Outlook PST size limits explained (and what to do with huge archives)

Two PST formats: ANSI and Unicode

There are two versions of the PST format, and the limit depends almost entirely on which one you have.

ANSI PST (Outlook 97–2002)

The original format, used by Outlook 97, 98, 2000 and 2002. It has a hard limit of about 2 GB and supports only limited character sets. An ANSI file that gets close to 2 GB is at real risk of corruption, so Outlook caps it slightly below that, at roughly 1.93 GB by default. You’ll mostly meet ANSI files in very old archives.

Unicode PST (Outlook 2003 and later)

Outlook 2003 introduced the Unicode (“large”) format. It handles any language and has a far higher theoretical ceiling; the practical limit is a setting in Outlook rather than a property of the file. Every current Outlook for Windows creates Unicode PSTs by default.

The limit is enforced by the Outlook that opens the file. A 30 GB Unicode PST created in Outlook 2016 is fine in today’s Outlook, but an old Outlook 2007 installation with its 20 GB default would refuse to add anything to it.

How to check your PST’s format and size

Format

The PST has to be open in classic Outlook for Windows. Then:

1. Go to File → Account Settings → Account Settings and open the Data Files tab.
2. Select the PST and click Settings (or double-click it).
3. Look at the Format line. “Outlook Data File” (or “Personal Folders File”) means Unicode. Anything with 97-2002 in the name, such as “Outlook Data File (97-2002)”, means ANSI.

You can reach the same window by right-clicking the top folder of the PST in the folder pane, choosing Data File Properties and then Advanced.

Size

The quickest check is the file itself: the Data Files tab shows its location, so open that folder in File Explorer and look at the size. Inside Outlook, Data File Properties → Folder Size shows how much each folder holds. The two numbers rarely match exactly, because a PST keeps free space from deleted items until it is compacted.

What happens when a PST is full

Outlook doesn’t corrupt a Unicode PST that reaches its limit; it simply stops adding data. In practice that means:

• Moving or copying items into the file fails with a message that the file “has reached its maximum size”.
• If sent mail is saved to that file, sending can fail.
• AutoArchive into a full archive PST fails.

You can still delete items. The fix is to permanently delete what you don’t need (empty Deleted Items too), compact the file, or move older mail into a new PST.

Raising the limit with registry settings

Microsoft documents four registry values that control the size of both PST and OST files, in its article “Configure size limit for PST and OST files in Outlook”. For Unicode files the two that matter are:

• MaxLargeFileSize: the absolute maximum size, in MB. Default 51200 (50 GB) in Outlook 2010 and later.
• WarnLargeFileSize: despite its name, the point at which Outlook stops adding new data, in MB. Default 48640 (47.5 GB). It does not show a warning.

Both are DWORD (32-bit) values, entered with the Decimal option. The ANSI equivalents are MaxFileSize and WarnFileSize, set in bytes; values above the 2 GB ANSI limit are ignored.

For Outlook 2016, 2019, 2021, 2024 and Microsoft 365 (all version 16.0), the values go in the PST key under one of these paths. You may need to create the key and values:

• Policy location: HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\PST
• User preference location: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST

Microsoft recommends keeping the Warn value at least 5% below the Max value so Outlook’s internal housekeeping can continue. For example, a 100 GB limit would be MaxLargeFileSize = 102400 and WarnLargeFileSize = 97280. Microsoft also notes that limits above the defaults are possible but not recommended, because performance can suffer.

These same values also apply to OST files, the offline cache Outlook keeps for Exchange and Microsoft 365 mailboxes. Raising the limit for a PST raises it for your OST as well.

Big PSTs: performance and reliability

A PST that is allowed to reach 50 GB will work, but large files are slower to open, search and back up, and a single damaged file affects more mail. A few habits keep them healthy:

• Keep PSTs on a local drive. Microsoft doesn’t support opening PSTs over a network share, and sync services such as OneDrive or Dropbox can corrupt a PST that is open in Outlook. Copy it to cloud storage for backup only while Outlook is closed.
• Compact after big clean-ups. Deleting mail doesn’t shrink the file by itself. Open Data File Properties → Advanced (or Account Settings → Data Files → Settings) and click Compact Now.
• Repair errors early. If Outlook reports problems with a data file, close Outlook and run the Inbox Repair Tool (SCANPST.EXE), which is installed with Office. Back up the PST first.
• Prefer several medium files to one giant one. Ten 5 GB archives are easier to back up, move and repair than one 50 GB file.

Splitting large archives into several PSTs

By year

Classic Outlook’s Archive feature moves older mail into a separate PST. Use File → Tools → Clean Up Old Items (named Archive in some versions), choose a folder, set Archive items older than a date and pick a new file such as archive-2019.pst. Repeat with different dates to create one file per year or per few years. AutoArchive can do this on a schedule.

By account or mailbox

Keep each old account in its own PST: one for a former job, one for a personal Gmail, and so on. That keeps every file smaller and makes it obvious where to look.

Open several PSTs side by side

Outlook can open many PSTs at once (File → Open & Export → Open Outlook Data File in classic Outlook). Each appears as its own folder tree, and search can cover all of them. Splitting costs you very little in convenience. See our PST import guide for opening versus importing.

Microsoft 365 alternatives

With a business Microsoft 365 subscription, you may not need large PSTs at all. Many plans include an online archive mailbox for older mail, and administrators can use Microsoft’s PST import service to upload PST files directly into mailboxes. Both depend on your plan and your administrator.

Converting a large mbox archive with MoveMyInbox

If your mail is in mbox format (Google Takeout, Thunderbird, Apple Mail or another app), MoveMyInbox converts it online into a Unicode PST, the format every current Outlook for Windows uses, which can also be imported in Outlook for Mac. Each conversion can be up to 50 GB, the same as Outlook’s default limit, so the file opens without registry changes.

For bigger archives, or simply to keep files manageable, split the job into several conversions:

• Per account: convert each old mailbox separately.
• Per group of Gmail labels: in Google Takeout you can select specific labels instead of all mail, so you can export and convert one set of labels at a time. Our Google Takeout guide shows where that option is.
• Per folder: Thunderbird and Apple Mail store each folder as a separate mbox file, so you can upload a few folders per conversion. See the Thunderbird guide.

Folders, attachments, read and flagged status come across, and you pay once based on total size (see pricing). For a broader look at the formats, read MBOX vs PST.

Frequently asked questions