Reduce the size of Outlook data files Your Microsoft Outlook mailbox grows as you create and receive items. Even when you delete items, the size of your data file might not decrease in proportion to the amount of data that you deleted. There are several ways to manage your mailbox and reduce the size of the data file. Manage the size of your mailbox You can use the Mailbox Cleanup tool, which allows you to view the size of your mailbox and individual folders. You can also archive older items to an archive Personal Folders file (.pst) and empty your Deleted Items folder. On the Tools menu, click Mailbox Cleanup . Do any of the following: To view the total size of your mailbox and of individual folders within it, click View Mailbox Size . To locate older items, select Find items older than n days , enter a number between 1 and 999, and then click Find . To locate large items, select Find items larger than n kilobytes , enter a number between 1 and 9999, and then click Find . Reduce the size of a Personal Folders file (.pst) If you delete items from a Personal Folders file (.pst), the file is automatically compacted in the background when you're not using your computer and Outlook is running. If you want to immediately reduce the size of the file, you can force a compaction. The procedure to compact the data file might take several minutes. On the File