How to Reduce the size of Outlook Data Files (.pst and .ost)?

Many organizations or enterprises prefer Microsoft Outlook to exchange the information over emails as Outlook is well known for data security and reliability. Outlook stores all its mailbox data in PST (Personal Storage Table) file format. In contrast the Exchange server generates an OST (Offline Storage Table) file instead of PST to save the mailbox items. OST files are the offline files on which users can work easily and the changes will get automatically saved once the internet connectivity is re-established. These OST and PST files not only store emails but the other mailbox items as well. So it becomes quite difficult for users to access the Outlook data file if the OST and PST file size increases beyond the limit. There might be a risk of corruption that can lead to severe data loss as well. Users have queries about “how to reduce Outlook data file size”. In this blog, we will discuss the most recommended methods to reduce Outlook mailbox size. We will provide the manual techniques and OST splitter professional software to split the oversized OST files. 

Outlook sometimes uses a conflict folder to store the conflicted items in it. When a copy of an item is saved on the email server and is not synchronized with the copy of the item stored in the local computer then, conflicts take place. Outlook Mailbox cleanup tool can delete the conflicts for the user. 

Steps to Run Mailbox Cleanup Tool to Compress Outlook Mailbox

  1. Login to your MS Outlook profile. 
  2. Go to the File tab.
  3. Click on Tools and go to Mailbox Cleanup.
  1. Select the options to compress Outlook data file folder as shown in the image below.

How to Reduce Outlook Data File Size (Compress OST File)

OST files are the offline storage files supported by MS Exchange to save the mailbox database. OST files are similar to PST files with an exception that one cannot import or export from the offline Outlook data file. Now follow the steps to compress OST file manually.

  1. Go to the File tab.
  2. Click on the Account Settings section and select Account Settings. 
  3. Now go to the Data Files section and choose the required OST data to compress. Click on the settings. 
  4. Click on the Advanced tab.
  5. Select Outlook Data file settings option.
  6. Click on compact now under Outlook data settings dialog box. 

How to Reduce Outlook Data File Size (Compress PST File Outlook 2010)

When your Outlook PST file becomes oversized and exceeds the mailbox size limit then, it affects the performance. Also there is a risk of corruption and ultimately a risk of severe data loss. In this scenario, the user needs to compress the PST file. How to reduce PST file size? Well, you just need to follow the manual steps provided for manual compression.

  1. Delete the unnecessary items within the PST and then empty the deleted items folder. 
  2. Go to the File section. 
  3. Click on Account Settings > Account Settings. 
  4. Select the PST file to compress under the data files tab.
  5. Click on the settings button. 
  6. Click on the Advanced tab and select Outlook data file settings. 
  7. Now hit the Compact now button.
  8. Exit the window once the PST compression process is completed successfully.

Note: These are the manual steps and sometimes manual strategies are more time consuming and complex and would not work properly. As an alternative, users can also opt for the professional advanced techniques available to split the large Outlook data files without any hassle or worry. 

How to Reduce Outlook Data File Size Professionally?

As these manual techniques discussed above are more time consuming and there is no data security guaranteed under the process. So the user is required to opt for a well known and trusted Outlook data splitter tool to avoid all the limitations. If the user needs to split the large OST file into smaller parts to compress OST file then, the advanced DRS OST Splitter tool can be used to securely split the oversized OST file into smaller PST files. It is capable of splitting an OST file on the basis of date, folder, size, etc. And to compress Outlook data file PST, DRS PST Splitter Tool can be used which is designed on advanced algorithms. The software can easily split the large PST file into multiple smaller PST files. There is no data manipulation risk during the splitting process. It supports both ANSI and UNICODE PST files. Compatible with all the Windows and Mac versions. The free demo versions are also available for both the utilities for the purpose to evaluate the performance of them. 


We have discussed the different strategies to solve the user’s query regarding “how to reduce Outlook data file size”. We have provided manual techniques as well as the automated techniques to compact Outlook data files. Users can follow any of the techniques as per their convenience. 

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