This article describes how to compact personal folders (.pst) files or offline store (.ost) files in Microsoft Outlook to eliminate deleted item space and recover room on your hard disk.

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To conserve space on your hard disk, you can delete unwanted items from the various .pst and .ost files, and then compact the files to make them smaller.

NOTE: When you delete the unwanted items from each folder, it moves them to the Deleted Items folder, which is in the same file. You must delete the items from the Deleted Items folder to permanently delete them so that Outlook can compact the file.

The compacting process does not remove all the free space from the file. It leaves either 16 kilobytes (KB) or 4 percent (%) of the file size (before compacting) whichever is greater.

To be compacted, a .pst or .ost file must meet the following requirements:

•	The file must be 16 KB or larger.
•	The file must have at least 16 KB of free space since the last time that you compacted it.

You can start the Outlook file compaction manually, or run it automatically in the background.

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Automatic Background Compaction

Automatic compaction takes place as an idle task in the background. The following conditions must be true for this background task to take place:

•	You must be running Outlook.
•	Your computer must not be engaged in other CPU-intensive tasks such as copying or downloading files.

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How to Manually Start PST Compaction

1.	On the File menu, click Data File Management.
2.	Click to select your Personal Folder, and then click Settings.
3.	On the General tab, click Compact Now.
4.	Click OK, and then click Close.

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How to Manually Start OST Compaction

Use one of the following methods:

Method 1: Microsoft Office Outlook 2007

1.	Delete any items that you do not want to keep, and then empty the Deleted Items folder.
2.	On the Tools menu, click Account Settings.
3.	In the list, select Microsoft Exchange Server, and then click Change.
4.	Click More Settings.
5.	On the Advanced tab, click Offline Folder File Settings.
6.	Click Compact Now.

Method 2: Microsoft Office Outlook 2003 and earlier versions of Outlook

1.	On the Tools menu, click to select E-mail Accounts.
2.	Click View or change existing e-mail account, and then click Next.
3.	Click Microsoft Exchange Server, and then click Change.
4.	Click More Settings.
5.	On the Advanced tab, click Offline Folder File Settings.
6.	Click Compact Now.
7.	When the compact operation is finished, click OK twice.
8.	Click Next, and then click Finish.

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