Am wondering why disk space in the Finder seems to dwindle over the course of several days, even when I haven’t added any big files?

Why does my Mac gradually lose disk space, even when I'm not adding files?

This can happen on macOS because the system and applications create and manage data in the background.

The trick is to work out what is using the space rather than simply deleting files to free it up.

Here is how to go about investigating it.

1. Use Disk Utility to check overall disk usage

Start with Disk Utility to get an overview of the internal drive's storage.

1. Open Applications >> Utilities >> Disk Utility.
2. Choose View >> Show All Devices.
3. Select your internal drive, then examine the relevant APFS container and Macintosh HD volume.
4. Note the capacity and the reported used and available space, where shown.
5. Record the figures and check again after a few days.

Disk Utility is useful for checking whether overall storage usage has changed, but it will not tell you which files or applications are responsible. Also, APFS volumes can share free space within a container, so figures for the physical drive, container and individual volume are not necessarily directly comparable.

For this investigation, the important thing is to compare the same volume over time. Disk Utility provides a useful cross-check rather than being inherently more accurate than every other measurement.

2. Check the storage categories in macOS

Go to Apple menu  >> System Settings >> General >> Storage.

Note the available space and the sizes of the main storage categories.

Pay attention to System Data, but treat it as a clue rather than a diagnosis. It includes various types of system and application data, so an increase doesn't necessarily identify the cause.

Apple's guide explains how to check and manage storage: Free up storage space on Mac.

3. Monitor disk usage from Terminal

For another perspective, open Terminal (Applications >> Utilities) and run: df -H /

This shows filesystem capacity, used space and available space. Record the results and repeat the check over the next few days.

The figures may differ from those in Disk Utility or Finder because macOS and APFS account for shared and reclaimable storage differently. What matters most is whether the same measurements show a consistent downward trend.

4. Look for folders that are growing

In Finder, select a folder and choose File >> Get Info to check its size. You can also switch to List view, choose View >> Show View Options, and enable Calculate all sizes.

Compare folder sizes over several days, particularly in your user Library and application data folders. Look for folders that are actually growing, rather than simply finding folders that are already large.

Apple's guide to locating and managing files is here: Find and delete files on your Mac.

Avoid deleting anything from System or Library folders until you've established what it does.

5. Check for Time Machine local snapshots

If you use Time Machine, open Terminal and run: tmutil listlocalsnapshots /

This lists any local snapshots; it doesn't delete anything.

However, snapshots should not automatically be assumed to be the cause. macOS manages them automatically, and local snapshot storage is generally treated as available because it can be reclaimed when needed.

For details, see Apple's guide: About Time Machine local snapshots.

6. What should you do if the space keeps disappearing?

The solution depends on what the checks reveal:

• A particular application is accumulating data: Check its storage settings or install any relevant updates.

• Downloads or attachments are growing: Remove unnecessary files through the relevant application.

• System Data is increasing: Investigate further rather than assuming that the entire increase represents unnecessary files.

• No obvious cause emerges: Continue monitoring the figures before resorting to cleanup utilities or deleting system files.

Small fluctuations in available space are not necessarily a concern. But if several gigabytes keep disappearing and the space does not recover, it's worth finding out why.

My recommendation: Start by recording the used and available space in Disk Utility, then check the same volume again after a few days. Use System Settings and Terminal as cross-checks. If the decline is consistent, compare folder sizes to establish where the data is accumulating rather than guessing at the cause.

The available space you see in Finder changes over time because macOS writes caches, makes Time Machine snapshots, and periodically purges those things. Importantly, on Tahoe ‘available’ space is not the same as free space (free space is what matters, and it’s less than ‘available’.

If your examples of 40 GB and 35 GB are real numbers, then you’re at the edge of a cliff and you need to free up space now.

The absolute minimum is ~30-40 GB free space, and as a rule of thumb you should aim to have 15-20% of your total internal storage as free space. You can check the amount of free space you have on your Mac in a variety of ways (but on Tahoe, the first two do NOT show how much free space you really have, and you’ve been using the first one):

• Select your internal drive then click File menu > Get Info, check Available space
• Go to System Settings > General > Storage, check the graphical display at the top of the settings pane
• In Disk Utility (in Applications > Utilities), by selecting the main container disk below the drive itself (you may need to select View menu > Show all devices to see it)
• In the System Report – hold the Option key and click the Apple menu > System Information (the top menu item), look for the name of your internal drive (Macintosh HD, unless you have renamed it as I did)

Note that on versions of macOS prior to Golden Gate, the ‘available’ space reported with the first two methods above was greater than the ‘free’ space reported by the second two methods.

That 90 GB difference between ‘available’ in Finder and the real free space isn’t significant for me, but would be if I had a 256 GB or 512 GB internal drive.

Apple corrected that confusion with Golden Gate, and as shown in the above screenshot, the available/free space is consistent across the different methods. For more details, see this User Tip.

Some steps to take to safely free up space:

Free up storage space on Mac - Apple Support

Optimize storage space on your Mac - Apple Support

How to free up ‘System Data’ and other st… - Apple Community

swap files.

If you open apps that consume a lot of memory (aka RAM), especially if you open them concurrently, and/or have lots and lots of browser tabs open, macOS is going to move memory to swap files.

swap files are dynamic and grow based on your memory usage. The storage for swap files comes from the shared file system free space.

swap files can shrink, but that happens less quickly that when they grow.

Rebooting will remove the swap files, but they will come back as you use apps with large memory demands.

Others have provided good advice about why space is being used. My advice is I find that we are back to the days when simply restarting your computer can free up some space as well as clear up any small issues (I find Finder starts getting "buggy" after a while).

If you do not want to restart your computer then at least restart your browser if you tend to have many tabs open.

Not to be left out from the recent good advise

• Amount of space available, which can include both free space and “purgeable space”—or space that macOS can free up when needed by removing files from your computer (you can’t manually remove the files that are designated purgeable, but macOS removes them as space is required)

Get detailed information about a disk in Disk Utility on Mac

You are welcome

As a follow-up to my previous posting, intended to highlight the symptoms that may occur if the computer is genuinely running low on free disk space.

The aim is to distinguish between the normal fluctuations in available storage that can occur during everyday use of a Mac and the outward signs that might indicate that insufficient free space is beginning to affect the computer's operation.

The following is a practical guide to those symptoms, arranged from the least serious to the most critical.

What happens when an Apple silicon iMac runs low on free disk space?

Apple does not specify a universal minimum amount of free disk space that every Mac must maintain. 

Recommendations such as keeping 30–40 GB free, or 15–20% of the drive's capacity, are general rules of thumb, not official Apple requirements.

How much space your iMac needs depends on what it is doing, which applications are running, and how much temporary storage is required.

Apple's own guidance focuses on whether the startup disk has enough space for the tasks being performed, rather than specifying one minimum figure for every Mac.

Its official guide to freeing up storage space explains how to check available storage and what to do when space becomes insufficient. 

The important distinction is between free space gradually declining and the Mac genuinely running short of the space it needs to operate reliably.

1. The outward signs, from least serious to critical

The following is a practical guide, not an official Apple severity scale.

Symptoms can occur in a different order, and some may have causes unrelated to storage.

Level 1 — Normal operation

• The Mac starts, runs applications and shuts down normally.
• Files open and save without problems.
• Available space fluctuates, even though you haven't deliberately added large files.

What it means: This is not necessarily a problem. macOS manages caches, temporary files and other storage during ordinary use.

Level 2 — Occasional sluggishness

• Applications take a little longer to open
• Large files take longer to load.
• You occasionally notice pauses or delays.

What it means: Low disk space can contribute, especially when macOS or an application needs temporary storage. However, slow performance can have other causes too. See Apple's guide, If your Mac runs slowly. 

Level 3 — Low-storage warnings

• macOS warns that the startup disk is almost full.
• Available space drops substantially over a short period.
• Downloads, large file copies or software updates fail because there isn't enough room.

What it means: Storage is becoming a practical limitation. The Mac may still seem perfectly normal, but some operations can no longer complete. Apple's storage guidance explains how to investigate and free up space. 

Level 4 — Everyday tasks begin to fail

• Documents cannot be saved.
• Applications report that they cannot write files or create temporary data.
• Photo libraries, email databases or other applications struggle to update.
• Backups or macOS updates fail because there isn't enough working space.

What it means: This is more serious than a warning. Save what you can, protect important files and free up space safely. If an update fails, Apple's guide If an error occurs while updating or installing macOS provides further steps. 

Level 5 — Significant system instability

• Several applications fail to launch or save work.
• Applications repeatedly quit.
• The Mac becomes unusually unresponsive.
• Important system operations fail.

What it means: Storage exhaustion may be contributing, although software or hardware problems can produce similar symptoms. Protect your data and address the problem promptly.

Level 6 — Critical: startup or recovery problems

• The Mac cannot start normally or repeatedly becomes unusable.
• A system update leaves macOS unable to start.
• Recovery or repair becomes necessary.

What it means: This requires urgent attention, but a full disk isn't necessarily the cause. For example, a flashing question-mark folder indicates that the startup disk is unavailable or doesn't contain a working macOS installation. See Apple's guide to a Mac starting up to a question mark. 

2. Does a Mac always progress through these stages?

No. Your iMac might operate perfectly well with relatively little free space, then encounter a problem when a particular task needs more room.

Equally, a Mac with 40 GB free might run out of working space during a demanding operation, while another with less available space might continue handling everyday tasks without visible trouble.

The amount of free space matters, but so does what the Mac needs to do with it.

3. How I would investigate it

There have been several suggestions,  each on their own have merit,  so there is no need , at this time to rehash them again, at this time 

arko2 wrote:

I’m just writing because I notice, in the day to day running of my Mac, that in the Finder, disk space seems to dwindle, often over the course of several days (for instance it might hold steady at say 40GB for a few days, then dwindle to 35GB, then dwindle a bit further, etc)…

I would suggest making sure the Free storage space value starts at 100GB. If you start your workloads with less, then you will always be fighting with storage. Even with 100GB Free, sometimes even that 100GB can disappear very quickly even with seemingly simple workloads. I had a pending software update eat up lots of storage until I installed it. My configuration was only to download updates, but it seems download means download & stage and wait to delete the original installation files.

I appreciate your explanation of why macOS gradually consumes disk space, and the different ways in which macOS displays free space, and when I have a bit of free time I’m going to be studying your post a lot more closely to understand the things you mentioned, and work out how it applies to my own system!

Thanks a bunch to everyone who gave me further good advice and info, it's appreciated, and this is something (when I have a little bit of time) that I'm going to be reading over very carefully, to understand what's been said, and work out what steps to take (thanks again)!

(Just wanted to add that obviously there's quite a bit to read through here, but I look forward to studying this really closely, and will definitely post back with any feedback or anything else I might have!!)

Thanks very much — appreciate your advice — will try out your recommendation and other steps, and once I have some ‘data’ from my troubleshooting, will post back here to let you know how I went!

Thanks for this, appreciate it!!