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How to Reduce Data File (.pst/.ost) Size in Outlook

The more you send and receive emails, add new calendar events, contacts, tasks, and notes; the bigger your mailbox will get overtime. Unless you archive old emails or delete older data, your Outlook Data File size will reach a limit where you can no longer store new data. 

Most email providers put size limitations on mailboxes to prevent performance issues. However, archiving or deleting data from the Outlook Data File will not always reduce the size of the .pst/.ost file because of "white spaces" left behind after you delete an item in Outlook. These will still take up space and thus add to the overall size of the Outlook data file.

That's where data file compacting comes in. Basically, it removes all those white spaces left by deletions and it can significantly reduce the size of your data file which, in turn, can improve the overall performance of Outlook.

How to compact a .pst file in Outlook?

Step 1. Launch your Outlook application and click the File button in the top left corner.

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Step 2. In the Info window, click the Account Settings button and then select Account Settings from the drop-down menu.

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Step 3. In the Account Settings window, select the Data Files tab.

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Step 4. Select the .pst file you want to compact and click the Settings... button.

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Step 5. In the Outlook Data File window, click the Compact Now button.

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Step 6. Depending on the size of your data file, the compacting process could take a while and during this time you will be unable to use the Outlook application. You just need to wait for the process to finish and the window will close once complete.

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How to compact a .ost file in Outlook?

Step 1. Launch your Outlook application and click the File button in the top left corner.

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Step 2. In the Info window, click the Account Settings button and then select Account Settings from the drop-down menu.

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Step 3. In the Account Settings window, select the Data Files tab.

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Step 4. Select the .ost file you want to compact and click the Settings... button.

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Step 5. In the window, click the Advanced tab.

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Step 6. In the Advanced tab, click the Outlook Data File Settings... button.

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Step 7. In the Outlook Data File Settings window, click the Compact Now button and wait for compacting process to finish. The window will close once compacting is complete. 

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Tips:

  • If you are a heavy Outlook user, do the compacting procedure every 4-5 months or if you feel that your mailbox performance has declined. 
  • Make sure to never come close to the size limit of your .pst/.ost file. Once the limit is reached, the data file will become inaccessible and you may end up losing data.

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