Outlook OST File Too Big – How to Fix it | 2023

PST and OST files are both important as they act as data containers. These files store all the Outlook data like emails, attachments, calendar, contacts, etc. The difference between PST and OST file is that PST file stores the data in online mode, and OST file stores the data in offline mode.

The OST file too large error might occur when your OST file is corrupted or when the huge data is piled up. In such cases, you will not be able to access your data. Now lets understand what is the actual size limit of an OST file.

Outlook Version

Size Limit of an OST file

Outlook Version 2002 The size limit is 2GB
Outlook Version 2007 The size limit is 20 GB
Outlook Version 2010 and above The size limit is 50GB

Why Does Having a Large OST File a Problem?

Here are some of the issues you might face when your OST file is too large.

  • If your OST file is huge then you will not be able to open your emails. It might take longer than usual time to open your emails in a large OST file.
  • Cannot add data to an existing Exchange .ost file.
  • You keep receiving Outlook is not responding, error message.
  • You will keep receiving errors during synchronisation of OST and MS Exchange Server.

Reasons Behind Microsoft Outlook OST File too Big

  • If your OST file is piled up with the large data then it might be a reason for the error.
  • Corrupted OST files might be one of the reasons for the OST file being too large an issue.
  • If your Outlook Add-ins are enabled
  • If you have not auto archived your OST file.

How to Fix Outlook OST File too Large?

Fix 1: Disable the Add-Ins in Outlook

Having Add-ins sometime might create a disruption in smooth performance of an Outlook. It is recommended to disable the Add-ins in Outlook.

  • Open the Outlook and navigate to the File menu, then select the Options and then click on the Add-ins.  check with the outlook options
  • From the left navigation panel under the Manage option and select Com Add-Ins and click on the  check with the com-addins
  • Now unselect all the Add-ins that are in use or not and click on the OK.  

Fix 2: Archive OST File in Outlook

Archiving the OST file might reduce the size of the OST file and can clear some of the unwanted junk that has consumed all of the space of the OST file.

  • Open the Outlook and go to the File tab and click on the Tools and then navigate to the Clean Up Old Itemsupdate the mailbox settings
  • Now you will see two available options
  • Option 1: You can move older items in the mailbox to the Archive folder by clicking on the AutoArchive
  • Option 2: You can click on the Empty and delete the items from the Deleted Items

Fix 3: Compressing Outlook Data

OST file stores the data in the offline mode, compressing the Outlook OST file manually might resolve the issue “Outlook OST file too big”.

  • Open the Outlook and click on the File tab and go to the Account Settings and select the Account Settings.
  • Then click on the Data File tab and select the data file to be compressed and click on the Settingsupdate the outlook data file
  • Now click on the Advanced tab and then click on the Outlook Data File Settings and in the Outlook Data File Settings window and click on the Compact Nowclick on the advance settings

Fix 4: Change the UNICODE Registry Value

Managing the UNICODE Registry will increase the OST file size limit. Follow the steps below to change the UNICODE Registry values.

  • Go to the Search Bar and type Regedit and select the Registry Editor. Now navigate to the registry key path.

Outlook Versions

Registry Values

For Outlook version 2007 follow the registry path HKEY_CURRENT_USER\Software\Policies\Microsoft\Policies\12.0\Outlook\PST for Outlook 2007.
For Outlook version 2010 follow the registry path HKEY_CURRENT_USER\Software\Policies\Microsoft\Policies\14.0\Outlook\PST for Outlook 2010.
For Outlook version 2013 and above follow the registry path HKEY_CURRENT_USER\Software\Policies\Microsoft\Policies\15.0\Outlook\PST for Outlook 2013.


  • You can now Right click on the MaxLargeFileSize and Modify and Type the values in MB.
  • Right click on the WarnLargeFileSize and Modify and Type the values in MB.
  • Finally, you can exit the Windows Registry Editor.

Changing the registry values might result in OST file corruption, hence we recommend you to take a backup before performing the above operations. If you have not taken a backup and encountered any issue like corruption in an OST file, then it is recommended to make use of a professional Outlook PST Repair tool.

Remo Recover for windowsSteps to Repair Oversized corrupt OST file

Step 1: Launch the tool, select from the three options Open Default PST file, Find All Your PST files, Select PST File Manually. Browse the OST file that needs to be repaired.

download and install remo repair outlook pst

Step 2: Now you can choose between two scan options, Normal and Smart scan.

Normal Scan: Normal scan comes in handy when the level of PST/OST corruption is low as it ensures quick OST repair.

Smart Scan: Smart Scan can help you recover emails, contacts, notes and other Outlook data items from a severely damaged PST/OST file. You can also choose this option if a normal scan fails to repair the OST file.

click on the scan

Step 3: Click on Repair to start the OST repair process. After completion of the OST repair process Browse the location of the OST file and, you can also click on the Preview to see the repaired OST file.


Hope this article was helpful in fixing the OST file that is too large. By the above discussion we got to know some of the reasons and the issues you might face with regards to oversized OST files.

Do share the article with your colleagues and comment down your thoughts below.

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