Configuring PST and OST File Size Limits in Outlook

The Outlook email client stores mailbox items on the user’s computer in special data format files—PST and OST. Each new email item that is saved into the Outlook data file increases its size. In some cases, when sending/receiving mail in Outlook, you may get an error:

The Outlook data file C:\Users\xxx\Outlook.pst has reached the maximum size. To reduce the amount of data in the file, permanently delete some items that you no longer need.

Not all users know that the maximum size of a PST or OST file in Outlook is limited. Different versions of Outlook have different limits on the maximum file size:

  • Outlook 2003 and Outlook 2007—20 GB is maximum OST/PST file size in the Unicode;
  • Outlook 2010, Outlook 2013, Outlook 2016—50 GB file size limit on PST file in Unicode.

This maximum PST file size is considered optimal by a Microsoft. Although technically you can increase these limits up to 4096 TB. However, before you increase the maximum file size, you need to consider the important factors:

  • With a large PST/OST file size (more than 10 GB and 100,000 objects), Outlook’s performance decreases significantly, the program opens slowly, user facing the delay in opening mailbox item (especially felt on HDDs; on SSD, the performance drops not so much);
  • The larger the size of the data file, the greater the likelihood of its damage due to an error on the disk or file system (you risk losing the entire data file).

Therefore, it is recommended to split one large PST/OST file into several PST archives.

For PST/OST files in ANSI format there is another hard limit—the file size is not more than 2 GB. ANSI PST files are quite rare: they were mainly used in older versions of Outlook. The probability to face the mail archive in ANSI format is now quite low. You can find out the format of the PST file in the Outlook by pressing File > Account Settings > Data Files > Outlook Data file > Settings. Check the Mailbox Mode value. In our case it is indicated here «Outlook is running in Unicode mode against Microsoft Exchange».

increase ost file size outlook 2016 to 100gb

Restrictions on the size of the PST data file can be configured through the registry. Depending on the version of Outlook, different registry keys are used:

Outlook 2016:

  • HKCU\Software\Microsoft\Office\16.0\Outlook\PST
  • HKCU\Software\Policies\Microsoft\Office\16.0\Outlook\PST

Outlook 2013:

  • HKCU\Software\Microsoft\Office\15.0\Outlook\PST
  • HKCU\Software\Policies\Microsoft\Office\15.0\Outlook\PST

Outlook 2010:

  • HKCU\Software\Microsoft\Office\14.0\Outlook\PST
  • HKCU\Software\Policies\Microsoft\Office\14.0\Outlook\PST

Outlook 2007:

  • HKCU\Software\Microsoft\Office\12.0\Outlook\PST
  • HKCU\Software\Policies\Microsoft\Office\12.0\Outlook\PST

Outlook 2003:

  • HKCU\Software\Microsoft\Office\11.0\Outlook\PST
  • HKCU\Software\Policies\Microsoft\Office\11.0\Outlook\PST

In each of these branches, you can use the following registry parameters:

  • MaxLargeFileSize—maximum size of PST and OST file in Unicode format. When this value is reached, Outlook prohibits an increase of file size;
  • WarnLargeFileSize—PST/OST file size when reached is prohibited to add new items to the file, but the file size on the disk may increase due to the various internal processes;
  • MaxFileSize—maximum size of PST files in the ANSI file format;
  • WarnFileSize—maximum amount of data in PST file in ANSI format.

When the maximum values of the data file are reached, Outlook stops sending and receiving emails, prompting the user to clear the place using the built-in mailbox cleanup wizard. You can find and delete old items or archive them by moving some email to a separate PST file.


increase ost file size outlook 2016

To increase the limit to the maximum PST file size, for example, up to 100 GB, open the regedit.exe, go to the key corresponding to your version of Outlook, and create two REG_DWORD parameters:

  • MaxLargeFileSize—100000 (decimal) or 186a0 (hexadecimal);
  • WarnLargeFileSize—950000 (dec) or e7ef0 (hex).

Restart Outlook. Now the maximum size of a PST file is limited to 95 GB, but its size on the disk can grow up to 100 GB due to internal processes.

In the Active Directory domain environment, you can manage restrictions on the size of OST and PST files using GPOs. To do this, you need to download and install the ADMX/ADML files of administrative templates for your Office version.
You can then control the PST file max size using the following policies in the section

User Configuration > Administrative Templates > Microsoft Outlook 2016 > Miscellaneous > PST Settings:

  • Large PST: Absolute maximum size;
  • Large PST: Size to disable adding new content;
  • Legacy PST: Absolute maximum size;
  • Legacy PST: Size to disable adding new content.

ost file size limit

It is enough to enable the policy and specify the maximum size of the PST file. Policy settings on clients are stored in the registry key HKCU\Software\Policies\Microsoft\Office\XX.0\Outlook\PST.

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