Increase OST File Size Limit Outlook 365 Registry
Published: April 29th, 2025 • 5 Min Read
Microsoft Outlook uses an OST file to store a synchronized copy of mailbox data locally. By default, Outlook limits the size of OST files to maintain Outlook performance locally. Increase OST File Size Limit Outlook 365 Registry to prevent issues and file corruption. When your OST file increases with frequent email sending or receiving with large attachments. You may have performance issues, sync errors, and warnings Outlook Oversized OST problem
To avoid these issues, many users look to increase OST File size limit Outlook 365 using Windows Registry Method. Modifying a specific registry allows an increased OST file size limit with smoother Outlook performance and uninterrupted access to your mailbox.
In this guide, you’ll learn Why OST File limits matter, how to modify through registry and best practices to follow during the process.
Understanding OST File Size Limits in Outlook 365
OST contains duplicate copies of Outlook mailbox data in a local drive. It allows you to access emails, contacts, and calendars offline. But as OST file size closes to its limit then the file corruption chances are too high. Let’s know OST file size limit:
Default OST File Size Limit by Outlook Version
- Outlook 2010, 2013, 2016, 2019, 2021 and Microsoft 365:
Default limit is 50 GB. - Older version(Outlook 2007 and earlier):
Default limit was 20 GB.
When an OST file closes to its limits, Outlook may begin to show warnings, lag during use, or fail to synchronize with Exchange Server. That’s why increasing the OST file limit is necessary for large mailbox data or heavy attachment usage.
You can raise this limit by modifying specific values in Windows Registry, but it should be done carefully to avoid data corruption.
Symptoms of an Oversized OST File in Outlook 365
- Outlook becomes slower when sending or receiving emails. Opening different folders may also lag.
- Frequent freezes or unexpected crashes are common when OST becomes oversize.
- You may notice an error message that says Outlook cannot synchronize folders or delay in syncing new emails and calendar updates.
- Emails might disappear temporally or become unreadable due to synchronized failure or file corruption.
- Outlook shows error OST reached maximum size or exceeded its size limit.
Methods to Increase OST File Size Limit Outlook 365 Registry
Increase OST file size limit Outlook 365 involves carefully updating the Windows Registry. Follow approaches as we have explained below:
Method 1. Locate the Registry Editor in Windows
- Press Windows+R type Regedit and click on Enter.
- If prompted by user account control, Click Yes to allow.
Method 2. Backup your Registry
- Click on file and press Export.
- Save a backup of your registry in case you need to restore it later.
Method 3. Navigate to Outlook Registry Path
- Open Outlook Registry Path and enter HKEY_CURRENT_USER\Software\Microsoft\Office\2503\Outlook\PST
- Replace Version with your Outlook 365 Version number.
Method 4. Create or Modify Max File Size and WarnFileSize Values
- Right-click on the right pane, Select New > DWord (32-bit Value).
- Create Two Values:
- Max File Size: Control maximum OST size limit.
- WarnFileSize: Display warning before exceeding its limit.
- Set the values in bytes. Such as for 100 GB = 107374182400.
- After saving changes, restart Outlook to reflect changes.
How to Take Backup Before Increase OST File Size Limit Outlook 365 Registry
Any modifications can affect the OST files, so you must take the necessary backup of the OST file before any modification. For backup, we recommend you to opt for the BitRecover OST Manager Toolkit, as this software allows you to backup, repair, split, and convert OST files into 30+ saving formats. This software is completely reliable for managing and maintaining healthy OST files and also offers full data assurance to prevent any bit of data leakage.
Often Inquired Questions
How to Increase Outlook mailbox size to 100GB Regedit?
Open Registry Editor, go to HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST and create max file size and Warn File Size entries. Set MaxFileSize to 107374182400(FOR 100 GB). Relaunch Outlook to apply changes.
Can Outlook allow OST files larger than 50GB?
Yes, Outlook allows OST files larger than 50 GB by modifying the registry. You must create or modify MaxFileSize and WarnFileValues under the PST key in the registry and relaunch Outlook to reflect changes.
What is the maximum size of Outlook 365 OST has reached?
Outlook sets as default OST limits of 50 GB, but users have extended it to 100 GB via registry edits. Exceeding this without adjustment may cause performance issues or sync errors. Always monitor file growth and apply limits carefully.
Overall Reflection
In this guide, we have learned effective methods to Increase OST File Size Limit Outlook 365 Registry. As any modification via Registry can lead to file corruption. We have also mentioned that automated tools are a reliable choice for backup and can also repair damaged OST files. So, before the increased OST file size limit, take backup to keep a healthy OST file and prevent any corruption or data loss.