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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11You can raise the configured size ceiling for a Unicode Outlook data file in some classic Outlook versions, but Microsoft warns that higher limits can reduce performance. The supported registry guidance retrieved here covers Outlook 2003 through Outlook 2016; it does not establish that the same procedure applies to every current edition or to new Outlook for Windows. Before changing the registry, identify your Outlook version and whether your file is an ANSI or Unicode PST/OST. For many users, cleaning the file or moving older mail into additional PST archives is safer than making one file larger.
What the Outlook PST and OST size limits mean
Outlook has different limits for older ANSI data files and Unicode data files. Microsoft’s documented defaults for Outlook 2016, 2013, and 2010 are a 50 GB maximum and a 47.5 GB warning threshold for Unicode PST and OST files. For Outlook 2007 and 2003, the corresponding Unicode defaults are 20 GB and 19 GB. The warning threshold is not the absolute file-size ceiling: it marks the content level at which Outlook can stop allowing additional data, even though internal processing may still change the physical file size.
| Outlook versions covered by Microsoft’s registry article | Unicode maximum | Unicode warning threshold | ANSI ceiling |
|---|---|---|---|
| Outlook 2016, 2013, 2010 | 50 GB (51,200 MB) | 47.5 GB (48,640 MB) | 2 GB |
| Outlook 2007, 2003 | 20 GB (20,480 MB) | 19 GB (19,456 MB) | 2 GB |
These are Microsoft-published defaults; the support article does not give a publication year. ANSI files cannot be made larger by setting a higher MaxFileSize value: Microsoft says values over 2 GB are ignored to prevent corruption. The registry values for ANSI files are expressed in bytes, while Unicode values are expressed in megabytes. Microsoft’s size-limit article lists the values and version-specific paths.
Should you raise the limit?
For a Unicode file in a supported classic Outlook version, Microsoft says the configured maximum can be set above the listed defaults, but cautions: “You can set the UNICODE limits beyond the values that are listed in the table. However, we do not recommend doing this because performance can decrease.” A higher ceiling does not make a large file faster, nor does it remove the risks of relying on one very large local data file.
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OST size can affect responsiveness well before the configured maximum is reached. Microsoft’s troubleshooting guidance says most hardware begins to experience short pauses when an OST exceeds 10 GB; at 25 GB or larger, pauses become more frequent, especially while downloading new mail. Those are thresholds in Microsoft’s guidance, not a performance guarantee for every computer. Shared folders and other synchronized content can also contribute to OST growth.
Outlook’s registry guidance is explicitly for older generations, so do not assume these keys apply to Microsoft 365, Outlook 2024, Outlook 2021, Outlook 2019, or new Outlook for Windows. Microsoft’s current support pages include tasks for those clients, but that is not confirmation that the legacy registry procedure works identically in them. Check the exact client and version before editing registry settings.
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How to change the limit in supported classic Outlook versions
Use this only after confirming that you have a Unicode data file and a version covered by Microsoft’s legacy guidance. Registry edits affect Outlook behavior and an incorrect value can cause problems; back up important PST data first. If your organization manages Office settings, a policy key may control the setting instead of the user preference key.
- Confirm the Outlook version and file format. The paths below are for classic Outlook releases. Do not use them as universal instructions for new Outlook or a version not listed in Microsoft’s article.
- Open Registry Editor and go to the Outlook PST key for your version. For Outlook 2016, the user preference path is
HKEY_CURRENT_USERSoftwareMicrosoftOffice16.0OutlookPST. The policy path isHKEY_CURRENT_USERSoftwarePoliciesMicrosoftOffice16.0OutlookPST. Microsoft lists corresponding version branches for Outlook 2013, 2010, 2007, and 2003 in its version-specific instructions. - Create or edit the Unicode DWORD values. Set
MaxLargeFileSizeto the maximum size in MB andWarnLargeFileSizeto the warning threshold in MB. If the values are absent, Microsoft notes that DWORDs may need to be created. Do not use the ANSI value names for a Unicode file. - Keep a margin between warning and maximum. Microsoft recommends that the warning value be at least 5% below the maximum. This gives Outlook room to warn before reaching the hard ceiling.
- Close Registry Editor and restart Outlook. Confirm that the intended values are in the correct version and policy branch before relying on a changed ceiling.
For ANSI data files, the names are MaxFileSize and WarnFileSize, and their values are in bytes. Because ANSI files remain limited to 2 GB, raising their maximum beyond that will not work and risks file problems. Avoid converting these instructions into a guessed numeric target: use Microsoft’s table and the value units for the format and version that actually apply.
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Safer ways to manage a file that is nearly full
The right alternative depends on what the file is for. A PST can hold archives, exports, or mail for some POP accounts. An OST is a synchronized local copy associated with Microsoft 365, Exchange, IMAP, or Outlook.com accounts; reducing or rebuilding local synchronized content is different from managing a standalone archive.
| Approach | Changes the ceiling for one file? | Best fit | Trade-off |
|---|---|---|---|
| Raise the configured ceiling | Yes, for supported Unicode files and Outlook versions | A specific classic Outlook installation that genuinely needs one larger file | Microsoft warns performance can decrease; very large OSTs can pause more often |
| Use additional PST archives | No | Keeping older or infrequently used mail available without concentrating it in one archive | Creates multiple files to organize and manage |
| Clean and compact the existing file | No | Removing unwanted content and reclaiming unused space | Deleting messages does not necessarily shrink the physical file until compaction occurs |
Find and remove large or unnecessary content
Use Outlook’s Mailbox Cleanup to locate large or old messages, remove redundant mail, and delete items you no longer need. If an OST is growing quickly, consider whether synchronized shared folders or other mailbox content are contributing, rather than treating the local file as an isolated archive.
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Archive older mail into another PST
For material you want to retain but do not need in the active data file, move or archive it into an additional PST. This does not raise the limit on any one file, but avoids forcing a single PST to hold every year of mail. An OST is synchronized from the account, so it should not be treated as a substitute for an archive PST.
Compact after deleting content
Deleting items can leave unused space inside a data file. Microsoft says Outlook may compact a PST or OST in the background during idle time when more than 20% of the file is unused; manual compaction is also available. Compaction can reduce the file’s physical size, but it does not increase the per-file ceiling. See Microsoft’s steps for reducing mailbox and Outlook Data File size.
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Choose the remedy based on the file
- Unicode PST, classic Outlook 2003–2016: a higher configured ceiling is possible, but weigh performance against the need for one larger file; multiple archive PSTs or cleanup may be preferable.
- ANSI PST or OST: do not try to exceed 2 GB by raising the registry value; move or export content into Unicode files where appropriate.
- Large OST: first consider synchronized mailbox scope and cleanup. Increasing the ceiling does not resolve pauses associated with large OST files.
- Current Outlook or new Outlook: verify version-specific Microsoft guidance before editing registry values; the cited legacy procedure does not establish universal support.
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