What Is a PST File? Outlook Email Storage, Size Limits and How to Open It
A PST file is the Outlook Data File that keeps your email on your own computer instead of on a mail server. Jump to your question below, or read it through in order.
A PST file is the Outlook Data File that keeps your email on your own computer instead of on a mail server. Jump to your question below, or read it through in order.
A PST file is an Outlook Data File. PST stands for Personal Storage Table. Classic Outlook for Windows uses it to store email, calendar events, contacts, tasks and notes locally on your own computer instead of on a mail server.
On Windows 10 and 11 the default folder is Documents\Outlook Files. A modern Unicode PST file can reach 50 GB; an old ANSI one stops at 2 GB. Only classic Outlook opens a PST file on its own.
A PST file works like a single-file mailbox. Everything you see in Outlook sits inside it: email messages and attachments, the folder tree, contact cards, calendar events, tasks and notes. That is why one file can grow to several gigabytes. In File Explorer it still looks like one ordinary file.
Microsoft has used three names for the same thing. Personal Storage Table is the technical name. Personal Folders File is what older Microsoft guides call it. Outlook Data File is the label you see in Outlook's menus today. All three mean a file with the .pst file extension.
Most people meet their first PST file in one of two ways. An IT team hands over a backup and says "your old email is in here". Or Outlook starts warning that the file has run out of space. Either way you are holding a complete mailbox that only Outlook fully understands.
Almost every step on this page needs classic Outlook for Windows, so check which one you have before you start. Classic Outlook has a File tab in the top-left corner and a ribbon of tabs across the top. New Outlook has no File tab, and it shows a New Outlook toggle switch in the top-right corner.
If that toggle is there and switched on, turn it off to get back to classic Outlook. Put simply: if you cannot find the File tab in Outlook, you are in new Outlook, and none of the PST steps below will appear.
Outlook creates a PST file in four situations: a POP account, AutoArchive, an export and IMAP accounts on Outlook 2013 and earlier.
Microsoft 365, Exchange and Outlook.com accounts behave differently. Their email stays on the server. Outlook keeps a cached copy locally in a different file, and that one has its own guide: the OST file format. If your email comes from an Exchange Server database mailbox, you will usually have no PST file at all unless you made one yourself.
A PST file holds every Outlook item type in one place. That means email and attachments, the whole folder tree including custom folders, contacts and distribution lists, calendar appointments and meetings, tasks, notes and journal entries. It even keeps per-folder settings such as views and rules.
Single email messages pulled out of a PST file are saved as .msg files. When one of those refuses to open, our guide to Outlook MSG file errors covers the usual causes.
A PST file gives you an offline backup of your email that you own, that works with no internet connection and that frees up space on the mail server. The trade is that nothing backs it up for you, and one damaged file can take the whole mailbox with it.
Whether you need one at all depends on your account type. This is the first thing to check, because half the questions people ask about PST files are really questions about which kind of account they have.
| Account type | Where your email lives | Do you get a PST file? |
|---|---|---|
| POP3 | Downloaded to your computer | Yes, always. The PST file is the only copy |
| IMAP, Outlook 2013 and earlier | Server, with a local copy | Yes |
| IMAP, Outlook 2016 and later | Server, with a local copy | No, unless you export one |
| Microsoft 365 or Exchange | Server, cached locally | No, unless you export or archive |
| Outlook.com | Server, cached locally | No, unless you export one |
A Unicode PST file defaults to 50 GB in Outlook 2010 and later, and 20 GB in Outlook 2003 and 2007. An ANSI PST file is hard-capped at 2 GB.
The ceiling is not the target. Outlook slows down and repairs get common well before it, usually past about 10 GB. Treat that as your real limit.
| Outlook version | Default format | Maximum size | Warns at |
|---|---|---|---|
| 2002 and earlier | ANSI | 2 GB | 1.82 GB |
| 2003 and 2007 | Unicode | 20 GB | 19 GB |
| 2010, 2013, 2016 and later | Unicode | 50 GB | 47.5 GB |
Two details decide what happens in practice. First, the ANSI cap is hard. The default MaxFileSize is 2,075,149,312 bytes. Set a bigger value and Outlook ignores it, holding the file at 2 GB rather than risk damage.
Second, the Unicode numbers are ceilings, not targets. Microsoft asks for a 5 percent gap between the maximum and the warning value so its own clean-up jobs have room to work.
In real use the practical limit is much lower than the ceiling. A PST file past about 10 GB makes Outlook slow to search and slow to start, and it is the size range where most repair jobs begin. If your file is heading that way, the shrink a PST file section below is the fix.
When it does hit the wall, Outlook does not fail quietly. The usual symptoms are Outlook error 0x8004060C when the store cannot grow, and Outlook mailbox export limit exceeded when an export runs past the cap.
On Windows 10 and 11, PST files you create or export are stored in C:\Users\<username>\Documents\Outlook Files.
The exact storage location depends on the Outlook version, the account type and the settings used when the file was made. That is why so many older guides send people to folders that no longer exist. These are the typical default paths on a current Windows computer.
archive.pst were written here on Windows Vista, 7 and 8. The folder is hidden, so type the path into the File Explorer address bar. Outlook does not save new PST files here today.
Some PST files sit in folders no Outlook profile knows about. Open File Explorer on Windows 11 or Windows 10, turn on hidden items, then search your whole system drive for *.pst. Old backups handed over by an IT team often sit on the desktop or in a folder nobody documented.
Do not keep a working PST file on a network share. Microsoft states plainly that "moving a .pst file to a network share is not supported" and that "connecting to .pst files over LAN/WAN links is not supported".
The same goes for a folder that OneDrive, Dropbox or Google Drive syncs. Outlook writes to the file all the time while it is open. A sync client copying it mid-write is one of the most common ways a healthy file turns into a broken one. Keep it on the local disk and back up a copy instead.
In classic Outlook go to File → Open & Export → Open Outlook Data File and pick the .pst file. It attaches as its own folder tree and leaves your existing mailbox alone.
Double-clicking a PST file in File Explorer does not work. Windows has no handler for the file type, so you get an error or a dialog asking which program to use. The file has to be opened from inside Outlook: open classic Outlook, go to File, choose Open & Export, choose Open Outlook Data File, then browse to the file.
.pst file and select it. It appears as a new top-level folder in the sidebar, named after whatever the file was called when it was made.
This is how to access PST files you have been handed without mixing them into your live mailbox. Nothing is copied and nothing is merged. To remove it later, right-click the folder and choose Close. That detaches the file without deleting it.
If Outlook refuses to open the PST file, the error text tells you which problem you have. The Outlook PST permission denied error usually means the file is read-only, or another program has it locked. Outlook error 0x80040600 points at damage inside the file that SCANPST.EXE can often repair.
Windows has no built-in PST reader. To open a PST file without Outlook you need a third-party PST viewer, or you upload the file into a mail service such as Gmail and read it there.
This trips people up. A PST file looks like an ordinary file, so double-clicking it feels like it should work. It does not. File Explorer has no PST handler and no preview, and renaming the file extension only changes the label. These are the routes that do work.
.msg files.New Outlook can import a PST file, but only when classic Outlook is on the same computer and both builds match on 32-bit or 64-bit. On a machine with new Outlook alone there is no PST import at all. If new Outlook is throwing errors during the import, our New Outlook error fixes cover the common ones.
From the Inbox, select New Items → More Items → Outlook Data File, give the file a name and click OK.
Documents\Outlook Files unless you browse somewhere else.A new PST file made this way is always Unicode, so it gets the 50 GB ceiling and not the old 2 GB one. That is why "make a new PST file and move everything across" is the standard fix for an ANSI file that has run out of space.
Go to File → Open & Export → Import/Export → Import from another program or file → Outlook Data File (.pst).
People mix these two up constantly, and picking the wrong one is why a mailbox ends up with everything duplicated.
Attaches the file as its own folder tree in the sidebar. Nothing is copied and your mailbox is untouched. Right-click and Close to detach it again.
Merges the contents into the mailbox you already use. The two sets of email end up mixed together, and you cannot undo it with one click.
Two things go wrong often enough to name. If the wizard cannot see the file type you expect, check the Outlook service provider does not support it error. If the import starts and then stops partway, the source file is usually the damaged one, not the destination. Run SCANPST.EXE on a copy first.
Deleting email does not shrink your PST file on its own. You have to delete the items, empty the Deleted Items folder, then run Compact Now to release the free space.
This is the part that confuses almost everyone. Outlook marks deleted blocks as reusable inside the file rather than handing the space back to Windows, so the file size on disk does not move.
Compacting rewrites the file without those gaps. On a large file it can take an hour or more, so start it when you do not need Outlook.
Use this when you have already deleted email and the file size has not moved.
Outlook also compacts in the background when it is idle and the file has more than about 20 percent wasted space. A file that has been open and untouched for a while may already be smaller than you expect.
Use this when the file is full of email you still want to keep.
Compacting recovers space from email you deleted. AutoArchive gets space back from email you want to keep, by moving anything older than a set age out into a separate archive PST file.
Archiving moves email, it does not copy it. After AutoArchive runs, the only copy of that email is in the archive file on your computer. Back that file up before you rely on it.
You can set a password on your PST file, but it does not encrypt anything. Microsoft's own specification says the format's encoding provides data obfuscation only.
Almost every guide online calls this "encrypting your PST file". That is wrong, and it matters. People store sensitive email in a file they believe is protected. Here is the honest split.
Stops someone opening the file in Outlook on a shared computer without the password. That is a genuine, if small, benefit.
It does not encrypt your email. Anything that reads the file directly, instead of going through Outlook, never sees the password at all.
"This file format specification uses two keyless cipher algorithms to encode the data blocks in the PST. These algorithms only provide data obfuscation and can be conveniently decoded once the exact encoding algorithm is understood. [...] In summary, the strength of the encoded PST data blocks provides no additional security beyond data obfuscation."Microsoft, [MS-PST] Strength of Encoded PST Data Blocks
There are two separate parts, and neither is real encryption. The password is stored as a CRC-32 hash. Outlook checks it on screen before showing you the folders. It is not a key and it encrypts nothing.
The block encoding, which Outlook calls Compressible Encryption or High Encryption, scrambles only the user data blocks, using a fixed algorithm with no key. Microsoft notes that the header, the allocation pages and the B-tree pages are stored plain. Anything that reads the file directly, instead of going through Outlook, walks past both.
Microsoft says it in plain language too. PST passwords "aren't intended to provide security against intentional malicious attempts to access your information."
Treat the password as a speed bump for a shared family computer, nothing more. Never treat it as protection for regulated or confidential email.
Microsoft ships no recovery tool and no back door, and support cannot reset it for you. There are two realistic routes. If the PST file is still open in a working Outlook profile, create a new Outlook Data File and drag everything across into it. The new file carries no password.
If it is not open anywhere, a third-party password removal tool is the only option left. Those work precisely because the password is a stored hash rather than an encryption key.
Encrypt the drive or folder the file sits in with BitLocker. Use a password-protected Windows account so the file cannot be read from another profile. Encrypt individual email messages that carry sensitive content. These work at a level the file format cannot reach, and none of them rely on the PST password.
A PST file is one large file that Outlook writes to constantly, so almost every cause of corruption comes down to something interrupting or duplicating those writes. SCANPST.EXE, the free Inbox Repair Tool that ships with Office, fixes most of the damage.
SCANPST.EXE in your Office install folder, such as C:\Program Files\Microsoft Office\root\Office16. Searching the drive for the filename finds it on any version.Seeing a specific code rather than a general failure? Our Outlook PST error fixes cover the common ones step by step. That includes a missing Outlook export download link and the Outlook 0x80040600 unknown error.
The PST file format is a binary B-tree database with 512-byte nodes. Every PST file starts with the four-byte signature !BDN, a four-byte CRC and the two-byte marker SM.
You do not need this section to use your PST file. It is here for two reasons. Microsoft published the format as an open specification, so the layout is documented rather than guessed at.
And the structure explains one thing that surprises people: one damaged block inside a big file can make the whole file unreadable. That is why repair tools work on the index and not on the email itself.
.pst!BDN + 4-byte CRC + SMapplication/vnd.ms-outlook-pst (in common use, not IANA registered)[MS-PST]Microsoft's specification describes it as three stacked layers, and knowing the names makes repair-tool output much easier to read.
Damage almost always starts at the NDB layer. That is why a single bad block stops the whole file rather than losing one email, and why SCANPST.EXE spends its passes rebuilding the node and block trees.
The PST file format comes in two versions, and the version sets how much the file can hold. ANSI is the old one, used by Outlook 2002 and everything before it. It stores text as extended ASCII with a codepage, so it cannot handle every character set, and it stops at 2 GB.
Unicode came in with Outlook 2003. It stores text as UTF-16 little endian and uses 64-bit pointers, which raised the ceiling more than twenty times.
A PST file keeps the format it was born with. A backup made in Outlook 2002 is still an ANSI file today, whichever Outlook opens it. Export its contents into a new Outlook Data File and it becomes Unicode, which removes the 2 GB wall.
.pst file. It appears as its own folder tree in the sidebar and your existing mailbox is left untouched. Double-clicking the file in File Explorer does not work.C:\Users\<username>\Documents\Outlook Files. The exact location depends on the Outlook version, the account type and the settings used. To confirm the real path, open Outlook and go to File, Account Settings, Account Settings, the Data Files tab, then Open File Location.Is Outlook refusing to open the PST file, or showing a specific code? Start with the matching guide in our Outlook PST import and export error guides. Not finding your problem? Send us the exact wording and we will add it.