Reduce the disk space used by Outlook for Mac

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Outlook for Mac is a disk space hog but the macOS has a way to recover some of that.

Mac computers can have relatively small hard drives like 128GB or 265GB which Outlook 2016 for Mac can gobble up.

Unfortunately, Outlook for Mac, even the latest Outlook 2016 doesn’t have some of the space saving options available to Office for Windows.  In particular, the ability to only sync the most recent messages.  Outlook for Mac is ‘sync all’ software.

The result can be many gigabytes used up on a relatively small drive.  Before you buy a new Mac or get a third-party drive upgrade, check out a somewhat hidden feature in the macOS – HFS+ compression.

macOS HFS+ compression

Windows users have a ‘Compress contents’ options to shrink the disk space used by files. NTFS Compress works seamlessly in the background.  It’s easy to use and available on the Properties dialog for any file, folder or entire NTFS drive. It’s not used a lot on Windows computers now because hard drives have become a lot larger and cheaper.  The compression doesn’t save as much disk space as it once did because many file formats are now already compressed (e.g. docx, xlsx, pptx, jpg etc).

MacOS has a similar feature, HFS+ compression which has been available since the Snow Leopard release.  Note: macOS also has a feature called ‘Compress’ which is a different thing entirely.

Unfortunately, HFS+ compression isn’t easy to use.  The Apple supplied method is terminal command lines!  That’s a right PITA and really strange for the usually user friendly Mac.

If you want to try HFS+ compression from the command line – check out here.

We found a nifty and cheap tool to use HFS+ compression, but it’s disappeared.  See below.

Outlook for Mac data folder location

Outlook 2016 for Mac puts the data folders in the users Library folder

~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile

Then drill down to /Data/ and three ‘Message’ folders:

  • Messages
  • Message Sources
  • Message Attachment

Outlook 2011 for Mac is different.  Data is stored in Documents/Microsoft User Data then in ‘Identities’.

What’s taking up Mac disk space?

We used Disk Inventory X to look at the entire drive and see which folders took up the most space.  Sure enough, Outlook is using 20GB.

Unlike Outlook for Windows (which has a single enormous data file .PST or .OST), Outlook for Mac saves messages and attachments in many different files.  That’s another reason why Outlook for Mac uses up so much disk space.

Now you’ve identified the Outlook for Mac data folders, you can compress them.

HFS+ Compression with Outlook 2016 for Mac

Our tests of HFS+ compression saved 9GB of disk space from 20GB of Outlook data – a whopping 47% improvement.

Here’s the results of some folder HFS+ compression.

The three Outlook 2016 message folders get shrunk nicely by HFS+ compression.

Messages was 4.6GB and now uses less than 1GB.

Message Sources was 4.5GB and is now about 3GB.

The 10.5GB Message Attachments folder is now about 7GB.  That was a welcome surprise since many of the attachments are already compress formats like .docx .pdf and .jpg

The top three folders are non-Outlook examples for reference. The Video and Top Shots folders have video and JPG files which are already compressed, so the disk space saved is tiny.  The Documents folder has a mix of many files types with about 10% disk spaced saved by HFS+ compression.


We turned to Clusters which is an app which puts a friendly face on HFS+ .  We bought it in late September 2017 for a mere US$15. A darn lot less than a Mac hard drive upgrade.

Alas, Clusters is no more.  According to LateNightSoftClusters has been discontinued and is no longer available for sale.” .  A download is still available which might only be the trial version.

We’ve asked LateNightSoft for more detail and will post here if we find out more.

Apple File System AFS

Keep in mind that Apple is changing the file system used in macOS.  HFS+ is being replaced by Apple File System APFS which does NOT support real-time compression.

Newly installed Mac will have AFS installed by default.  Older Macs have HFS+.  You can tell which from the Finder, Info pane for your hard drive.  If the Format is ‘Mac OS Extended’ then you have HFS+ and compression is available.


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