How to Configure OST File Size Limit in Outlook?

Every OST files has definite file size that maintains the saving space in the Exchange Server and also in the user’s system. For better system and Exchange Server functionality, it is always advised to set the maximum OST file size to 1 GB. Beyond 1 GB the system performance gets affected i.e. speed may get slow, there is low response time and some more ill effects such as synchronization errors, failure in loading or opening .ost files, etc.

Usually, when mailbox reaches 2 GB limit, it stops saving the mails. Consequently, user cannot save the upcoming mails and it leads to technical mess. For any user with some technical guidance it is very easy to make the way for limiting the OST file size. There are two ways to configure OST file size limit via Windows Registry value settings without affecting the file content. First learn about two crucial Windows Registry settings for file size.

For that, user should first learn about two crucial Windows Registry settings for file size.

Via Windows Registry setting method

You can try some basic solutions at first like compacting the OST file size to compress the file removing the extra space by visiting the OST file location through Account Settings and then from the Advanced Settings, using the Compact Now option to compress the file size and second one, using the Mailbox Cleanup tool to empty the deleted items permanently and making some space. One more way to reduce the OST file size is using the Archive feature by visiting File>Info>Clean up old items in Outlook to archive OST file emails to the PST file.

But that is not enough, you are required to limit the file size yourself. There are two Windows Registry settings that configure OST file size limit without affecting the file content. First learn about two crucial Windows Registry settings for file size.

  • WarnFileSize Registry Entry: The registry determines the allowed file size which can be saved to some extent. Once the saving saturation capacity is reached, no more files could be saved either from OST or PST. However, user can increase the physical file size as it is a completely independent domain. The default value for WarnFileSize Registry Entry is 1.816 GB.
  • MaxFileSize Registry Entry: This registry limits the maximum file size of OST file to be saved. When the maximum the saving limit is reached, Outlook stops saving the OST files further.

In some cases it is observed that registry values do not exist and it is user’s responsibility to create it according to the Outlook version. The default value for MaxFileSize Registry Entry is 1.933 GB.

How to create Outlook Registry Values?

To create the registry values in Outlook, it is advised to follow these steps:

  1. Click ‘Start’ button followed by clicking ‘Run’ or press Windows button and R alphabet together. In command line, type Regedit click on OK and expand these registry keys:
    Version Path
    Outlook 2003 HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\Outlook
    Outlook 2007 HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook
    Outlook 2010 HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\14.0\Outlook
    Outlook 2013 HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\15.0\Outlook

    And same path location can be traced for next Outlook versions, i.e. Outlook 2016 and Outlook 2019.

  2. Hit Edit menu and select ‘New’ then click ‘Key’.
  3. Type OST file name which is to be limited.
  4. Now click DWORD value.
  5. Write MaxFileSize and hit Enter key twice.
  6. In Edit DWORD value window, enter the required value (in MB) and click OK.
  7. Create another DWORD Value WarnFileSize and repeat the above steps.
  8. Now, close the Windows registry.

Once the Outlook registry is created everything is under the user’s control. It means users can now configure OST file size limit in Outlook anytime using Windows Registry settings. However, close attention has to be paid while making changes in the registry values. This is because during making the changes in the registry values through manual process, there is possibility of deletion of some important files. Therefore, it is recommended to take back-up of the registry values before making any changes. To get the effects of the modifications in the Windows Registry, reboot your system once and then use the OST file in Outlook.

For new users or not-so-technical ones, who do not have any knowledge of the making registry changes, it is advisable to take assistance of somebody professional. Once OST file size limit is configured in the registry values, the Outlook would activate the compact function so that unutilized space can be utilized for saving the OST files. In case the process seems complicated user can go ahead with SysTools OST to PST Converter Tool. It not only manages the time but also sorts out every OST-related query or issue. The tool recovers data from corrupt, inaccessible and orphaned OST files and saves desired data to the PST file format for managing data efficiently. Just download the trial version of the tool to know more about it. Upon satisfaction over the functioning of the tool, user can purchase the licensed version from the website buy page easily.

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Seeking the issues due to grown OST file size in the Outlook application, a manual procedure to configure OST file size limit in Outlook via modifications in Windows Registry settings is explained which can be not so useful for novice users. For compensating that, a reliable Best OST to PST Converter software is recommended too for resolving all OST related issues and queries.

About Barrett Clay

Barrett is passionate about technology and an admirer who believes in offering instant computing solutions to everyone. He solves personal and business technical issues related to IT products, applications, IT environments, platforms, technologies, etc., through well-documented blogs and articles, informative, authentic and descriptive content. He participates in technical content development, content publishing and marketing.


  1. I was facing the issue of opening the OST file while using Outlook. I was not aware of the issue of the OST file size limit when I asked my friend about it. He advised me to visit this blog and apply the methods suggested by the blog. After reading the blog and using these methods, I have successfully resolved this issue. Thank you for sharing this amazing blog.

  2. While continuously getting synchronization error and slow Outlook performance issue, I started to find out the reason of the issue that was the oversized OST file. So I configured OST file size limit by creating Outlook registry values. I also setup the Windows registry settings to mange OST file size limit. Such a great post for Outlook users who want to avoid OST file size limit error.

  3. It was needed to configure OST file size limit for proper functioning the system. And this solution helps to understand OST file limitation through manual ways. After setting of file size limit, my mailbox responded well and opening emails in the shortest time period.

  4. My mailbox responding slow because of bulky size of OST files and I configures OST file size limit by above manual methods. After creating Outlook registry, it is easy to control OST file size limit.

  5. From long time period I am seeking for free method to limit OST file size in Outlook. I faced many issues due to oversize of mailbox and this reliable method help me figure out issues and solve issues. Thanks to the entire team.

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