How to Fix

How to Fix 'Outlook Data File Has Reached Maximum Size' (50GB Limit)

Your Microsoft Outlook client suddenly stops sending or receiving emails.

You try to delete an old message, but Windows throws an immediate blocker:

"The file C:\Users\\AppData\Local\Microsoft\Outlook\archive.pst has reached its maximum size. To reduce the amount of data in this file, select some items that you no longer need, and permanently delete them."

You can't delete anything because Outlook needs temporary buffer space to execute deletions in the first place. You are caught in a classic MAPI deadlock.

By default, modern Microsoft Outlook (2024, 2021, 2019, 2016, and Microsoft 365) sets a hard upper ceiling of 50 GB (51,200 MB) on Unicode .pst and .ost data files. Once your database crosses 49.5 GB, Outlook locks into read-only mode to prevent index corruption.

Here is the complete engineering walkthrough to resolve the 50GB Outlook data file limit: increasing capacity through Windows Registry tweaks, compacting unallocated space, and splitting monolithic archives into stable 10GB–20GB databases using the Prism PST Converter & Optimization Engine.

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* 🔍 Run Free Client-Side PST Header Integrity Diagnostic ➔

* 🚀 Download Standalone 64-Bit PST Splitting & Optimization Tool ➔

* 📊 Calculate Mailbox Compaction & Cloud Transfer Timing ➔

Technical Overview: Why Outlook Imposes the 50GB Barrier

Microsoft introduced the 50GB default cap in Outlook 2010 (up from the 20GB limit in Outlook 2003/2007 and the legacy 2GB limit in ANSI Outlook 2000).

While the underlying 64-bit NDB (Node Database) structure theoretically supports archives up to 4,096 GB (4 TB), desktop storage controllers experience severe performance degradation once a single PST file exceeds 25 GB. Searches lag, calendar reminders delay, and sudden power outages trigger immediate B-Tree corruption.

Method 1: Increase Outlook PST/OST Size Limits via Windows Registry

If you need emergency access to send or receive emails immediately, you can increase Outlook's database capacity up to 100 GB by adding two DWORD values into the Windows Registry.

Step 1: Close Microsoft Outlook

Ensure OUTLOOK.EXE is fully terminated. Check Windows Task Manager (Ctrl + Shift + Esc) to verify no background MAPI processes are active.

Step 2: Open Windows Registry Editor

Press Windows + R, type regedit, and hit Enter.

Step 3: Navigate to Your Outlook Registry Path

Depending on your installed Office version, browse to the corresponding hive:

• Outlook 2016, 2019, 2021, 2024 & Microsoft 365:

HKEYCURRENTUSER\Software\Microsoft\Office\16.0\Outlook\PST

• Outlook 2013:

HKEYCURRENTUSER\Software\Microsoft\Office\15.0\Outlook\PST

• Outlook 2010:

HKEYCURRENTUSER\Software\Microsoft\Office\14.0\Outlook\PST

(Note: If the PST sub-key does not exist under Outlook, right-click Outlook, select New > Key, and name it PST.)

Step 4: Create the MaxLargeFileSize Value

1. Right-click inside the right pane, select New > DWORD (32-bit) Value.
2. Name the value: MaxLargeFileSize.
3. Double-click it, select the Decimal radio button, and set the value to your desired cap in Megabytes (MB):

• For 100 GB: Enter 102400 (Hex: 0x00019000)
• For 80 GB: Enter 81920 (Hex: 0x00014000)

Step 5: Create the WarnLargeFileSize Value

Outlook requires a safety buffer threshold (at least 5% lower than MaxLargeFileSize):

1. Right-click in the right pane, select New > DWORD (32-bit) Value.
2. Name the value: WarnLargeFileSize.
3. Double-click it, select Decimal, and enter a value roughly 95% of your max size:

• For 100 GB Max: Enter 97280 (95 GB)
• For 80 GB Max: Enter 77824 (76 GB)

Registry Value Name: MaxLargeFileSize
Type: REG_DWORD
Value (Decimal): 102400 (Allows up to 100 GB)

Registry Value Name: WarnLargeFileSize
Type: REG_DWORD
Value (Decimal): 97280  (Warns at 95 GB)

Step 6: Restart Outlook

Reopen Outlook. The client will immediately detect the new 100GB ceiling, mount the data file, and resume normal email syncing.

Method 2: Compact the Data File to Recover White Space

When you delete thousands of emails or move folders, Outlook does not automatically shrink the .pst file on disk. Instead, it marks the clusters as "unallocated white space."

Running an in-place compaction reclaims this unused disk space.

Step-by-Step Compaction Workflow:

1. In Outlook, click File > Account Settings > Account Settings.
2. Switch to the Data Files tab.
3. Select your oversized .pst or .ost file and click Settings.
4. In the dialog box, click Compact Now.

[Outlook Account Settings]
       │
       ▼
[Select Data File (.pst/.ost)]
       │
       ▼
[Click "Settings" ➔ "Compact Now"]
       │
       ▼
(Reclaims deleted cluster pages — takes 15–45 minutes for 50GB files)

Outlook will scan all B-tree nodes and release unreferenced clusters back to your Windows file system. If your archive contained 15GB of deleted items, your 50GB file will shrink to 35GB.

Method 3: Split Oversized PST Files into Safe 10GB–20GB Archives (Permanent Fix)

The only permanent, enterprise-grade resolution to the 50GB limit is splitting the monolithic file into smaller, year-wise or size-wise chunks.

Microsoft's native archive tool frequently freezes during this process because it relies on standard Windows MAPI buffers. The Prism PST Converter & Recovery Engine executes low-level binary B-tree carving directly without touching Outlook.

How to Split a 50GB+ PST File with Prism Engine:

1. Launch Prism PST Converter on your Windows workstation or Apple Silicon Mac.
2. Click Add File and load your oversized 50GB .pst or .ost file (even if Outlook refuses to open it).
3. Under Split Options, choose your target threshold:

• Split by Size: Automatically slice into clean 5 GB, 10 GB, or 20 GB chunks.
• Split by Year / Date Range: Create separate archives (e.g., Archive2024.pst, Archive2025.pst).
• Split by Folder: Export discrete PST files for Inbox, Sent Items, and Projects.

1. Choose your destination directory and click Convert & Split.
2. The 64-bit multi-threaded engine parses the file at up to 25 GB per hour, generating verified, error-free Unicode PST files with 100% metadata and attachment integrity.

Technical Comparison: Fixing the 50GB Limit

Frequently Asked Questions (FAQs)

Q1. What is the hard maximum size limit for Outlook PST files?

Ans: By default, Microsoft Outlook 2016, 2019, 2021, 2024, and Microsoft 365 limit Unicode PST and OST files to 50 GB (51,200 MB). Through Windows Registry configuration (MaxLargeFileSize), you can raise this threshold up to 100 GB or 4,096 GB, though operational stability degrades significantly above 50 GB.

Q2. Why can't I delete emails when Outlook says the data file reached maximum size?

Ans: Deleting an email in Outlook actually moves the item into the Deleted Items folder and updates MAPI transaction logs. This action requires temporary scratch buffer memory inside the database. When a file is at 100% capacity (50.0 GB), Outlook blocks all write operations, preventing deletions. You must either increase the registry limit temporarily or use a third-party splitter.

Q3. Does the 50GB limit apply to Microsoft 365 and Exchange OST cache files?

Ans: Yes. Local OST cache files share the exact same 50GB Unicode limit as local PST files. If your cloud Microsoft 365 mailbox has a 100GB license (Exchange Online Plan 2) but your local OST reaches 50GB, Outlook will fail to synchronize new items until you adjust the local cache slider in Account Settings or configure registry expansion.

Q4. How do I fix Outlook error code 0x80040600 on an oversized PST file?

Ans: Error 0x80040600 indicates that the internal B-tree index tables are damaged due to exceeding file size thresholds. First, terminate Outlook and run ScanPST.exe or apply the registry tweak. If ScanPST hangs in Phase 7, use the Prism PST Recovery Tool to extract all messages directly into fresh 10GB PST files.

Q5. Can I split an oversized PST file without having Microsoft Outlook installed?

Ans: Yes. The Prism PST Converter Engine operates completely standalone on Windows 11/10 and macOS Sequoia/Sonoma. It reads the raw disk sectors of .pst and .ost files directly, splitting archives without needing an active Microsoft Outlook installation or Exchange Server connection.

The Bottom Line

Hitting the 50GB Outlook data file ceiling can bring business communications to a dead stop.

While tweaking the Windows Registry (MaxLargeFileSize) provides instant emergency relief to get emails flowing, maintaining multi-gigabyte monolithic databases is a major corruption hazard.

The permanent, industry-standard solution is to compact unallocated white space and split bloated archives into distinct annual databases using the Prism PST Converter & Splitter Engine.

Related Guides: If your data file was corrupted when hitting the 50GB limit, follow How to Repair Corrupted Outlook PST Files. For splitting archives on macOS, check out our PST for Mac Outlook Solution Guide.