How to Reduce Outlook Offline OST File Size?

admin | May 9th, 2017 | Outlook

As a user creates his mailbox and keeps on piling data as he receives emails items frequently, then his MS Outlook mailbox tends to grow heavily in size. This is about the offline OST file i.e. the Outlook Data file, which gets increased in volume when more & more data keeps piling up to their mailboxes. In the UNICODE format, the size limitation is up to 50 GB and when this reaches, it apparently shows up that the OST file has become very large in size, whereas in the ANSI format, the file size can go up to even 2 GB. Also, an unfortunate aspect is that if any data is deleted, there won’t be a considerable difference in the file size and it will not get reduced much in the proportion to that of the deleted volume. The reason for this is that it happens as the free space gets possessed by some white spaces there after. Still, there exist several methods that can help you to manage/reduce the size of your OST file.

Explaining OST File

OST file is the ditto of your MS Exchange mailbox, which is stored locally on your system whenever the Cached Mode in MS Exchange gets enabled. An OST file is formed in an automatic fashion through Outlook’s feature of the offline folders and becomes active whenever MS Exchange Server goes in offline mode. Whenever the internet connectivity is once again retained, these OST files get synched in an automatic way to your MS Exchange email account and all your email data get updated soon after.

Why OST Gets Too Big in Size?

The OST files contain all the data generated/received since the time your account was created, unless those items that might have been deleted by you. The data storage capacity of OST files depend on the Microsoft Outlook version being used/installed. In Outlook 2002 or its previous versions, the file size limit is 2GB; whereas, in MS Outlook 2007 version, this default storage limit is 20GB. In Outlook 2010 & version later to it, the storage capacity is 50GB. If, at some point of time, you are noticing that the OST file has become too big in size; then, it becomes an imperative task for you to manage your mailbox, else it might lead to OST corruption. Outsized OST files can make your mailboxes to be completely inaccessible, wherein, you won’t be able to access your any of your email items.

How to Compact or Reduce the Size of Outlook OST File?

  1. Delete those email items that you do not need any longer
  2. Now, open the Folder List view
  3. And then, click the icon for the Folder List there at the lower side of the appearing Navigation pane
  4. After that, click File tab
  5. And then, click Account Settings
  6. Now, select Account Settings
  7. And next to this, select that particular OST file which you need to reduce the size of under the Data Files tab
  8. And after that, click Settings
  9. Lastly, click Compact Now

Another Way of Reducing OST File Size

  1. First, click File tab
  2. And then, click Cleanup Tools
  3. After that, select Mailbox Cleanup
  4. And next to this, you can do either of the below mentioned steps:
    • For moving older email items that were located in your mailbox to the archive folder, you can click on the AutoArchive option
    • For deleting email items there inside your Deleted Items folder, you have to click Empty


If your OST file gets enormously and abnormally big, it might start affecting your system; and is also not a favorable thing for your Server as well. So, it is very important that you reduce the size of your Outlook OST file and manage your mailbox regularly to make sure that your mailbox continues to function appropriately. Reducing OST file size would probably improve your mailbox and system performance. The above methods might seem good to be use but some they might be lengthy. So, a better workable alternative to this is to call the experts from some service offering vendor near to you.