How to Increase The Size Limit Of PST And OST Files In Outlook For Windows?

Important: This article only applies to Outlook for Windows.

PST and OST are Outlook file formats. PST is the format used by Outlook for backups (mailbox data that is exported and then imported). OST is the format used by Outlook to keep a local cached copy of mailbox data. By default, when you try to open, you will receive the following error, Starting from Outlook 2010 version the desktop application is able to open .pst files and create .ost whose size does not exceed 50 GB. If you try to open a .pst file larger than 50 GB, you will receive the following error:

Important: It is not recommended to have a .ost file larger than 50 GB or open a .pst file larger than 50 GB as it may corrupt the Outlook profile. If your .ost file has reached the limit, it is recommended to reduce the .ost file size instead of increasing the file limit. However, you may need to do this temporarily to upload the data from the .pst file to the server and you will find instructions for this below.

To increase the maximum size of .pst and .ost files that your Outlook can open or create, you will need to add or modify the following Windows registry entries: · MaxLargeFileSize registry entry (default value is 51,200 MB (50 GB)): Sets the absolute maximum size that both .pst and .ost files can grow to. After reaching this maximum size, Outlook does not allow the file size to grow beyond this size. · The WarnLargeFileSize registry entry (default value is 48,640 MB (47.5 GB)): determines the maximum data that both the .pst and the .ost files can have. After this maximum data is reached, neither the .pst nor the .ost files are permitted to add any more data. However, the size of the physical file may still increase because of internal processes. These settings apply to both .ost and .pst files. Important: It is recommended that the value between the MaxLargeFileSize registry entry and the WarnLargeFileSize registry entry differ by at least 5 percent (%) so as not to interfere with the continuation of internal processes.

To modify Windows registry entries, follow these steps:

1. Click Windows Start, type regedit and click Registry Editor to open it.

2. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\PST (x.0 corresponds to the Outlook version: 16.0 = Outlook 2016, 2019 and 365, 15.0 = Outlook 2013, 14.0 = Outlook 2010)

Note: If the PST key of the above path does not exist, create it by right-clicking on the previous key in the Registry (Outlook) > New > Key > Enter the name PST > Press Enter.

3. Click the PST, then right-click the MaxLargeFileSize entry in the right pane and click Modify.

Note: if the entry doesn’t exist, create it by right-clicking on the empty space in the right pane > New > DWORD (32-bit) Value > type in MaxLargeFileSize > press Enter

4. Select Decimal, type the range (102400 for 100 GB) in the Value data and click OK.

Note: The value of registry entries must be set in bytes or megabytes and you will need to calculate it first. You can use any online converter to calculate the value, for example, This – Enter the Gigabyte value, click Convert – the value you will need is in binary. We do not recommend increasing the limit to more than 100GB due to the nature of our shared hosted Exchange environment.

5. Right-click WarnLargeFileSize, and repeat step 4 with a different value (set WarnLargeFileSize to 97280 – 95 GB if you set the value for MaxLargeSize to 100GB).

Note: If this entry does not exist, create it following the same instructions as Step 3 but changing the name to DWORD.

Despite the fact that after making the registry changes described above you will be able to open .pst files larger than 50 GB, it may significantly reduce the performance of your Outlook application.