Want to increase the OST file size limit in Outlook to store more data on email profiles? Then read this article from start to end. With this write-up, we will show you the process to increase OST file size in Outlook 2021, 2019, 2016, 2013, 2010, and other editions.

MS Outlook email client allows users to send/receive emails from their desktops. Outlook performs emailing process and saves the user’s profile data locally on their computer. For locally saving email account data Outlook uses PST and OST data files.

Each sent or received item of their Email accounts Outlook preserves in this OST file. Outlook older editions create OST files in ANSI format which has the limitation of storing data up to 2GB size.

With newer editions of the Outlook application, Microsoft has introduced Unicode OST files. In comparison to the ANSI format, the UNICODE OST file has the ability to store more data.

Outlook 2003 and 2007 editions, can create Unicode OST files with a size limit of 20 GB. In newer Outlook editions (like- 2021, 2019, 2016, 2013 & 2010) users get the OST file size limit created of 50 GB.

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Why do users need to Increase OST file size in Outlook?

By default, Outlook’s newer editions have a pre-configured OST file size limit of 50 GB. But by downloading data from the mail server, OST files increase their size and sometimes cross their size limit. After crossing this size limitation, OST files start creating different problems in Outlook.

Challenges users face with large-size OST files: –

Because of these problems with OST file size, users require to increase its limitation.

Method to Increase Outlook OST file size in Outlook 2021, 2019, 2016, 2013, 2010

By default, Outlook sets the OST file size limit up to 50 GB, but users can manually increase its size. The size limit for OST files users can extend up to 4096 GB (4 PB). ANSI OST files don’t support size increasing, users can only increase the size limit of Unicode OST files.

So, before proceeding with steps for increasing the OST file size limit check the format of your OST files.

How to Check the format of Outlook OST files?

Apply these steps to check the format of the OST file: –

  • Launch MS Outlook, click on File > Info > Account settings > Account Settings.
  • Click on Data File tab from the next opened window.
  • Select your email profile/outlook data file and click on “Open file location”.
  • In windows explorer view the OST file, right-click on the Outlook OST file, and select “Properties”.
  • Click on the “Details” tab and check the type.

Note: – The Type users will get an Outlook data file (this indicates that your OST file is in UNICODE format). If the Type is as Personal Folders File (97 – 2002) or Outlook Data File (97 – 2002), it means your OST file is in ANSI format.

Solution: Increase OST file size via Windows Registry Editor

With the windows Registry Editor, users can safely modify registry keys and also can create new registry keys or files to increase the size limit of OST files. Apply these steps to increase the size of OST files:-

With the windows registry editor tool users can safely modify registry keys and also can create new registry keys or files to increase the size limit of OST files. Apply these steps to increase the size of OST files: –

  • Press Windows + R key from your keyboard, type regedit and press Enter key.
  • According to your Outlook application’s version, visit the following location in the Registry editor.
  • Now create or set these two values –
    • WarnLargeFilSize set this value up to 4090445042.
    • MaxLargeFileSize set this value up to 4294967295
  • Save the changes and restart your computer.

After restarting the computer, open Outlook and click on File > Info, and there you will see the increased size limit of OST file.


The default size limit which is defined by Outlook for storing data in OST files is 50 GB. This size restriction becomes a huge problem for users. Also, after exceeding this limit Outlook generates several errors and effects in the functioning of Outlook. As a result, users should increase the OST file size limit in Outlook. In the above section, we have shown the complete process and steps, by which users can safely increase OST file size.

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