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Outlook data files can get very large over time, especially if you get a lot of attachments. Even if you delete old messages, the size of your Outlook data file is not reduced accordingly.

Your Outlook data files (.pst and .ost files) include all your emails (in all your folders, including sent emails), contacts, calendar appointments, tasks, and notes, as well as their associated file attachments. As you send and receive more emails and attachments, add more appointments to your calendar, and create more tasks and notes, your Outlook data files can easily grow to several gigabytes in size. Mine is one gigabyte, but that’s still pretty large.

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RELATED: How to Automatically Empty the Deleted Items Folder When Exiting Outlook

Even if you empty the Deleted Items folder, or have it automatically emptied when you exit Outlook, the space occupied in the data file is not released. You can, however, reclaim that space by compacting your Outlook data file. Here’s how to do that.

Step One: Examine Your Accounts and Delete What You Don’t Need

Before you go through the compacting process, you’ll want to go through Outlook and delete anything you don’t need. We recommend checking how big your various folders are, so you know what items will free up the most space. Remember, you can also delete calendar, tasks, and notes items in addition to emails. To check the sizes of your folders, select the Inbox (or any other folder) in the account (Outlook data file) you want to compact, and click the “File” tab.

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On the backstage screen, click “Tools” and select “Mailbox Cleanup” from the drop-down menu.

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On the Mailbox Cleanup dialog box, click “View Mailbox Size”.

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The Folder Size dialog box shows you the size of each folder in the currently selected account. Scroll through and see which folders are bigger, where you might want to delete items. Click “Close” when you’re done.

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To see the size of the Deleted Items folder, click “View Deleted Items Size”.

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Our Deleted Items folder is not huge, but we should empty it anyway before compacting our data file. Click “Close”.

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You are returned to the Mailbox Cleanup dialog box. If you want to delete more items from your account, click “Close” to return to your account. Select any emails, calendar, tasks, and notes items you don’t want and press Delete. You can also press Shift+Delete to permanently delete the selected items, bypassing the Deleted Items folder.

Step Two: Empty the Deleted Items Folder

You’ll also want to empty the trash, or “Deleted Items” folder, before continuing. If there are still items in the Deleted Items folder, there are two ways you can empty the folder. One way is by clicking “Empty” on the Mailbox Cleanup dialog box, if it’s still open.

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A confirmation dialog box displays making sure you want to permanently delete the items in the Deleted Items folder. Click “Yes”.

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If the Mailbox Cleanup dialog box is not open, you can also empty the Deleted Items folder directly in your account on the Mail pane. To do this, first make sure the Mail pane is active. Press Ctrl+1 to open the Mail pane, if it’s not. Then, right-click on the Deleted Items folder in the account you want to compact and select “Empty Folder” from the popup menu. The same confirmation dialog box pictured above displays.

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Step Three: Compact Your Outlook Data FIle

Now that you’ve deleted items you don’t want and emptied the Deleted Items folder, it’s time to compact your Outlook data file. In the main Outlook window, click the “File” tab again. Then, make sure the Account Information screen is active. If not, click “Info” in the list of items on the left. Click “Account Settings” and then select “Account Settings” from the drop-down menu.

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On the Account Settings dialog box, click the “Data Files” tab.

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Select the Outlook data file you want to compact in the list and then click “Settings”.

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If the currently selected account is a POP3 account (.pst file), the following dialog box displays. Click “Compact Now”.

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Otherwise, if the selected account is an IMAP account (.ost file), you will see the following dialog box. Click “Compact Now”.

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The following dialog box displays while the Outlook data file is compacted.

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When the compacting process is done, you are returned to the Outlook Data File (or Outlook Data File Settings) dialog box. Click “OK” or “Cancel” to close it.

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Click “Close” on the Account Settings dialog box.

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Our Outlook data file went from about 951MB down to about 845MB, so we got back a little over 100MB of disk space. The larger your data file, the more space you’ll probably get back.

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Compacting your Outlook data files not only help you save disk space, but may also improve the performance of Outlook. A large Outlook data file can make Outlook take longer to start up, work with, and shut down. So, periodically compacting your Outlook data file is a smart thing to do.