📖 What is a PST File?
A PST file is an Outlook data file that keeps a copy of your mailbox on your own computer. The letters stand for Personal Storage Table, and the file holds your email along with contacts, calendar events, tasks and notes, all bundled into one file with the .pst extension.
Think of a PST as a filing cabinet for Outlook that lives on your hard drive rather than on a mail server. When you archive old email, export a mailbox or set up an older account type, Outlook writes everything into a PST so you can keep it, move it or back it up on your own terms. Because it sits on your machine, a PST is something you can copy to another computer, hand to a colleague or store on a drive for safe keeping.
The most useful thing to understand is that a PST is a standalone archive. It does not stay in step with a mail server, which is exactly what sets it apart from the other Outlook data file you may have heard of, the OST.
Key Characteristics
- An Outlook data file made by Microsoft
- Stores a whole mailbox, not just email
- Lives on your computer, not a server
- Portable, so you can copy and move it
Good to Know
- Does not sync with a mail server
- Opens in Outlook on Windows
- Has a size limit that depends on its age
- Can be repaired with a built-in tool
⚡ Quick Facts
| File Extension | .pst |
| Full Name | Personal Storage Table |
| Developer | Microsoft |
| Opens In | Microsoft Outlook |
| Category | Outlook data file |
| Syncs to Server | No, it is a local archive |
| Formats | ANSI (older) and Unicode (modern) |
| Repair Tool | ScanPST (Inbox Repair Tool) |
🗂️ What a PST Stores
A PST is not only for email. Outlook keeps every kind of item you work with inside it, which is why a single file can carry years of your digital life.
Email Messages
Your inbox, sent items and every folder of mail, along with the attachments inside them.
Contacts
Your address book, with names, email addresses, phone numbers and other details.
Calendar Events
Appointments, meetings and reminders from your Outlook calendar.
Tasks and Notes
Your to-do items, notes and any journal entries you keep in Outlook.
Attachments
The files attached to your messages, stored right alongside the mail they belong to.
Folder Structure
The way your folders and subfolders are arranged, kept intact inside the file.
⚖️ PST vs OST
Outlook uses two kinds of data file, and telling them apart clears up most of the confusion around the .pst extension. They look similar but play very different roles.
A PST is a standalone archive that you create or export yourself, and it does not talk to a mail server. An OST, short for Offline Storage Table, is a copy that Outlook builds on its own when you connect an Exchange or Microsoft 365 account. The OST mirrors your mailbox on the server so Outlook keeps working when you are offline, and any change syncs back once you reconnect. That difference matters most when you try to move a file, since a PST travels well while an OST is tied to one account and profile and will not simply open on another machine.
| Trait | PST | OST |
|---|---|---|
| Full name | Personal Storage Table | Offline Storage Table |
| Created by | You, on purpose | Outlook, automatically |
| Syncs with server | ✗ No | ✓ Yes |
| Portable | ✓ Yes | ✗ Tied to a profile |
| Typical use | Archive, backup, export | Offline cache of a live account |
🔬 Inside the File
A PST is a single binary file that holds a small database of your mailbox. Opening it in a plain editor shows little of use, but its layout is easy to picture as a tree of folders.
How a PST is organised inside
Outlook Data File (.pst) ├─ Inbox messages and attachments ├─ Sent Items mail you have sent ├─ Contacts address book entries ├─ Calendar appointments and meetings ├─ Tasks to-do items and notes └─ Archive older folders you filed away
Inside, the file keeps a set of internal tables that act as an index to your folders and items, which is how Outlook finds a message quickly in a file that may hold many thousands of them. Microsoft publishes the format as an open specification known as MS-PST, but the internal structure is involved enough that reading a PST outside Outlook needs a purpose-built tool rather than a text editor.
🔢 ANSI and Unicode
Not every PST is the same under the hood. There are two generations of the format, and which one you have decides how large the file can grow before it runs into trouble.
Older versions of Outlook, from 2002 and earlier, wrote PST files in the ANSI format, which is capped at around 2 GB and becomes fragile as it nears that ceiling. Outlook 2003 introduced the Unicode format, which lifts the limit to 20 GB or more and is far sturdier. Almost every PST made in the last two decades is Unicode, but a very old archive may still be an ANSI file, and if it is close to 2 GB that alone can be the cause of errors. Converting an old ANSI file to Unicode, or splitting it, removes that particular ceiling.
📂 How to Open a PST File
How you open a PST depends on whether you have Outlook to hand. With it, the file slots straight in; without it, you need a viewer or a migration tool.
Open in Outlook
Go to File, then Open and Export, then Open Outlook Data File, and pick your .pst. It appears in the folder pane.
Use a PST viewer
A standalone PST viewer reads the mailbox without Outlook, which is handy on a machine that has no Outlook installed.
Migrate the data
A migration tool can move a PST into another mail service, such as Gmail or Microsoft 365, importing its contents.
In Outlook the file loads as an extra set of folders next to your live account, so you can read, search and move items freely. On a computer without Outlook, a dedicated PST viewer opens the file for reading, and a migration tool is the route when you want the mail to end up in another mailbox rather than just to look at it. Which path fits depends on whether you need to browse the archive or bring it into a working account. It helps to know the difference between opening and importing a PST. Opening it adds the file as a separate set of folders that sits alongside your mailbox, leaving your existing folders untouched. Importing it, through File then Open and Export then Import and Export, merges the messages into your current folders instead. Opening is the safer choice when you only want to read an archive, while importing suits moving old mail into your live account.
📍 Where PST Files Are Stored
A common question is simply where Outlook keeps a PST on the disk, often because someone wants to back it up or move it to a new computer. The answer depends on the Outlook version, but a couple of folders cover most cases.
Typical PST locations on Windows
# PST files you create or export usually sit here C:\Users\<you>\Documents\Outlook Files\ # auto-created data files often live here (hidden folder) C:\Users\<you>\AppData\Local\Microsoft\Outlook\
The surest way to find the exact path is inside Outlook itself, through File then Account Settings then Data Files, which lists every PST in use and can open its folder for you. One catch is that the AppData folder is hidden by default, so you may need to turn on hidden items in File Explorer before you can browse to it. If you plan to move a PST, close Outlook first and copy rather than cut, since Outlook expects to find the file where it left it.
📈 Size Limits and Compacting
PST files have a habit of growing, and past a point that growth causes problems of its own. Two behaviours are worth knowing so a large archive does not catch you out.
The first is the size limit covered above, where a modern Unicode PST can reach 20 GB or more, and when it hits its ceiling Outlook stops saving new mail to it and warns that the file is full. The second is a quirk of how the file manages space. When you delete items, a PST does not shrink on its own; it holds on to the freed space and reuses it later, so the file stays large even after a big cleanup. To reclaim that space you run the Compact Now option in Outlook, which rebuilds the file smaller. For an archive that keeps swelling, the lasting fix is to move older mail into a separate PST, delete what you do not need and compact what remains.
🔧 Repairing a Corrupt PST
A corrupt PST is one of the more alarming things an Outlook user meets, often showing up as an error that the file is not a personal folders file, or Outlook freezing as it tries to open. The good news is that Microsoft ships a repair tool for exactly this.
The built-in fix is the Inbox Repair Tool, known by its filename ScanPST.exe, which comes with every copy of Outlook. You close Outlook, point the tool at the troubled file and let it scan and rebuild the internal tables. It handles minor damage well and is free. Two cautions are worth heeding. Always work on a backup copy, since the tool changes the file as it repairs and a bad run can lose data. It also has its limits, since ScanPST mends the file structure but not permanently deleted messages, and very severe damage may need a specialist recovery tool instead. Running it more than once is normal, as each pass can clear a further layer of errors.
Back up first
Copy the .pst somewhere safe before you touch it, so you always have the original to fall back on.
Run ScanPST
Close Outlook, open the Inbox Repair Tool, point it at the file and let it scan and repair.
Repeat if needed
Run it again for stubborn damage, and turn to a specialist tool if the file is badly broken.
🔄 Converting a PST File
People convert PST files for two main reasons, either to read the mail in a different program or to move a whole mailbox to another service. The right approach depends on where the email needs to end up.
To keep the messages as ordinary files, a converter can turn a PST into formats such as PDF for a fixed record, or MBOX and EML, which many other mail programs read. To move a mailbox wholesale, a migration tool imports the PST straight into a service like Gmail or Microsoft 365, folders and all. A closely related job is converting an OST to a PST, since an OST cannot be moved on its own; turning it into a PST first makes the mail portable. In each case the work is done by a dedicated PST tool rather than by Outlook alone.
🏛️ A Short History
The PST format has been around almost as long as Outlook itself. Microsoft introduced it with Outlook 97, in the mid-1990s, as the place to keep mail on a local machine.
For its first years the format was ANSI based, with the roughly 2 GB ceiling that later caused so much grief for heavy email users. The big change came with Outlook 2003, which brought the Unicode format and its far larger capacity. In 2010 Microsoft published the format as an open specification, MS-PST, so other software could work with PST files in a documented way. Through all of this the .pst extension stayed put, which is why a file made today shares its name with archives created a quarter of a century ago.
🛠️ Common Issues
Most trouble with PST files falls into a handful of familiar patterns, and each points to a clear next step.
| Not a personal folders file | A classic sign of corruption. Back up the file, then run the Inbox Repair Tool to rebuild it. |
| File has reached its limit | The PST is full. Archive older mail to a new file, delete what you can and compact to reclaim space. |
| File stays large after cleanup | A PST reuses freed space. Run Compact Now in Outlook to shrink it on disk. |
| Cannot open on another PC | Check it really is a PST and not an OST, since an OST is tied to one profile and will not travel. |
| Password will not protect it | PST password protection is weak and easily bypassed. For real security, encrypt the file or the drive instead. |
❓ Frequently Asked Questions
A PST file is a Microsoft Outlook data file that stores a copy of your mailbox on your own computer. PST stands for Personal Storage Table, and the file holds email messages along with contacts, calendar events, tasks and notes, all under the .pst extension. It is a standalone archive that lives on your machine rather than on a mail server, which is what makes it portable enough to copy, move or back up. Outlook creates one when you archive or export mail, or for certain older account types.
In Outlook, go to File, then Open and Export, then Open Outlook Data File, and select your .pst. It shows up as an extra set of folders in the folder pane, where you can read and search the mail. If you do not have Outlook on the computer, a standalone PST viewer opens the file for reading instead. To move the mail into another service such as Gmail or Microsoft 365, you use a migration tool rather than a viewer, since that imports the contents into a working mailbox.
A PST is a standalone archive you create or export yourself, and it does not sync with a mail server. An OST, or Offline Storage Table, is a copy that Outlook builds automatically for an Exchange or Microsoft 365 account, mirroring the server mailbox so you can work offline, with changes syncing back when you reconnect. The practical difference is portability. A PST moves easily between computers, while an OST is tied to one account and profile and cannot simply be opened on another machine.
Use the Inbox Repair Tool, known by its filename ScanPST.exe, which comes with Outlook. Close Outlook, point the tool at the file and let it scan and rebuild the internal tables. Always make a backup copy of the PST first, because the tool changes the file as it repairs and a bad run could lose data. ScanPST fixes the file structure but not permanently deleted messages, and it is normal to run it more than once. For very severe damage, a specialist recovery tool may be needed instead.
A PST does not shrink on its own when you delete messages. Instead it holds on to the freed space and reuses it later, so the file can stay large even after a big cleanup. To actually reduce its size on disk, run the Compact Now option in Outlook, which rebuilds the file smaller. If the archive keeps growing, the lasting fix is to move older mail into a separate PST, delete what you no longer need and then compact what remains.
It depends on which format the file uses. Older ANSI PST files, from Outlook 2002 and earlier, are capped at around 2 GB and become fragile as they approach it. Modern Unicode PST files, from Outlook 2003 onward, support 20 GB or more. Almost every PST made in the last two decades is Unicode, so the higher limit applies. When a PST reaches its ceiling, Outlook stops saving new mail to it and warns that the file is full, at which point archiving or splitting the file is the way forward.
Yes, with the right tool. A standalone PST viewer reads the mailbox and lets you browse and search the messages without Outlook installed, which is useful for checking an archive on a computer that does not have Outlook. If your aim is to move the mail rather than just read it, a migration tool imports the PST into another service such as Gmail or Microsoft 365. What you cannot do is open a PST properly in an ordinary text editor, since it is a binary database that needs a purpose-built reader.
A PST converter can turn the file into other formats depending on what you need. To keep the messages as ordinary files, it can produce PDF for a fixed record or MBOX and EML, which many other mail programs read. To move a whole mailbox, a migration tool imports the PST into a service such as Gmail or Microsoft 365. A closely related task is converting an OST to a PST, which makes an otherwise tied offline file portable. Two target formats come up often. MBOX is the open mailbox format that Thunderbird and Apple Mail read, so converting a PST to MBOX is the usual route to open Outlook mail on a Mac or on Linux, while EML keeps each message as a separate file that almost any mail program can read. In each case a dedicated tool does the work rather than Outlook by itself.
On Windows, PST files you create or export usually sit in your Documents folder under Outlook Files, while data files Outlook makes on its own often live in the hidden AppData Local Microsoft Outlook folder. The surest way to find the exact path is inside Outlook, through File then Account Settings then Data Files, which lists every PST in use and can open its folder for you. Because the AppData folder is hidden by default, you may need to turn on hidden items in File Explorer before you can browse to it.
Opening a PST adds it to Outlook as a separate set of folders that sits alongside your mailbox, leaving your own folders untouched, which is ideal when you just want to read an archive. Importing a PST, through File then Open and Export then Import and Export, merges its messages into your existing folders instead, which suits moving old mail into your live account. So opening keeps the archive apart, while importing blends it in. Choose opening to browse, and importing to combine.
Outlook lets you set a password on a PST through the data file properties, but that password is weak protection rather than real security. It is not proper encryption, and freely available tools can bypass it, so you should not rely on it to guard sensitive mail. If a PST holds confidential data, a far stronger approach is to encrypt the file itself or the whole drive, for example with full-disk encryption, and to keep backups in a secure place. Treat the built-in password as a mild deterrent at most.
An OST is tied to one Outlook profile and cannot simply be moved, so turning it into a PST is how you make that mail portable. If Outlook still opens the account, the simplest route is to export the mailbox to a PST through File then Open and Export then Import and Export then Export to a file. If the account has been removed and the OST is orphaned, a dedicated OST to PST converter reads the offline file and writes out a standalone PST you can open anywhere. Once it is a PST, the mail moves between computers freely.
PST is Microsoft Outlook's own mailbox format, while MBOX is an open format that many other mail programs use, including Thunderbird and Apple Mail. A PST holds email plus contacts, calendar and tasks in one Outlook data file, whereas MBOX stores plain email messages in a widely readable form. Because MBOX is not tied to Outlook, converting a PST to MBOX is the usual way to open Outlook mail on a Mac or on Linux. Which you want depends on the program that needs to read the mail.
📝 Summary
A PST file is a Microsoft Outlook data file that keeps a copy of your mailbox on your own computer, holding email, contacts, calendar events, tasks and notes in a single file under the .pst extension. Its defining trait is that it is a standalone, portable archive that does not sync with a mail server, which sets it apart from an OST, the offline copy Outlook builds automatically for an Exchange or Microsoft 365 account and ties to one profile. Under the surface a PST is a small binary database indexed by internal tables, published by Microsoft as the open MS-PST specification, and it comes in two generations, the older ANSI format capped near 2 GB and the Unicode format from Outlook 2003 that reaches 20 GB or more. You open a PST in Outlook through File then Open and Export, or with a standalone viewer when Outlook is not available, and you move it into another service with a migration tool. Two habits keep a PST healthy. Because deleting mail does not shrink the file, you run Compact Now to reclaim space, and because a PST can turn corrupt, you repair it with the built-in ScanPST tool, always on a backup copy. Introduced with Outlook 97, the format has carried the .pst extension for a quarter of a century.