What is the OST file size limit in Outlook 2016?
The OST (Offline Storage Table) file is a local copy of your mailbox data that Outlook 2016 creates when using an Exchange, Office 365, or IMAP account. It lets you work with email, calendar, contacts, and other Outlook data offline. Changes are automatically synchronized to the server when you reconnect. Understanding OST files is critical for mailbox performance management and ensuring you don’t exceed the default OST file size limit in Outlook 2016.
Difference Between OST and PST Files
Although both OST and PST files store Outlook data, they serve different purposes:
OST file: Automatically created when using Exchange/IMAP accounts. It syncs with the mail server and cannot be imported into Outlook from another profile.
PST file: Created manually, usually for backups or archiving. It can be moved and imported into any Outlook profile and is ideal for reducing OST file size by offloading older data.
Knowing when to use PST files can help manage OST size limits in Outlook 2016.
Purpose of OST in Cached Exchange Mode
In Outlook 2016, Cache Exchange Mode uses an OST file to improve performance. Instead of constantly accessing the server, Outlook reads data from the OST file, providing faster access to emails and calendar events. This setup is especially helpful in low-bandwidth environments or when working remotely.
However, as mailbox data grows, the OST file can become large, leading to slow performance or corruption. That’s why it’s important to monitor and manage the OST file size limit in Outlook 2016 proactively.
What is the Default OST File Size Limit for Outlook 2016?
Microsoft Outlook 2016 supports large mailboxes but has storage limits, especially for OST files. When your OST file grows too big, it can degrade performance, cause sync issues, or get corrupted. Understanding the default size limit can help you avoid these issues and manage your mailbox more effectively.
Default Size Limits for ANSI vs. Unicode OST Files
Outlook supports two types of data file formats:
ANSI format (used in older Outlook versions): Maximum size is 2 GB.
Unicode format (used in Outlook 2003 and later, including 2016): Default limit is 50 GB.
Outlook 2016 uses Unicode format, so the default OST file size limit is 50 GB, configurable via the Windows Registry.
Official Microsoft Limits for Outlook 2016
According to Microsoft documentation:
Default maximum size: 50 GB for both OST and PST files.
Configurable limits: You can adjust limits via Registry:
MaxLargeFileSize – absolute maximum size (MB)
WarnLargeFileSize – warning threshold (MB), usually around 47.5 GB
Keeping your OST file below the warning threshold optimizes performance and Outlook stability.
When Should You Consider Converting OST to PST?
While OST files are great for offline access in Outlook 2016, they’re not ideal for backup, recovery, or migration. OST Converter may be necessary in these scenarios:
Orphaned OST file: If your Outlook profile is deleted, corrupted, or the Exchange server is unavailable, OST becomes unusable without conversion.
Exceeded OST size limit: If Outlook won’t start or sync properly due to file size, converting can offload older data.
Portability needs: OST files are tied to one profile and not portable. Converting to PST allows easy import to another computer or profile.
Migrating to other platforms: PST files are widely supported by third-party tools and facilitate migration from Exchange or Office 365.
Selective recovery: Conversion tools often allow recovering specific folders from a corrupted OST file.
How to Back Up OST Files Offline
Using an OST to PST Converter is the best way to maintain metadata integrity and prevent data loss. This software is a tested utility for restoring and converting damaged, corrupted, or inaccessible OST files.
Steps to Save OST Files Offline:
Download and launch the automated software.
Select “Customized Outlook mailbox data” if needed.
Choose mailbox folders to export.
Select “Outlook PST file” as the destination format.
Choose the saving location and click Next to begin the export process.
Signs Your OST File is Approaching the Limit
When your OST file nears or exceeds the limit, you may encounter:
Slow Outlook performance
Delayed sending/receiving of emails
Calendar sync issues
File size warning messages
Unexpected Outlook hangs or crashes
These signs indicate it’s time to reduce OST size, archive old data, or increase the storage limit manually.
Risks of Hitting the OST File Size Limit in Outlook 2016
When the OST file gets too large, it can severely affect Outlook and data integrity. Although Outlook 2016 supports OSTs up to 50 GB, surpassing the limit may cause:
Slow loading and syncing of mail items, impacting productivity
Incomplete server synchronization, leading to outdated emails or calendars
Corruption risk, which may render the OST unreadable
Outlook crashes, hangs, or refusal to open
Backup and recovery complications, since OST files are not portable without conversion
How to Check OST File Size in Outlook 2016
1. Check via Outlook:
Open Outlook 2016.
Go to File → Info.
Under Mailbox Settings, click Tools → Mailbox Cleanup, then View Mailbox Size.
Review the total size (including subfolders) in KB. Around 47,500,000 KB (~47.5 GB) means you’re nearing the warning limit.
2. Check via File Explorer:
Open File Explorer.
Navigate to:
C:\Users\<YourUsername>\AppData\Local\Microsoft\Outlook
Right‑click the OST file → Properties.
If AppData is hidden, enable hidden items in Explorer’s View menu.
How to Increase the OST File Size Limit in Outlook 2016
Outlook 2016 defaults to a 50 GB OST file limit, but you can increase it via the Windows Registry. (Warning: Incorrect registry edits can harm your system, so back up before proceeding.)
Steps:
Press Win + R, type regedit, and press Enter.
Navigate to:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST
Create or modify two DWORD values (in MB):
MaxLargeFileSize – maximum OST size
WarnLargeFileSize – warning threshold
Recommended settings:
MaxLargeFileSize = 102400 (100 GB)
WarnLargeFileSize = 97280 (95 GB)
Close Registry Editor and restart Outlook 2016 to apply changes.
Tips for Managing OST File Size Effectively
Even with a higher limit, proactive management keeps OST files healthy:
Archive old emails regularly
File → Options → Advanced → AutoArchive → Move items older than a specified time to PST.
Clean Deleted and Junk folders
Right-click those folders → Empty Folder to free up space.
Compact the OST file
File → Account Settings → Data Files → select OST → Settings → Advanced → Compact Now.
Find and archive large attachments
Use search to locate emails with big attachments and move them to a PST.
Disable shared folder syncing
In Account Settings → Mailbox Settings → uncheck Download shared folders if not needed.
Conclusion
This guide covers the OST file size limits in Outlook 2016, how to monitor file size and location, and techniques to preserve your data. If you found this useful, you now have the insights you need to prevent OST limit issues and ensure a smooth offline mail experience.