth, but it also stops backups and sync. Prefer the setting that keeps the feature and limits its space, such as Optimize Mac Storage or Keep messages.
Pasting Terminal commands from a web page
Guides that tell you to paste a command to turn off snapshots or clear everything at once are ones to close. Commands that delete files can't be undone, and fake cleanup guides are a known way to install malware. Everything here works through System Settings, Finder and the apps themselves.
Reinstalling macOS
A clean install rarely fixes a disk that keeps filling. The same apps, sync settings and tools fill it again, usually within weeks. Finding the source is quicker, and it keeps your setup as it is.
How the app does it
Wombit shows what's using your Mac's storage, explains every item in plain language, and clears it safely: Trash first, undoable and logged. It reads only names, sizes and dates, and file names and paths never leave your Mac.
A small part of the app can stay in the menu bar, showing available space and keeping a daily history from the day you install it. It measures free space every hour and, once a day at low priority, the places the catalog knows by a fixed path, such as caches, simulators, Docker, package caches, AI models and backups.
What Changed shows the last 7 days, the last 30 days or the time since your last scan. It starts with the answer, such as how much used space grew, then shows each day and which places grew or shrank. Alerts can tell you when space is low, or when something grows by 10 GB or more in a day.
Select anything in the map to see what it is and how safe it is to remove:
- Safe: app caches, logs and Xcode's DerivedData. The owner recreates them, they start checked, and only they count in the headline number.
- Usually safe: iCloud Drive downloads and Docker's disk image, which come back at a cost or need the owner's own tool.
- Review first: Messages attachments, iPhone backups and AI models, which may be the only copy. Nothing starts checked.
- Managed by macOS: snapshots, swap and the Photos library are explained and handled through macOS's own mechanism, never moved.
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
~/
Library/ Caches/ * Data that apps and macOS services keep so they can load things faster. Apps create it again when they need it.
Safe · Moves it to the Trash
~/
Library/ Messages/ Attachments Photos, videos, audio messages and files sent and received in Messages on this Mac.
Review first · Explained only: the app shows you how
~/
Library/ Application Support/ MobileSync/ Backup/ * Backups of an iPhone, iPad or iPod touch that were made on this Mac, one folder for each device.
Review first · Moves it to the Trash
~/
.ollama/ models Language models you've pulled or created with Ollama, stored as shared layers.
Review first · Explained only: never removed
The menu bar item and its meter are free, and history is kept for everyone from the first day. What Changed and its alerts, Cleanup, the Apple Data actions and 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. Upgrading shows the history already kept.
Frequently asked questions
Why is my Mac storage full when I haven't downloaded anything?
Plenty is downloaded and saved for you: caches, Time Machine's hourly snapshots, Messages attachments, photos from iCloud Photos, update files and parts of macOS. Open System Settings, click General, then Storage, and note the largest categories. Check again in a few days to see which one is growing.
Why does my Mac's System Data keep growing?
System Data holds everything without a category of its own, so it grows with snapshots, swap, caches, logs, developer tools and AI models. Snapshots and swap leave by themselves within a day or at a restart. Steady growth over weeks usually comes from a tool or app that never clears its files.
Does Time Machine use space on my Mac when the backup disk isn't connected?
Yes. Time Machine keeps a snapshot about every hour on the Mac itself, so you can restore files without the backup disk. Each one is kept for 24 hours, and all of them are deleted when the space is needed, so they don't pile up for weeks.
How do I stop my Mac from downloading updates automatically?
Open System Settings, click General, then Software Update, and click the information button next to Automatic Updates. Turn off Download new updates when available. Updates then download only when you ask, so install them regularly, because they carry security fixes.
Why did my free space go up after restarting my Mac?
Restarting clears the swap files macOS uses as extra memory, and that space shows up as free again. Snapshots that expired and caches macOS cleared can add to it. The space fills again gradually as you open apps, which is normal.
Will clearing caches stop my Mac from filling up?
Only for a few days. Apps rebuild the caches they use, often within a day, so clearing them on a schedule mostly costs time. Clear the caches of apps you've removed, then limit the real sources with settings such as Keep messages and Optimize Mac Storage.
Why is my Mac storage filling up after I got a new iPhone?
A new iPhone often brings a backup to the Mac, a software file of several gigabytes from setting it up, and new photos through iCloud Photos. Check the iOS Files category in Storage settings and delete the old device's backup once the new one is backed up.