Change the Maximum Size Limit for Outlook PST and OST Files

Microsoft Outlook stores mailbox data in PST and OST files. By default, modern versions of Outlook typically have a file size limit of 50 GB for Unicode PST and OST files. If Outlook reaches this limit, users may see errors stating that the Outlook data file has reached its maximum size. Microsoft provides registry values that can be used to increase or adjust this limit.

Registry Location

For Outlook 2016, Outlook 2019, Outlook 2021, Outlook 2024, and Microsoft 365 Apps, use the following registry path:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

If the PST key does not exist, you may need to create it manually.

Registry Values to Create

Create the following DWORD (32-bit) Values:

MaxLargeFileSize
WarnLargeFileSize

Set them using Decimal values:

MaxLargeFileSize   = 102400
WarnLargeFileSize  = 97280

These values are measured in megabytes. A value of 102400 MB sets the maximum file size to approximately 100 GB, while 97280 MB sets the warning threshold to approximately 95 GB. Microsoft notes that MaxLargeFileSize controls the absolute maximum size of the PST or OST file, while WarnLargeFileSize controls the point where Outlook stops allowing additional user data to be added.

Steps

1. Close Outlook.

2. Open the Start Menu.

3. Type regedit and open Registry Editor.

4. Navigate to:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

5. Right-click in the right pane and select:

New > DWORD (32-bit) Value

6. Create the first value:

MaxLargeFileSize

7. Double-click it, select Decimal, and enter:

102400

8. Create the second value:

WarnLargeFileSize

9. Double-click it, select Decimal, and enter:

97280

10. Close Registry Editor and reopen Outlook.

Important Notes

Increasing the PST or OST file size limit can be useful as a temporary workaround, but it is not always the best long-term fix. Very large Outlook data files can cause performance issues, corruption risk, slow searches, sync problems, and profile instability.

For OST files connected to Microsoft 365 or Exchange, it is usually better to reduce the cached mailbox size, archive older mail, or recreate the Outlook profile when needed. For PST files, consider splitting old mail into archive files instead of allowing a single PST file to grow indefinitely.

Microsoft also recommends keeping a gap between the warning size and the maximum size so Outlook has room for internal file operations.

Copy-and-Paste Registry File

You can also create a .reg file with the following contents:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST]
"MaxLargeFileSize"=dword:00019000
"WarnLargeFileSize"=dword:00017c00

This sets:

MaxLargeFileSize  = 102400 MB
WarnLargeFileSize = 97280 MB