Outlook compacts a PST or OST file on its own after you delete items. It acts only when unused space passes 20 percent of the file. When the file remains large, delete items and empty Deleted Items first and then select Compact Now. Compact Now takes several minutes and Outlook can remain open.
You delete thousands of emails and the data file has the same size on disk. Outlook compacts the file only under set conditions so the file may not shrink at once.
Steps follow the Microsoft Support page on reducing mailbox and data file size. Menu names fit current Outlook for Windows.
When does Outlook Compact a Data File?
After you delete items from a PST or OST file Outlook compacts the file in the background during idle time while Outlook is open. Microsoft states that Outlook compacts only if the file has more than 20 percent of white space.
White space is unused space left behind by deleted items. Deleting a small number of items rarely reaches that limit.
Which Data File Fits Your Account?
| Account type | File |
|---|---|
| POP | PST file |
| Microsoft 365, Exchange, IMAP and Outlook.com | OST file with a copy of the mailbox |
How do You Compact a Data File Manually?
- Delete unneeded items and clear Deleted Items.
- Select File then Account Settings then Account Settings.
- Open the Data Files tab, select the file and choose Settings.
- For Exchange or Outlook.com accounts open the Advanced tab and choose Outlook Data File Settings.
- Select Compact Now then OK.
Microsoft states the step can take several minutes and Outlook does not need to close.
What Else Shrinks a Mailbox?
- Mailbox Cleanup. Go to File then Tools then Mailbox Cleanup to check folder sizes and empty Deleted Items.
- Conversation Clean Up. It removes redundant messages from long threads.
- Large messages. Search for
messagesize:>5 mband delete them or move them to another PST file.
Microsoft notes that large mailboxes can open and search slower. Related reading: free Outlook PST viewers and how to recover an unsaved Word document.
Frequently Asked Questions
Why is My Outlook Data File Not Shrinking?
Outlook compacts the file only when unused space passes 20 percent. Delete more items or use Compact Now.
How do I Compact a PST File?
Open File then Account Settings then Data Files and select Settings then Compact Now.
How Long does Compact Now Take?
Microsoft states it can take several minutes.
Do I Need to Close Outlook?
No. Microsoft states Outlook does not need to close.
Which Accounts Use PST Files?
POP accounts. Other account types use OST files.
Key Takeaways
- Outlook compacts files in the background during idle time.
- Compacting needs more than 20 percent white space.
- Delete items first and then select Compact Now.
- Compact Now takes several minutes.
- POP accounts use PST and other accounts use OST.
Sources and References
Steps follow the Microsoft Support page.
- Reduce the size of your mailbox and Outlook Data Files (Microsoft Support) support.microsoft.com