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Detailed Steps on How To Reduce the Size Of Oversized OST File

Published By Nilesh Kumar
Debasish Pramanik
Approved By Debasish Pramanik
Published On January 25th, 2024
Reading Time 5 Minutes Reading
Category Outlook Tips

OST is an offline storage file of Outlook that stores mailbox data that is an exact replica of Exchange Server. It is saved on the local machine and synchronizes with the server whenever online. Emails can be read and deleted even when offline using this data file. Edits will be saved on the storage file and updated on the server as soon as Outlook is connected to the internet. People often keep on communicating for years and do not bother with the mailbox size. The size of offline storage keeps on increasing and reaches its limit. This causes errors, bugs and hangs in Outlook email client and may also lead to corruption in OST file. To decrease the chances of failure, immediate measures need to be undertaken to maintain data saved in a storage file. Hence, some of the ways on how to reduce size of OST File of Outlook have been discussed in this post.

Regular Maintenance to Reduce Size of OST File in Outlook

Outlook .ost file is a data file that stores emails, contacts, calendars, notes, journals, tasks etc. The size of the OST file mainly depends on the mailbox data stored within it. Regular maintenance of mailbox items can ensure that offline storage size limit is not reached. Thereby reducing any chances of corruption. Following are some of the measure to check size of OST file.

  • Regularly deleted unwanted emails and other mailbox items.
  • Empty deleted items folder like Trash and Spam folders to reduce the size of the offline folder file(.ost).
  • Perform hard deletion of data items if they need to be deleted from the mailbox.
  • Ensure that mailbox folders are compacted frequently.

How to Shrink Size of OST File of Outlook ?

MS Outlook provides a unique way to bring down the size of mailbox folders by compacting them. This reduces the size of the .ost file and uncovers space for more data. There is no limitation to how many times offline folders can be compacted. To ensure smooth working, make sure that the mailbox is compacted at least twice a year. Repeat the steps below to reduce size of Outlook OST file.

  1.  Firstly, delete any unnecessary items that need not be compacted or saved in the mailbox.
  2.  Now, press Ctrl+6 key to open the folder list view.
  3.  From the Navigation Pane, right-click on the Deleted Items folder and click Empty Folder.
  4.  Next, go to File>>Account Settings>>Account Settings.
  5. Then, go to the Data File Tab and click once to highlight the offline storage that needs to be compacted.
  6. Following that, click on Settings.
  7. From the Settings box, go to Advanced tab and click on Outlook Data File Settings.
  8. From here, click on the Compact Now button and click OK to reduce size of OST file.

Additional Measures to Reduce the Size of Outlook Offline Folders

There are other ways to remove large OST file issues. These can be performed as an alternative to the compaction method. These are manual approaches to shrink the offline storage in Outlook. They are performed if the maximum OST file size is approaching or has already approach and errors are generating because of it. Some of the ways on how to reduce size of oversized OST file have been described below.

  • Recreate OST : Delete or rename the current offline data file from its default location in Outlook Mail folder on the local machine. Next time the email application is launched all the errors will be removed.
  • Disable Add-on : Often there are additional programs added to Outlook that take up reasonable size in offline storage file and may cause corruption.
  • Remove duplication : There are data redundancies found in many mailboxes. Emails are saved in more than one folder or contacts are repeated in address book. These may take up space in an offline data file and can be easily removed to save space.

Expert Solution to Reducing Outlook OST File Size

Sometimes it becomes difficult to manage space in offline storage of data even after using all the manual methods. The mailbox has been ongoing for too long and conversations are important that just can be deleted. To resolve such issues often customers look for workarounds in third party tools. These are professional solutions that can split offline file size. Furthermore, these effective tools split a large offline data file into smaller parts that are manageable. One such tool is the OST Splitter Software. It can break down a .ost file into multiple parts according to date, size, year and folder. It divides data from an offline file and saves it in a personal storage file called PST file. The tool is compatible with all versions of Outlook 2016 and below. It works with all versions of Windows operating system including Windows 10.


The size of an offline storage can reach its maximum limit without the knowledge of users. People start facing issues like errors, bugs, hangs and corruption of data. They are not aware of what is causing these problems. Therefore, it is highly recommended that users perform regular compaction of data. This will make sure that data is not corrupted because of the increase in file size. Some of the ways on how to reduce size of oversized OST file have been discussed in this post. Use these methods regularly to avoid data loss.


By Nilesh Kumar

As a Chief Technical Writer, I know the technical issues faced by home and professional users. So, I decided to share all my knowledge via this blog. I love to help you with challenges while dealing with technical jargon.