Outlook PST OST File Size Limit

Outlook PST and OST files have a built-in size limit that varies by Outlook version: the classic limit is 20 GB for Outlook 2007 and earlier, 50 GB for Outlook 2010 through 2016, and 100 GB for Outlook 2019 and Microsoft 365 (with a registry tweak to raise it further). This article explains the exact limits, how to check your current file size, what happens when you hit the cap, and how to keep your mailbox healthy.

What Are PST and OST Files?

Outlook stores your email, calendar, contacts, and other items in two types of data files:

• PST (Personal Storage Table) – Used with POP3 and IMAP accounts, and for archiving. This file lives on your computer and is the only copy of the data.
• OST (Offline Storage Table) – Used with Exchange, Microsoft 365, and Outlook.com accounts. It’s a cached copy of what’s on the server, so the server remains the primary source.

Both file types have size limits, but they behave differently when full. An OST that hits the limit can cause Outlook to freeze or crash, while a PST that’s full may simply refuse to accept new items.

Exact Size Limits by Version

The default maximum size for PST and OST files depends on the Outlook version you’re running. Here’s the breakdown:

• Outlook 2003 and earlier: 2 GB (hard limit, no workaround)
• Outlook 2007: 20 GB
• Outlook 2010, 2013, 2016: 50 GB
• Outlook 2019 and Microsoft 365 (Office 365): 100 GB (can be increased to 150 GB via registry)

These limits apply to both PST and OST files. The 100 GB limit for newer versions is a significant improvement, but it’s still possible to hit it if you have large attachments or years of email.

How to Check Your Current File Size

Before you worry about hitting a limit, you should know how large your PST or OST file already is. Here’s how to find it:

1. Close Outlook.
2. Press Win + R, type %localappdata%\Microsoft\Outlook and press Enter.
3. Look for files with .pst or .ost extensions. The size column shows the current file size.

Alternatively, in Outlook: File > Account Settings > Account Settings > Data Files tab. The size is shown in the “Size” column.

What Happens When You Hit the Limit?

When a PST or OST file reaches its size cap, Outlook behaves unpredictably. Common symptoms include:

• Outlook crashes or freezes when you try to send or receive mail.
• You get an error like “The file is too large” or “Cannot expand the folder.”
• New emails stop downloading, or you can’t send messages.
• Outlook may start in a degraded mode, showing only cached items.

For OST files, the server still has all your data, so you can usually recover by recreating the OST (delete it and let Outlook rebuild). For PST files, the data is only local, so hitting the limit can mean losing access to older items unless you archive or split the file.

How to Increase the Limit (Registry Tweak)

If you’re using Outlook 2019 or Microsoft 365, you can raise the limit from 100 GB to 150 GB by editing the Windows registry. This is a supported Microsoft configuration, but it requires careful steps:

1. Close Outlook.
2. Press Win + R, type regedit, and press Enter.
3. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST (if the PST key doesn’t exist, create it).
4. Create a new DWORD value named MaxLargeFileSize and set it to 150 (in GB).
5. Also create a DWORD named WarnLargeFileSize and set it to 150 to avoid warnings.
6. Close regedit and restart Outlook.

This tweak only works for Outlook 2019 and Microsoft 365. Earlier versions ignore these registry values.

How to Reduce Your PST/OST Size

If you’re approaching the limit, you have several options to shrink your data file:

• Archive old items: Use File > Cleanup Tools > Archive to move old emails to a separate PST.
• Empty deleted items: Right-click the Deleted Items folder and choose “Empty Folder.”
• Compact the file: In Account Settings > Data Files, select the file and click Settings > Compact Now. This reduces the physical size after deletions.
• Remove large attachments: Use the search tool to find emails with attachments larger than 10 MB and delete or save them elsewhere.
• Use a different account type: If you’re on POP3, consider switching to IMAP or Exchange to keep the server as the primary store.

Frequently Asked Questions

Is the limit the same for PST and OST?

Yes, the default size limit is identical for both file types in a given Outlook version. However, OST files are typically smaller because they only contain a subset of server data (the cache).

Can I exceed the limit if I use a 64-bit version of Outlook?

No, the limit is a software setting, not a memory limitation. Both 32-bit and 64-bit versions of Outlook enforce the same default limits. The registry tweak works on both.

What is the recommended maximum size for optimal performance?

Microsoft recommends keeping PST files under 10 GB for best performance, even though the limit is higher. Large files slow down searches, syncing, and opening Outlook.

Does the limit apply to OneDrive or network drives?

It’s not recommended to store PST files on network or cloud drives because of file locking and sync issues. The size limit still applies, but you may encounter additional errors.

How do I know if my OST is corrupted due to size?

If Outlook crashes repeatedly and the OST file is near the limit, you can run the Inbox Repair Tool (scanpst.exe) for PST files, but for OST, the simplest fix is to delete the OST and let Outlook recreate it from the server.

Can I split a PST file into multiple files?

Yes, you can create a new PST and move folders into it. Use File > New > Outlook Data File, then drag and drop folders from the old PST to the new one.

Related

• Block Changing Date and Time on iPhone: Full Guide
• New Apple Account: What Happens to Your Existing One?
• Why Streaming Layoffs Happen After Failed Acquisitions