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4.5. Increase maximum size for pst-files and ost-files

In Outlook 2003 and Outlook 2007, the maximum recommended size of a Unicode pst-file and ost-file has been limited to 20GB.

In Outlook 2010, 2013, 2016, 2019 and Microsoft 365, this recommended  limit has been set to 50GB.

You can increase (but also decrease) this limit via the Registry or via Group Policies.

Note: This tip does not apply to pst-files and ost-files which are in the Outlook 97-2002 format (ANSI formatted pst-files).

Via the Registry

To increase the maximum size of pst-files and ost-files in Outlook, you’ll need to create and set 2 values in the following location in the Registry;

• Outlook 2003
  HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\PST
• Outlook 2007
  HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST
• Outlook 2010
  HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\PST
• Outlook 2013
  HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST
• Outlook 2016 / Outlook 2019 / Microsoft 365
  HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

The following 2 new DWORD values must be created or set;

• WarnLargeFileSize
  Don’t set this higher than 4090445042 (decimal) or f3cf3cf2 (hexadecimal)
• MaxLargeFileSize
  Don’t set this higher than 4294967295 (decimal) or ffffffff (hexadecimal)

The first value is how many MB a user can write to a pst-file or ost-file before receiving a warning that the file is full. The second value is how many MB the system can write to a pst-file or ost-file. This difference has to be at least 5% since there is more written to a pst-file and ost-file than just user data.

In this example the maximum size of pst-files and ost-files has been set to 100GB with a warning at 95GB.

Note: Although the Registry path says “PST”, it also applies to ost-files.

Via Group Policies

If you are a corporate administrator and want to adjust the maximum allowed sizes of pst-files and ost-files, then you can use Group Policies as well. For detailed instructions on how to use and deploy Group Policies see this guide.

The settings for adjusting the maximum file size for pst-files and ost-files can be found in the section;

• User Configuration-> Administrative Templates-> Microsoft Outlook <version>-> Miscellaneous-> PST Settings.

The following 2 settings need to be modified;

• Large PST: Absolute maximum size
  Don’t set this higher than 4294967295
• Large PST: Size to disable adding new content
  Don’t set this higher than 4090445042

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