Outlook data files can grow large over time, consuming significant disk space and potentially slowing down the application. This is typically caused by years of accumulated emails, attachments, and calendar items stored in a local PST or OST file. This article will show you how to locate these large files and provide clear methods to reduce their size and free up storage.
Key Takeaways: Managing Outlook Storage
- File > Account Settings > Data Files: Shows the location and size of your primary Outlook Data File, which is usually the largest.
- Compact Now in Data File Settings: Reclaims unused space from deleted items within a PST file without removing any data.
- Archive feature or moving items to a new PST: Moves older items out of your primary mailbox file to a separate, smaller data file.
Why Outlook Data Files Become Large
Outlook stores all your local email, calendar, contacts, and tasks in a single data file. For POP and IMAP accounts, this is a PST file. For Microsoft 365 or Exchange accounts using Cached Exchange Mode, it is an OST file. Every sent and received email, especially those with large attachments like images or documents, adds to the file size. Even when you delete items, the space they occupied is often not immediately freed up by the system, leading to file bloat.
The default location for these files is within your user profile on the C: drive. Without periodic maintenance, they can grow to many gigabytes. A very large data file can cause Outlook to start slowly, search to perform poorly, and may even risk file corruption if it approaches the size limit, which is 50 GB for newer Unicode PST files.
Steps to Find and Reduce Outlook Data File Size
Locate and Check the Size of Your Data Files
- Open Account Settings
In Outlook, go to File > Account Settings and select Account Settings again from the dropdown menu. - View Data Files
In the Account Settings window, click the Data Files tab. You will see a list of all data files associated with your profile, their types, and their current sizes. - Open File Location
Select your primary data file, usually named “Outlook Data File,” and click Open File Location. This opens a File Explorer window showing the actual PST or OST file on your disk, confirming its size.
Compact Your PST File to Reclaim Space
Compacting removes the empty space left behind by deleted items. This only works for PST files, not for OST files connected to an Exchange server.
- Access Data File Properties
Return to the Data Files tab in Account Settings. Select your PST file and click Settings. - Start the Compact Process
In the Settings dialog box, click Compact Now. A progress bar will appear. Do not use Outlook during this process, which can take several minutes for a large file.
Archive Old Items to a New Data File
- Open the Archive Dialog
Go to File > Info > Tools and select Clean up old items. Alternatively, use the Archive command directly from the Folder tab in the ribbon. - Configure Archive Settings
In the Archive dialog, choose the folder to archive, such as your entire mailbox. Select a date using the “Archive items older than” option, like items older than 1 year. - Choose Archive File Location
Click Browse to choose where to save the new archive PST file, ideally on a drive with more free space. Click OK to move all qualifying items out of your primary data file.
If Outlook File Size Remains Unchanged After Cleaning
OST File Size Not Decreasing After Deleting Emails
OST files synchronize with a mail server. Deleting items locally only marks them for deletion on the server. To reduce the OST file size, you must compact it, which requires a specific registry tweak or recreating the profile. A safer method is to switch your account to Online Mode temporarily, which deletes the local OST cache, then switch back to Cached Mode to download a fresh, smaller copy.
Search Folder or Large Attachment Cache
Outlook’s Instant Search creates a hidden index that can grow large. You can rebuild it via Windows Search settings. Also, check the size of your Temporary Internet Files folder, as Outlook caches linked image attachments there. Use Windows Disk Cleanup to clear this cache.
Multiple Large PST Files from Old Profiles
You may have old PST files still connected to Outlook from previous projects or accounts. Review the list in File > Account Settings > Data Files. For files you no longer need, close them by selecting the file and clicking Remove. This disconnects the file from Outlook but does not delete it from your hard drive. You can then manually delete the PST file from its folder location.
Manual Cleanup vs. Automatic Archiving: Methods Compared
| Item | Manual Cleanup (Delete/Archive) | Automatic Archiving (AutoArchive) |
|---|---|---|
| Primary Action | User manually selects folders and items to move or delete | Outlook automatically moves items based on age rules you set |
| Control Level | High, allows for item-by-item review | Low, processes items in bulk without user review |
| Best For | One-time major cleanup or managing specific large projects | Ongoing maintenance to prevent mailbox growth |
| Risk of Data Loss | Low if user is careful | Medium, if rules are misconfigured |
| Impact on Primary PST | Immediate size reduction after archiving | Gradual size reduction over time |
You now have the tools to identify which Outlook data file is consuming disk space and multiple strategies to reduce it. Start by checking the file size in Account Settings and running the Compact Now tool for PST files. For long-term management, set up the Archive feature to move old emails automatically. An advanced tip is to use Windows File Explorer’s search for *.pst to find all Outlook data files on your computer, including hidden ones from old profiles.