How to Increase Outlook OST File Size Limit Using Registry – Outlook 2013, 2016, 2019, 2021 & Microsoft 365

Quick Answer

If Microsoft Outlook is showing that your .ost file has reached its maximum size, you can increase the OST file-size limit by changing the MaxLargeFileSize and WarnLargeFileSize registry values.

The registry path depends on the Outlook/Office generation:

Microsoft documents the registry settings for both .pst and .ost files. The same MaxLargeFileSize and WarnLargeFileSize values are used for Unicode OST files.

What Is the Default Outlook OST Size Limit?

For Outlook 2013 and Outlook 2016, Microsoft documents a default Unicode OST maximum of 51,200 MB (50 GB), with a WarnLargeFileSize value of 48,640 MB (47.5 GB).

Outlook 2019, Outlook 2021 and Microsoft 365 Classic Outlook use the 16.0 Office registry branch. Microsoft community guidance also identifies the same 16.0 registry location for Outlook 2016, 2019, 2021 and Microsoft 365.

These are local Outlook data-file limits. They should not be confused with the mailbox storage limit imposed by an email provider or Exchange/Microsoft 365 service.

Which Registry Values Control the OST Size?

For Unicode .ost and .pst files, the important registry values are:

Important: Despite the name WarnLargeFileSize, Microsoft states that this registry value does not simply enable a warning popup before the file reaches its limit. It represents a data threshold at which Outlook can restrict adding more data.

Registry Path for Outlook 2013

For Outlook 2013, use:

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST

If the PST key does not exist, it can be created manually.

Registry Path for Outlook 2016, 2019, 2021 and Microsoft 365

For Outlook 2016, Outlook 2019, Outlook LTSC 2021 and Classic Outlook for Microsoft 365, use:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

The important point is that Outlook 2019 and Outlook 2021 do not use 19.0 or 21.0 registry branches for these settings. They use the Office 16.0 registry branch.

How to Increase the OST Limit

Step 1: Close Outlook

Completely close Outlook before modifying the registry.

It is also recommended to make sure that OUTLOOK.EXE is no longer running in Task Manager before making the change.

Step 2: Open Registry Editor

Press:

Windows + R

Type:

regedit

Press Enter.

Step 3: Open the Correct Outlook Registry Path

Use the appropriate path:

Outlook 2013:

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST

Outlook 2016 / 2019 / 2021 / Microsoft 365 Classic Outlook:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

Step 4: Create the PST Key if Necessary

If the PST key does not exist:

1. Right-click the Outlook key.
2. Select New > Key.
3. Name the new key PST.

Step 5: Create MaxLargeFileSize

Inside the PST key:

1. Right-click an empty area in the right pane.
2. Select New > DWORD (32-bit) Value.
3. Name it MaxLargeFileSize.
4. Double-click it.
5. Select Decimal.
6. Enter the desired maximum size in MB.

Step 6: Create WarnLargeFileSize

Create another DWORD (32-bit) Value named:

WarnLargeFileSize

Set its value in Decimal, using MB.

Microsoft recommends maintaining at least a 5% difference between the maximum and threshold values so that Outlook's internal processes are not unnecessarily restricted.

Example: Increase OST Limit to 100 GB

If you want the maximum OST size to be 100 GB:

100 GB × 1024 = 102,400 MB

A 5% lower threshold is:

95 GB × 1024 = 97,280 MB

Therefore, the final registry configuration would be:

[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST]
"MaxLargeFileSize"=dword:00019000
"WarnLargeFileSize"=dword:00017c00

For Outlook 2013, replace 16.0 with 15.0:

[HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST]
"MaxLargeFileSize"=dword:00019000
"WarnLargeFileSize"=dword:00017c00

What If My Outlook Version Is 2019 or 2021?

Do not look for:

Office\19.0
Office\21.0

For these desktop Office versions, the relevant registry branch is:

Office\16.0

For example, Outlook 2019 uses:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

Outlook LTSC 2021 also uses the 16.0 Office branch for these Outlook registry settings. Microsoft documentation for later Outlook versions likewise identifies 16.0 as the applicable Office version for Outlook 2016 and later desktop releases.

What About Microsoft 365 Outlook?

For Classic Outlook for Microsoft 365, the Office registry branch is generally:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

However, Microsoft 365 is continuously updated, so the exact Outlook build and account configuration should be considered before applying registry changes.

The registry modification discussed in this article applies to the traditional Windows Outlook application that uses local Outlook data files. It should not be confused with the newer new Outlook for Windows application, which has a different architecture and does not use the traditional OST workflow in the same way.

Does This Work for IMAP OST Files?

Yes. The registry settings documented by Microsoft apply to both .pst and .ost files. Microsoft specifically documents the same registry entries for Outlook data files, including Unicode OST files.

For an IMAP account, Outlook can maintain a local .ost file containing synchronized mailbox data. Microsoft also notes that Microsoft 365, Exchange, IMAP and Outlook.com accounts can use an Offline Outlook Data File (.ost).

What Happens When the OST Reaches the Limit?

When an OST reaches the configured threshold or maximum, Outlook can restrict operations that add more data to the file.

Depending on the situation, you may experience:

• Messages cannot be moved or copied into the data file.
• New messages may fail to be stored in the affected data file.
• Synchronization may produce errors.
• Outlook may become slow or temporarily unresponsive.
• In some situations, Outlook may experience crashes or synchronization problems.

Microsoft documents these behaviors for Outlook data files that reach their configured limits.

Should You Increase the OST to 100 GB?

Increasing the limit can solve a genuine storage-limit problem, but a larger OST is not automatically better.

Microsoft's Outlook performance guidance indicates that large OST files can cause increasingly noticeable pauses. Microsoft specifically notes that when an OST reaches more than 10 GB, short pauses can begin on most hardware, while OST files of 25 GB or larger can experience more frequent pauses, particularly when downloading new email.

Therefore, before setting the limit to 100 GB or higher, consider whether you actually need the entire mailbox cached locally.

Alternative: Reduce the OST Size

If the problem is caused by an unnecessarily large local cache, increasing the registry limit may not be the best solution.

Consider:

• Removing unnecessary messages and attachments.
• Emptying folders that contain unwanted data.
• Archiving older information where appropriate.
• Reducing the amount of mailbox data synchronized locally when the Outlook account type and version provide that option.
• Compacting the Outlook data file where appropriate.

Microsoft provides guidance for reducing the size of Outlook data files and notes that large mailboxes can take longer to open and search.

Registry Backup Before Making Changes

Before changing the Outlook registry settings, it is advisable to export the relevant PST registry key.

1. Open regedit.
2. Navigate to the applicable PST key.
3. Right-click PST.
4. Select Export.
5. Save the .reg backup file in a safe location.

If the change causes an unexpected problem, the exported registry file can be used to restore the previous configuration.

How to Verify the Registry Change

After changing the values:

1. Close Registry Editor.
2. Restart Windows or at least completely restart Outlook.
3. Open Outlook.
4. Allow the mailbox to synchronize.
5. Monitor the OST file size.
6. Check whether the previous maximum-size error has disappeared.

The OST location can normally be viewed from Outlook's account/data-file settings. Do not manually move or rename an active OST file unless you understand the consequences and have a recovery plan.

How to Roll Back the Change

If you want to return Outlook to its default configuration, close Outlook and remove the custom:

MaxLargeFileSize
WarnLargeFileSize

values that you created, or restore the registry backup taken before the modification.

Do not arbitrarily set the values to zero. Removing an unnecessary custom registry value is generally preferable when returning to Outlook's built-in configuration.

Important Difference Between User and Policy Registry Paths

Microsoft also documents a policy-based registry location:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\PST

For Outlook 2013, the corresponding policy branch is:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\15.0\Outlook\PST

In managed business environments, administrator policies can take precedence over ordinary user preferences. If the computer is managed by an organization, changing the normal user-preference key may not override an existing policy.

Frequently Asked Questions

What is the default OST size limit in Outlook 2013?

Microsoft documents the default Unicode OST maximum as 51,200 MB, or approximately 50 GB, for Outlook 2013. The documented WarnLargeFileSize value is 48,640 MB, or approximately 47.5 GB.

What registry path does Outlook 2013 use?

Use:

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\PST

What registry path does Outlook 2016 use?

Use:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

Does Outlook 2019 use Office 19.0?

No. For these Outlook data-file registry settings, Outlook 2019 uses the 16.0 Office registry branch.

Does Outlook 2021 use Office 21.0?

No. Outlook LTSC 2021 uses the 16.0 Office registry branch for these settings.

Does Microsoft 365 use the 16.0 registry path?

Classic Outlook for Microsoft 365 uses the Office 16.0 registry branch for these Outlook data-file settings.

Can I set the OST limit above 50 GB?

Yes. Microsoft states that Unicode limits can be configured beyond the documented default values. However, Microsoft does not recommend unnecessarily increasing the limit because performance can decrease as the data file becomes very large.

Does increasing MaxLargeFileSize increase my email server quota?

No. MaxLargeFileSize controls the local Outlook data-file limit. It does not increase the mailbox quota configured by an IMAP provider, Exchange server or Microsoft 365 service.

Can I use the same registry setting for an IMAP OST?

Yes. Microsoft documents these registry settings for both .pst and .ost files.

Is 100 GB OST recommended?

Not generally. It may be technically useful when a legitimate mailbox requires it, but very large OST files can negatively affect Outlook performance. Microsoft specifically warns about increasing pauses as OST files become large.

Quick Reference

Sources

• Microsoft Learn — Configure Size Limit for PST and OST Files in Outlook
• Microsoft Support — How to troubleshoot performance issues in Outlook
• Microsoft Support — Reduce the size of your mailbox and Outlook Data Files
• Microsoft Learn — Outlook attachment and Internet email account registry settings