If your Outlook OST file grows too large, it can slow down performance, cause sync errors, or even prevent Outlook from opening. The OST file stores a local copy of your Exchange or Microsoft 365 mailbox data, and its size is limited only by disk space by default. This article explains how to use the MaxLargeFileSize registry value to set a hard cap on the OST file size, preventing it from exceeding a specific limit. You will learn the exact registry path, the DWORD value to create, and how to apply the change without reinstalling Outlook.
Key Takeaways: Set a Maximum OST File Size in Outlook
- HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\OST: Add a DWORD named MaxLargeFileSize to cap the OST file size in megabytes.
- MaxLargeFileSize DWORD value: Enter the maximum OST size in megabytes. For a 10 GB cap, use 10240.
- Restart Outlook: The new limit takes effect only after you close and reopen Outlook completely.
Why MaxLargeFileSize Controls OST Growth
Outlook creates an OST file for any Exchange, Microsoft 365, or IMAP account configured in Cached Exchange Mode. The OST file mirrors the server mailbox locally so you can work offline. By default, Outlook does not enforce a size limit on the OST file. It grows as you sync more folders, attachments, and older email items.
When the OST file exceeds 50 GB on a 32-bit version of Outlook or 100 GB on a 64-bit version, performance degrades noticeably. Outlook may become unresponsive during sync, take longer to start, or display the error “Cannot expand the folder.” The MaxLargeFileSize registry value tells Outlook to stop synchronizing new items when the OST file reaches the size you specify. This prevents the file from growing indefinitely and protects system resources.
The MaxLargeFileSize value applies to Outlook 2013, 2016, 2019, 2021, and Outlook for Microsoft 365. It works for all account types that use an OST file. The limit is measured in megabytes. For example, a value of 20480 sets a 20 GB cap.
Steps to Set the MaxLargeFileSize Registry Value
- Close Outlook
Make sure Outlook is not running. If Outlook is open, the registry change may not apply until the next restart. - Open Registry Editor
Press Windows + R, type regedit, and press Enter. Confirm the User Account Control prompt if it appears. - Navigate to the Outlook OST key
In Registry Editor, go to:HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\OST
If the OST folder does not exist under Outlook, right-click the Outlook key, select New > Key, and name it OST. - Create the MaxLargeFileSize DWORD
Right-click the OST key, select New > DWORD (32-bit) Value. Name the new value MaxLargeFileSize. Do not use spaces or special characters. - Set the size limit in megabytes
Double-click MaxLargeFileSize. Select Decimal as the base. Enter the maximum OST file size in megabytes. For a 10 GB limit, enter 10240. For 20 GB, enter 20480. Click OK. - Close Registry Editor
Exit regedit. No reboot is required. - Restart Outlook
Open Outlook. The new size cap is now active. Outlook will not download new items once the OST file reaches the limit.
What Happens When the OST File Hits the Cap
When the OST file reaches the MaxLargeFileSize limit, Outlook stops synchronizing new items from the server. Existing items remain accessible. You will not see an error message immediately. Instead, new emails, calendar updates, and other changes on the server will not appear in the local OST file until you reduce the OST size or increase the cap.
To check if the cap is active, open the Outlook data file properties. Go to File > Account Settings > Account Settings. Select your Exchange or Microsoft 365 account and click Change. Under Offline Settings, the OST file path shows the current file location. Right-click the file in File Explorer and select Properties to see its current size.
If Outlook Ignores the MaxLargeFileSize Setting
Outlook version mismatch
The registry path 16.0 applies to Outlook 2016, 2019, 2021, and Outlook for Microsoft 365. If you use Outlook 2013, replace 16.0 with 15.0. For Outlook 2010, use 14.0. Verify your Outlook version in File > Office Account > About Outlook.
Wrong registry hive
The MaxLargeFileSize value must be under HKEY_CURRENT_USER, not HKEY_LOCAL_MACHINE. The per-user hive applies the setting only to the current Windows user account. If you set it under HKEY_LOCAL_MACHINE, Outlook ignores it.
OST file already larger than the cap
If the OST file is already larger than the MaxLargeFileSize value you set, Outlook will not shrink it. The cap only prevents further growth. To reduce an oversized OST file, create a new Outlook profile or use the Mail applet in Control Panel to remove and recreate the OST file.
MaxLargeFileSize vs Other OST Size Controls
| Item | MaxLargeFileSize Registry | Outlook AutoArchive / Retention Policy |
|---|---|---|
| Description | Hard size cap on the OST file in megabytes | Moves or deletes old items from the mailbox to reduce size |
| Effect on existing data | Does not reduce current OST size | Removes items from OST after archiving or purging |
| When limit is reached | Sync stops for new items | Items are moved or deleted based on age rules |
| Configuration method | Registry Editor | File > Options > Advanced > AutoArchive Settings |
| Supported Outlook versions | Outlook 2010 and later | All Outlook versions |
The MaxLargeFileSize registry value is a direct size limit. AutoArchive and retention policies reduce the number of items stored, which indirectly shrinks the OST file. For best results, combine both methods. Set a reasonable MaxLargeFileSize cap and configure AutoArchive to delete or move items older than a specific date.
After setting the registry value, you can monitor the OST file growth in File Explorer. If the file approaches the cap, adjust the value or enable AutoArchive to remove old items. A 10 GB to 20 GB cap works well for most users with a 64-bit version of Outlook. For 32-bit Outlook, keep the cap at 50 GB or lower to avoid performance problems.