Microsoft Outlook PST File Too Large – How to Reduce PST Size
A Microsoft Outlook PST file can become very large after years of storing emails, attachments, calendar entries, contacts and other Outlook data. A large PST...
A Microsoft Outlook PST file can become very large after years of storing emails, attachments, calendar entries, contacts and other Outlook data. A large PST may consume significant disk space and can also contribute to slower Outlook operations, particularly when large numbers of items are stored in individual folders.
Simply deleting emails does not necessarily make the physical PST file immediately smaller. Outlook may leave unused space inside the data file. To actually reclaim that disk space, you may need to delete or move unnecessary data and then compact the PST file.
This guide explains how to safely reduce the size of a large Outlook PST file without unnecessarily deleting important email.
Quick Answer
If your Outlook PST file has become too large, use this sequence:
- Back up the PST file first.
- Check which Outlook folders consume the most space.
- Find emails with large attachments.
- Save important attachments outside Outlook where appropriate.
- Delete unnecessary large messages.
- Empty Deleted Items and Junk Email.
- Archive older messages to a separate PST file if appropriate.
- Consider creating additional PST files and moving older data into them.
- Finally, use Compact Now to reclaim unused space inside the original PST.
Important: Deleting 10 GB of email does not necessarily make a 40 GB PST become 30 GB immediately. The deleted space can remain as unused space inside the PST until Outlook compacts the file.
What Is an Outlook PST File?
PST stands for Personal Storage Table.
A PST is an Outlook data file stored locally on a Windows computer. Depending on how Outlook is configured, it can contain information such as:
- Emails
- Attachments
- Sent messages
- Deleted messages
- Contacts
- Calendar entries
- Tasks
- Notes
- Custom Outlook folders
PST files are commonly associated with POP accounts and are also used when users create local Outlook data files or archives.
A typical modern PST may be located under:
C:\Users\<Username>\Documents\Outlook Files\
The exact location can vary depending on the Outlook version, profile configuration and whether the PST was manually created or moved.
To identify the actual location of a PST in classic Outlook:
File → Account Settings → Account Settings → Data Files
Select the PST file and use Open File Location where available.
PST vs OST: Do Not Confuse the Two Files
Although both files are Outlook data files, PST and OST files serve different purposes.
| Feature | PST | OST |
|---|---|---|
| File extension | .pst |
.ost |
| Typical purpose | Local storage/archive/POP data | Offline synchronized mailbox |
| Common with POP | Yes | No |
| Common with Microsoft 365/Exchange | Can be used for archives/exports | Yes |
| Server can normally recreate it | No | Usually yes |
| Backup may be important | Yes | Usually not necessary for the OST itself |
| Can contain the only copy of mail | Yes | Normally no |
An OST generally represents a synchronized local copy of server-hosted mailbox information.
A PST can contain locally stored information that exists nowhere else.
Therefore, deleting, moving or damaging a PST can result in permanent data loss.
What Is the Maximum Outlook PST File Size?
For modern Unicode PST files, Microsoft documents the following default limits for Outlook 2010 and later:
| Setting | Default |
|---|---|
| Maximum Unicode PST/OST size | 50 GB |
| Warning/data threshold | 47.5 GB |
The 50 GB figure is the default maximum file-size configuration for modern Unicode PST/OST files.
Older Outlook formats can have substantially smaller limits. In particular, legacy ANSI PST files are subject to an approximately 2 GB limit.
Therefore, you should not assume that every PST has a 50 GB usable limit.
Is It Safe to Keep a PST Near 50 GB?
Technically, a modern Unicode PST can have a default maximum size of 50 GB, but treating the maximum as a recommended operating size is not good storage management.
A PST approaching its configured limit leaves less room for normal Outlook operations and continued mailbox growth.
Microsoft also allows administrators to modify PST/OST limits through registry settings, but Microsoft warns that increasing Unicode file limits beyond documented values can reduce performance.
Recommendation: If a PST is already approaching its limit, reduce or reorganize the data instead of simply increasing the limit.
How to Check the Size of Your PST File
Method 1: Check the PST File in Windows
Close Outlook or ensure the file is not being actively modified, then locate the PST.
Common location:
C:\Users\<Username>\Documents\Outlook Files\
Right-click the .pst file and select:
Properties
Check:
Size
For example:
Outlook.pst — 42.6 GB
A PST of this size deserves attention before it reaches its configured maximum.
Method 2: Check Folder Sizes from Outlook
Classic Outlook includes the Mailbox Cleanup tool.
Open:
File → Tools → Mailbox Cleanup
Depending on your Outlook version and account configuration, it can help you:
- View mailbox/folder sizes
- Find old messages
- Find large messages
- Inspect Deleted Items
- Empty Deleted Items
- Inspect conflicts
This is usually the best place to start because it helps identify what is consuming the space before you delete anything.
Method 1: Find and Delete Large Emails
Attachments are often responsible for a significant portion of PST growth.
Instead of randomly deleting thousands of small emails, start by identifying the largest messages.
In classic Outlook, click the Search box and try:
messagesize:>5 MB
This searches for messages larger than approximately 5 MB.
You can adjust the value, for example:
messagesize:>10 MB
or:
messagesize:>20 MB
Review the results carefully.
Do not automatically delete all large emails. Some may contain invoices, contracts, customer records, licence information or other important business data.
Method 2: Save Large Attachments Outside Outlook
A single email may contain a relatively small message but a very large attachment.
For example:
- Email text: 50 KB
- ZIP attachment: 80 MB
If the attachment needs to be retained but does not need to remain inside Outlook, save it to an appropriate secure location first.
Depending on your environment, that might be:
- A protected local folder
- File server
- Document-management system
- Approved cloud storage
- OneDrive or SharePoint
After verifying that the saved copy opens correctly, you can decide whether the original email/attachment can safely be removed.
Important
Do not remove business-critical attachments merely to reduce storage.
Follow your organization's backup, legal, tax, compliance and retention requirements.
Method 3: Empty the Deleted Items Folder
This is one of the simplest but frequently overlooked steps.
Deleting an email normally moves it to:
Deleted Items
That means the message may still occupy space in the Outlook data store.
After verifying that you no longer need those messages:
Right-click:
Deleted Items → Empty Folder
Confirm the deletion.
Also inspect:
Junk Email
and remove unnecessary messages there.
Method 4: Check the Sent Items Folder
Sent Items can become one of the largest Outlook folders, especially for users who regularly send:
- Quotations
- PDF reports
- Scanned documents
- Presentations
- ZIP archives
- Excel files
- Product catalogues
- Images
Do not concentrate only on the Inbox.
Check:
Sent Items
and sort/search for large or old messages.
Deleting a few hundred very large sent messages can sometimes recover more storage than deleting thousands of ordinary text emails.
Method 5: Use Mailbox Cleanup
Classic Outlook provides a built-in Mailbox Cleanup utility.
Go to:
File → Tools → Mailbox Cleanup
You can use it to identify:
- Older messages
- Large messages
- Deleted Items
- Large folders
- Conflicts
This method is particularly useful when you do not know why the PST has become so large.
Method 6: Use Conversation Clean Up
Long email conversations frequently contain repeated copies of earlier messages.
For example:
Person A sends a message.
Person B replies while including the original message.
Person A replies again while including both earlier messages.
Over time, the conversation may contain significant duplicated text and attachments.
Outlook's Conversation Clean Up feature can remove redundant messages where Outlook determines that their content is already contained in later messages.
Review your organization's retention requirements before using automated cleanup features.
Method 7: Archive Old Emails to Another PST File
If you need to keep older messages but do not need them in your primary PST, archiving is often a better option than deleting them.
In classic Outlook, manual archiving can be accessed through:
File → Info → Tools → Clean Up Old Items
Select:
Archive this folder and all subfolders
Choose the required folder.
Specify:
Archive items older than:
Then select or create the archive PST.
For example:
This moves qualifying older items from the original storage location into the archive PST.
A Better Archive Structure for Large Mailboxes
Instead of allowing one PST to grow indefinitely, you can maintain logical archives.
For example:
Another option is to archive by purpose:
For most users, date-based archives are easier to understand and maintain.
Important
Creating ten unnecessarily small PST files can make Outlook management more complicated. The objective is sensible organization—not simply creating as many PST files as possible.
Method 8: Move Emails Manually to Another PST
You do not have to use AutoArchive.
You can create a separate Outlook Data File and manually move folders or messages into it.
In classic Outlook:
File → Account Settings → Account Settings → Data Files
or use the appropriate New Items / More Items / Outlook Data File option available in your Outlook version.
Create a file such as:
The new data file appears in Outlook's folder pane.
You can then move appropriate messages or folders from the large PST into the new PST.
This gives you more control than automatic archiving.
Method 9: Configure AutoArchive
Classic Outlook can automatically archive older items.
Go to:
File → Options → Advanced → AutoArchive Settings
You can configure options such as:
- How frequently AutoArchive runs
- How old an item must be
- Where archived items are stored
- Whether old items are moved or deleted
Important: New Outlook Limitation
Microsoft states that AutoArchive is not available in new Outlook for Windows.
Therefore, instructions involving classic Outlook AutoArchive should not be assumed to work in new Outlook.
AutoArchive may also be unavailable or restricted in some Exchange environments because organizational retention policies or administrator settings can override it.
Method 10: Compact the PST File
This is one of the most important steps when the goal is to reduce the physical size of the PST file on disk.
Suppose your PST is:
40 GB
You delete or move:
15 GB
You might expect Windows to immediately show:
25 GB
But it may continue showing a file close to the original size.
Why?
The deleted information may have created unused internal space inside the PST.
Compacting allows Outlook to reclaim that unused space.
How to Manually Compact a PST File
In classic Outlook:
- Open File.
- Select Account Settings.
- Select Account Settings again.
- Open the Data Files tab.
- Select the required PST file.
- Click Settings.
- Select Compact Now.
- Wait for the process to complete.
Depending on the PST size, storage speed and amount of reclaimable space, this can take time.
Do not force-close Outlook or restart the computer while the operation is in progress.
Why Compact Now Sometimes Does Not Reduce the PST Much
Compaction only removes reclaimable unused space.
It does not magically compress all active email.
For example:
Before cleanup
Active data: 38 GB
Unused space: 2 GB
Physical PST: approximately 40 GB
If you compact it, the reduction may be relatively small.
But suppose you first remove 15 GB of unnecessary data.
After cleanup
Active data: 23 GB
Unused space: approximately 17 GB
Now compaction has much more unused space that potentially can be reclaimed.
Therefore, the correct sequence is:
Clean → Delete/Move → Empty Deleted Items → Compact
not simply:
Compact
Microsoft notes that Outlook can also compact PST/OST files automatically in the background during idle time when sufficient unused space exists.
Why Deleting Emails Does Not Immediately Reduce PST Size
This behavior often confuses Outlook users.
Think of a PST as a storage container.
Deleting an item may free space inside the container, but it does not necessarily make the container itself immediately smaller.
Compaction reorganizes the data file and can return unused space to Windows.
Therefore:
Deleting email reduces the data stored inside the PST.
Compacting can reduce the physical PST file size on the disk.
Both steps may be necessary.
Should You Increase the 50 GB PST Limit?
Outlook supports registry settings including:
MaxLargeFileSize
and:
WarnLargeFileSize
These can control Unicode PST/OST file-size thresholds.
For Outlook using the Office 16.0 registry branch, relevant settings can exist below:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST
or administrator-controlled policy locations.
However, increasing the maximum PST size should not normally be the first solution to an oversized PST.
Better approach
Reduce unnecessary data, archive older mail and reorganize the PST.
Why?
Increasing the limit:
- Does not remove unnecessary data
- Does not solve poor mailbox organization
- Can allow the PST to grow even larger
- Can increase backup requirements
- May negatively affect Outlook performance
Microsoft specifically notes that although Unicode limits can be configured beyond the documented values, doing so is not recommended because performance can decrease.
Registry Warning
Incorrect Registry Editor changes can affect Outlook or Windows behavior.
Back up the registry and relevant Outlook data before making changes. In managed environments, PST size policies should preferably be configured by the administrator rather than individual users.
What If the PST Has Already Reached Its Maximum Size?
Symptoms may include:
- Outlook cannot add new items
- New messages cannot be moved into the PST
- Send/receive operations fail
- Outlook becomes slow or unresponsive
- Errors appear when moving or deleting messages
- Outlook reports that the data file has reached its maximum size
If the PST is at or very close to its limit:
- Stop adding unnecessary data.
- Back up the PST.
- Remove or move large unnecessary messages.
- Empty Deleted Items.
- Move older folders into another PST if possible.
- Compact the original PST.
- Check the PST for errors if Outlook continues behaving abnormally.
Do not immediately attempt unsupported file manipulation or third-party PST modification utilities on your only copy of important business data.
Should You Use ScanPST to Reduce PST Size?
No.
SCANPST.EXE, also known as the Microsoft Inbox Repair Tool, is designed to diagnose and repair certain structural problems in Outlook data files.
It is not a PST size-reduction tool.
Use Mailbox Cleanup, archiving, moving data and compaction when your objective is reducing PST size.
Use Inbox Repair Tool when there is reason to suspect PST corruption or structural problems.
Large PST and Outlook Performance
PST size is not the only factor affecting Outlook performance.
Performance can also be influenced by:
- Number of items in individual folders
- Number of folders
- Add-ins
- Antivirus integration
- Windows Search indexing
- Disk performance
- Available RAM
- Outlook profile condition
- Search index condition
- Mail synchronization
- Other Outlook data files
Therefore, a slow Outlook installation should not automatically be diagnosed as a PST-size problem merely because a large PST exists.
Important: Do Not Store Active PST Files in OneDrive-Synchronized Folders Without Understanding the Risks
PST files are frequently modified while Outlook is running.
Microsoft notes that large PST files can also affect OneDrive synchronization, including causing prolonged Processing changes or file in use conditions.
If you require PST backup, use a properly designed backup strategy rather than assuming that continuously synchronizing an actively opened PST is equivalent to a reliable Outlook backup.
Recommended Procedure for a 40–50 GB PST
If a user's PST has reached approximately 40–50 GB, a practical approach is:
Step 1 — Back up the PST
Close Outlook and make a verified backup before major cleanup or restructuring.
Step 2 — Identify the largest folders
Use Mailbox Cleanup and folder-size information.
Step 3 — Find large messages
Search:
messagesize:>10 MB
Review the largest messages first.
Step 4 — Deal with attachments
Save important attachments to an approved storage location where appropriate.
Step 5 — Clean unnecessary mail
Check:
- Inbox
- Sent Items
- Deleted Items
- Junk Email
- Old project folders
Step 6 — Archive old data
Move older years or completed projects to one or more archive PST files.
Step 7 — Empty Deleted Items
Do this only after confirming that nothing important is required.
Step 8 — Compact the original PST
Use:
File → Account Settings → Account Settings → Data Files → PST → Settings → Compact Now
Step 9 — Verify Outlook
Restart Outlook and confirm:
- Email opens correctly
- Search works
- Folders are accessible
- Archive PST files open
- Send/receive works as expected
Step 10 — Verify backups
Ensure important PST and archive files are included in an appropriate backup plan.
Common Mistakes When Reducing PST Size
Mistake 1: Deleting thousands of tiny emails first
A few messages with large attachments may consume more storage than thousands of text-only emails.
Start with the largest messages.
Mistake 2: Forgetting Deleted Items
Deleting messages without emptying Deleted Items may provide little immediate benefit.
Mistake 3: Expecting the Windows file size to change immediately
The PST may require compaction.
Mistake 4: Increasing the PST limit immediately
This postpones the storage-management problem instead of solving it.
Mistake 5: Deleting the PST manually
Never delete an active PST simply because it is large.
Mistake 6: Assuming a PST can always be recreated
Unlike an OST synchronized from a mail server, a PST may contain the only available copy of important information.
Mistake 7: Creating an archive without backing it up
Moving mail from one PST to another is organization—not necessarily backup.
If the archive PST exists on only one disk and that disk fails, the archive can be lost.
Best Practices to Prevent PST Files Becoming Too Large
For long-term Outlook reliability:
- Regularly check PST size.
- Monitor folders containing large attachments.
- Empty Deleted Items when appropriate.
- Remove unnecessary Junk Email.
- Archive old mail when appropriate.
- Use logical archive files rather than one endlessly growing PST.
- Keep reliable backups of important PST files.
- Avoid treating the maximum PST limit as the ideal working size.
- Periodically review Sent Items.
- Store large documents outside Outlook when business requirements permit.
- Avoid unnecessarily increasing PST limits through the registry.
- Keep Outlook and Microsoft 365 Apps updated.
Frequently Asked Questions
What is the maximum PST file size in Outlook?
For modern Unicode PST files in Outlook 2010 and later, Microsoft documents a default maximum size of 50 GB, with a default warning/data threshold of 47.5 GB. Older PST formats and Outlook versions can have different limits.
Why is my PST still large after deleting thousands of emails?
Deleting items can create unused space inside the PST without immediately reducing the physical file size. Empty Deleted Items and then compact the PST to reclaim eligible unused space.
How do I manually shrink an Outlook PST file?
First delete or move unnecessary data and empty Deleted Items. Then use:
File → Account Settings → Account Settings → Data Files → select PST → Settings → Compact Now
Does Compact Now delete emails?
Compaction is intended to reclaim unused space in the data file; it is not a command for selecting and deleting your normal active emails. Nevertheless, maintain a backup before significant PST maintenance.
Can I split one large PST into multiple PST files?
Yes. You can create additional PST files and move older folders or messages into them. Outlook does not provide a simple one-click "split PST into equal parts" function, so organization is normally performed by moving or archiving data.
Should I increase the Outlook PST limit above 50 GB?
It is technically possible to configure Unicode PST/OST size limits through registry settings, but it should not normally be your first solution. Microsoft warns that performance can decrease when limits are increased beyond documented values.
Can I reduce PST size without deleting emails?
Yes. Move older messages to another PST or archive them. This reduces the amount of data in the original PST while retaining the messages elsewhere.
Does archiving reduce PST size immediately?
Moving messages to another PST reduces the active data in the source PST, but the physical source file may still need to be compacted before Windows reports a substantial size reduction.
Is a PST file a backup?
Not necessarily. A PST is a storage file. If the only copy of the PST is on one computer, disk failure, accidental deletion, corruption or ransomware could cause data loss. Important PST files require a separate backup strategy.
Can I delete my PST and let Outlook recreate it?
Generally, do not do this. A PST may contain locally stored data that cannot be downloaded again. This is an important difference from many OST scenarios.
Does new Outlook support AutoArchive?
Microsoft currently states that AutoArchive is not available in new Outlook for Windows. Classic Outlook is required for the traditional AutoArchive functionality described in this article.
Is ScanPST used to compact a PST?
No. ScanPST is primarily a repair utility. Use Outlook's Compact Now function for reclaiming unused PST space.
Can attachments make a PST very large?
Yes. Large and repeated email attachments are often major contributors to mailbox and PST growth.
Should I empty Deleted Items before compacting?
Yes, if you have confirmed that the deleted messages are no longer required. This creates more potentially reclaimable space before compaction.
Will a 45 GB PST definitely corrupt?
No. PST size alone does not prove corruption. However, a 45 GB PST is close to the default 50 GB maximum for modern Unicode PST files, so cleanup and storage management should be considered before it reaches the configured limit.
Conclusion
A large Outlook PST file should be managed before it reaches its configured maximum.
The safest approach is not simply to increase the PST limit. Instead, identify what is consuming storage, remove unnecessary messages, manage large attachments, archive older data, empty Deleted Items and then compact the PST.
The most important point to remember is:
Deleting messages and reducing the physical PST file size are not necessarily the same operation.
Deleting or moving messages creates unused space inside the PST. Compact Now can then reclaim eligible unused space and reduce the physical size of the file.
For users with many years of email, maintaining sensibly organized archive PST files and a reliable backup strategy can prevent a single Outlook data file from growing indefinitely.
References
Microsoft Support — Reduce the size of your mailbox and Outlook Data Files (.pst and .ost)
Microsoft Support — Archive older items automatically
Microsoft Support — Archive items manually
Microsoft Learn — Configure size limits for PST and OST files in Outlook
Microsoft Support — Create and manage Outlook Data Files (.pst)
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