Recommended pst-/ost-file size vs. Maximum technical size

I heard that pst-files can grow as large as 50GB in size but also heard that issues will occur when it grows beyond 2GB.

My guess is that the difference is a technical limit versus a recommended limit.

Currently, my pst-file is reaching the 2GB and I wonder what I should do. Obviously, I do not want to run into issues but I prefer to keep my mailbox in a single pst-file rather than to create many little ones.

Which maximum pst-file size would you recommend?

Pst- and ost-file size buttonBy default, a Unicode pst-file or ost-file can grow as large as 20GB in Outlook 2007 and 50GB in Outlook 2010, 2013, 2016, 2019 and Office 365. However, the technical limit lies at 4194304GB (which is 4096TB or 4PB) and the default limit can be adjusted.

The 2GB limit applies to ANSI formatted pst- and ost-files. Unless you are still using pst-files which were created in Outlook 2002/XP or previous, it is unlikely that your pst-file is still in the ANSI format, but it never hurts to check whether your pst-file is in the Unicode or ANSI format.

Performance degradation

Performance buttonObviously, the technical limit is (currently) anything but workable so no wonder the Outlook Team had set a limitation on how large a pst-file or ost-file can grow.

When you are using Outlook 2007 SP1 or later on an average computer that is less than 4 years old, noticeable performance degradation usually starts at around 12GB.

When you are still using Outlook 2003, you might notice it at 4GB already.

With slower “green” hard disks (HDD) or when you store a considerable amount of items within a single folder (like 20,000 or more), you might notice this performance degradation sooner. This is especially true when you do not sort that folder by date either.

When you use a modern Solid State Disk (SSD), you probably will notice it a lot later.

Recommendations for pst-file size

Pst-file buttonSo while it is safe to go beyond the 2GB barrier, you might not want to push things too far either even if your hardware is fully capable of it.

How far depends on what is still convenient to you. Main things to consider are:

  • Backup time.
  • Backup space.
  • Error checking time.

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 the unfortunate event that you need to run scanpst.exe to check your pst-file for errors, size really matters. Scanpst.exe is a very disk intensive operation as it will pretty much check every single bit of your pst-file for integrity issues and also cross-references several indexes.

Suffice to say 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 could mean that it will scan for an entire day!

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

Recommendations for ost-file size

Ost-file buttonAs ost-files are only a 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.

If the worst happens, you can delete the ost-file and Outlook will rebuild its cache the next time you start it.

Obviously, this could take a while when you have a larger mailbox, cache a lot of additional data or are on a slow connection.

In general, you can cache what you need and what your hardware is still capable of. If that means your ost-file will grow as large as 30GB; Go for it!

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, think about the following to reduce your cache size:

Obviously, if your hardware allows it, you can re-enable caching the above when you are on a fast/cheap connection again.