One surprising thing about the way Outlook stores information in data files is that the files don't automatically get smaller when you remove items from them. Without some sort of intervention on your part, your data files will only get bigger and bigger. To squeeze out the unused space in data files, you must manually compact them.

Compacting a data file reorganises the information in it, allowing it to shrink to occupy only the amount of space actually needed for the items it contains. Compacting a data file can take several minutes but can free a lot of disk space and perhaps improve Outlook performance.

NOTE: These procedures only apply if you have installed Outlook with the Internet Mail Only option. To determine your installation type, click About Microsoft Outlook on the Help menu. If you have the Internet Mail Only option installed, you see "Internet Mail Only".

The following describes how to compact the Personal Folders file(s) (PST) in Microsoft Outlook, to eliminate deleted item space and recover room on your hard disk.
To conserve space on your computer's hard disk, you can delete unwanted items from the various PST 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.

• 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.
• To Manually Start PST Compaction
  1. If the Folder List is not showing, click Folder List on the View menu.
  2. Right-click one of the Personal Folders.
  3. Click "Properties For Personal Folders."
  4. On the General tab, click Advanced, and then click Compact Now.
  5. Click OK twice.

To compact a data file, follow these steps:

1. Click File | Data File Management to open the Outlook Data Files dialog box shown in the illustration. You should see one or more data file names in the list. You can manage all your Outlook data files from this dialog box.
2. Select the data file you want to compact (Outlook.pst should appear in the list with a name of Personal Folders), then click Settings to open a settings dialog box for that data file like the one shown here.
3. Click Compact Now and Outlook will squeeze out the wasted space in your data file.