What are the recommended .ost and .pst file size in MS Outlook?

In this content we will discuss the recommended .ost and .pst file size in Microsoft Outlook.

  • In 2003 and 2007 of Outlook, the maximum recommended size of a Unicode
  • Personal storage file and ost file has been limited to 20GB.
  • In MS Outlook 2010, 2013 and 2016, the max limit is 50GB.

Variation in maximum limit over time

In the case of Outlook 2007 SP1 or later on an average computer that is less than

4 years old, noticeable performance declines at around 12GB.

And in the case of Outlook 2003, the decline is around 4GB.

If you are storing a considerable amount of items within a single folder (like 20,000 plus), the performance will decline much sooner.

Now let us see in detail the recommended .ost and .pst file size in MS Outlook

Recommeded file size for ost files:

As ost-files are only the cache of an online mailbox (but maybe not when using Outlook 2013, Outlook 2016, Outlook 2019 or Office 365 with an IMAP account), the above recommendations don’t really apply to the ost-file.

In the worst case, delete the ost-file and Outlook will rebuild its cache at the time when you start it again.

Obviously, this could take a while when you have a larger mailbox, caching a lot of additional data will slow down the connection.

So,your ost-file will grow as large as 30GB.

Note: Outlook 2013, Outlook 2016, Outlook 2019 and Office 365 even compress a lot of the data within the ost-file itself so caching a 10GB mailbox could take as little as 4GB of disk space.

However, when you start getting performance issues or when you need to rebuild your cache on a slow/expensive connection

So, you can reduce your cache size by doing the following:

  • Try disabling caching of shared mailboxes completely.
  • Cache the non-mail folders.
  • Disable caching of Public Folder Favorites.
  • Reduce the amount of Public Folder Favorites.
  • Configure the Sync Slider (Outlook 2013, Outlook 2016, Outlook 2019 and Office 365)
  • Configure Synchronization Filters.

You can re-enable caching if your hardware allows.

Recommended file size for pst files:

Recommended size is 2GB, you should avoid further pushing it.

You can increase the pst file size based on three factors:

Backup time

With pst-files, it is of course important to back them up.

Having 3 “closed” archives of 3GB and 1 “active” archive or mailbox of 1GB is easier and quicker to backup than having one large “active” archive of 10GB.

The 3x3GB ones only need to be backed up once and the 1GB one regularly which is quicker than regularly backing up the entire 10GB one.

Backup space

Building on the previous example, assume that you want to create daily backups of your pst-file and keep a history of 1 week.

When you have a 10GB pst-file, this would take 70GB.

When you have 3x3GB “archive” pst-files which no longer change, you don’t have to back them up on a daily basis anymore so they will only take up 9GB. Together with the 7 daily backups of your “active” 1GB pst-file this would only take up 9GB + 7GB = 16GB of space.

Error checking time

In case of any errors, you might be using scanpst.exe to check your pst-file for errors, then size is a big issue. Scanpst.exe is a very disk intensive operation as it will check every single bit of your pst-file for integrity issues and also cross-references several indexes.

Its given that the larger the pst-file and the more items it contains, the longer it will take for it to complete. With a very large pst-file, it will take a long time to scan.

Note: After a major cleanup of your mailbox, make sure you also compact it afterwards to actually reduce the pst-file size.

To Sum Up

In this blog post I have explained the Recommended .ost and .pst file size in MS Outlook to give you a brief idea about its size and working.