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Why isn't my Outlook PST getting smaller after deleting emails?


I archived and deleted a bunch of email messages in Outlook, in order to reduce the size of the PST file containing them. However after doing so, the PST’s no smaller. What gives?

There are two very common possibilities:

  1. there’s deleting, and then there’s DELETING
  2. and even after DELETING, a little patience is called for

Fortunately, there are ways to force things and bypass that whole pesky “patience thing”.

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Much like Windows itself, your PST has an equivalent to the Recycle Bin called the “Deleted Items” folder. By default, when you delete an email message (or just about anything that Outlook stores in your PST), it’s not deleted at all, but simply moved to the Deleted Items folder.

The whole point of Deleted Items is as a safety net; a way to recover from “oops, I didn’t mean to delete that” because things aren’t really deleted.

So if things aren’t really deleted, then of course no space will be freed up.

You can configure Outlook to empty the recycle bin when you exit Outlook, or you can simply empty it yourself. In the folder list, right-click on the Deleted Items folder and click on Empty “Deleted Items” Folder:

Empty Deleted Items folder selection in Outlook

This will permanently delete the items from your PST.

However, your PST won’t get smaller. At least, not right away.

“If you empty your Deleted Items folder, and just leave Outlook alone for a while, sometimes a long while, it will dutifully compact and shrink your PST.”

When Outlook actually deletes an email (i.e. when it does not put it in the Deleted Items folder, or when it removes it from that folder), the space taken up by that email is “freed”. In other words that space within the PST is marked as unused, and made available for other email messages that might arrive thereafter. So your PST won’t get smaller right away, but it’s also quite likely that it won’t get bigger as new mail arrives either. The space freed within the PST by emptying the recycle bin will simply get reused.

I keep saying “right away” because Outlook will start making your PST smaller, after some time.

If you leave Outlook running, after some amount of idle time, it will start to perform something called “compaction”. Compaction is very similar to defragmenting your hard disk. During compaction, it moves all of the email messages and other “real” information in the PST to the front of the PST, and moves all the unused or free space within the PST to the back. When compaction is done, Outlook can then actually make the PST file smaller by removing that portion of the PST that isn’t actually being used by anything.

That’s where patience comes in. If you empty your Deleted Items folder, and just leave Outlook alone for a while, sometimes a long while, it will dutifully compact and shrink your PST.

If you’re impatient, like I am, you can force compaction.

In folder view, right-click on Personal Folders:

Outlook Personal Folders context menu

And click on Properties for “Personal Folders” …. You’ll get this dialog:

Outlook Personal Folders Properties dialog

Click on the Advanced… button, and you’ll get this:

Outlook Personal Folders Properties Advanced dialog

And as you might guess, hit that Compact Now button. You’ll then see this small dialog for a while:

Compacting Now Status Dialog

Exactly how long will depend on the size of your PST, and how much free space there is within it. It can take quite a while for larger files.

When it’s done, your PST will be as small as it can be to hold what’s inside.

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17 comments on “Why isn't my Outlook PST getting smaller after deleting emails?”

  1. I use ‘SeaMonkey’ formally ‘Mozillasuite’ and the integrated email client. I used a program to back it up one time and it was only a few megabytes in size, then at another time I ran a second backup and the file was over 50MB in size.

    I always chose the option to delete the cache and couldn’t understand what was going on. I went to my email folder and found some very large files there like the one called ‘inbox’, there was no extension.

    I went online to see if I could safely delete this file, thats when I discovered what ‘compacting’ meant.

    With Seamonkey and Thunderbird, just rightclick on each folder you want to compact and chose the option from the menu.


  2. Thank you so much for the ‘compacting’ option. After the longest time, in order to do a backup of the .pst file over a network, I’ve been trying to figure out what made the file smaller and now, thanks to you, I know. Thanks again.

  3. Leo,

    Thanks. Great advise. My outlook file bloated to 9GB – while I have removed some older mails to a new pst – my active pst showed 9GB while the personal folders were less than 3GB – as you may have gathered, I receive a heavy e-mail load…

    How long does it take to compact the file?



  4. Hi there. I had a huge main outlook.pst file (3gig), and to start the year fresh I decided to break it up into several PSTs. After i was done, I chek the folder size in my Outlook and it says my outlook.pst file is less than 1gig, but when I see the same pst file size in windows explorer it says 7.7gigs.

    any info would be really appreciated

    Be sure to empty the recycle folder, and then leave Outlook running overnight and it’ll probably compact the file.


  5. I have a pst file that after archiving a bunch of things. and cleaning out the deleted items folder will not compact. When I run it the dialog “compacting” show for a couple seconds and goes away. Any thoughts?


  6. Never ceases to amaze me. If you want to find out how to accomplish tasks such as this one you will never find the answer by going to Microsoft for help. It takes people like Leo to give you a direct answer that does just what you asked for. Microsoft gives you pages and pages of information that sucks up all your time reading and never leads you to the answer you need.

    Thanks Leo

  7. After clearing your “Deleted Items” folder and compacting the PST, your PST maybe still be substantially larger than the folder sizes inside. The next step would be to create a new PST, and move all folders to it, one by one.

    Make sure to switch to “Folder List” view, so you can drag the contents of the calendar, tasks and notes as well.

    After completing the job, you should be happy to see a huge further reduction (depending on your case), but now you will have to configure all the display parameters for the different folders, including which columns are shown, group by, category list, etc. I need to find out where this configuration data is stored, so I could copy it to the new PST, without manually redoing it.

  8. Today is april 28th. I’ve just spent all day trying to understand why afer deleting ALL of my emails my pst file wouldn’t get smaller. It’s 4:53 pm and I’ve just seen this topic. I just wanna say THANK YOU, Leo!!! And [edited] Microsoft…

  9. If you don’t see the “Compact Now” option where Leo advised, here is the alternate location.
    Tools->Options->Mail Setup->Data Files->Choose the data file and double click -> Compact now.

  10. Hi, just note – in 2 days the outlook.pst reduced from 5.8 GB to 4.9 GB, so it seems it is doing something but very slowly , approx 0.5GB per day. So some little light at tunnel end. :-)

  11. I use Outlook 2007 with large file size. The .pst measured 4.4gb before I ran an archive for everything over 3 months old. Clicking the Folder Size button in Personal Folders Properties shows a total of 641424kb of data in all the folders/subfolders. After compacting, it reports a .pst file size of 2.9gb. Better, but nowhere near what it should be with
    Short of exporting the database to a new .pst file and tricking Outlook to accept it as the active .pst, any other suggestions?

    Check some of the other folders that you might not use often – for example journalling, if turned on, can eat up a lot of space.


  12. Leo,

    Is it possible to defrag the pst (outside of Outlook) to reduce the crazy wait time? My 10G file has been cranking away for 6 hours?

    The PST defrags like any other file, so if you defrag your machine that’ll help it. If you mean its internal structures, then compacting is what you want. And for the record, while I know it *should* work these days, a 10 gigabyte PST makes me nervous. Smile


  13. For all you impatient people (like me): Perhaps you are frustrated by the deafening silence of the process. If so, go to the place where your pst is stored and you will be able to see (using refresh) the progress of the compaction. My 10.3 G file is now 8.45 G and shrinking.

    …counting backwards from 8 billion is still a daunting task…

  14. Hi Leo,
    this is great post, My .pst file si 4GB in size and it make my outlook slow. i have deleted some file and run compact now… it become smaller now. great… stuff.

  15. It does not work. Neither deleting and EMPTYING the deleted folder nor compacting.

    After many many days and after having removed more than half of my old emails, the pst files are exactly the same size.


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