This article describes how to compact the Personal Folders file (PST) or Offline Store file (OST) in Microsoft Outlook, to eliminate deleted item space and recover room on your hard disk.

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To conserve space on your computer's hard disk, you can delete unwanted items from the various PST and/or OST folders and then compact the files to make them smaller.

NOTE: Deleting the unwanted items from each folder only 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 file must meet the following requirements:

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

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

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Automatic Background Compaction

PST files that are not on your computer may require manual compaction, due to interactions between the computer where the PST files are stored and your computer.

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

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

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To Manually Start PST Compaction

1.	On the Tools menu, click Services.
2.	Click to select Personal Folders, and then click Properties.
3.	On the General tab, click Compact Now.
4.	Click OK twice.

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To Manually Start OST Compaction

1.	On the Tools menu, click Services.
2.	Click to select Microsoft Exchange Server, and click Properties.
3.	On the Advanced tab, click "Offline Folder File Settings."
4.	Click Compact Now.
5.	Click OK three times.

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