A PST file is Outlook's way of storing your email, calendar, and contacts on your computer

PST stands for Personal Storage Table. It's a single file that holds copies of your messages, calendar events, contact lists, and notes — everything you see in Outlook. When you set up Outlook to work with a local email account (rather than connecting to Microsoft 365 or an Exchange server), Outlook creates a PST file and saves it somewhere on your hard drive, usually in a folder called AppData on Windows or Library on Mac.

Think of it like a filing cabinet. Instead of your email living on a company server somewhere, it lives in this one file on your machine. You can open it, search through it, and back it up. The tradeoff is that if your computer crashes and you haven't backed up that file, your email is gone.

Key Takeaways

  • A PST file is a single container that stores all your Outlook email, calendar events, contacts, and notes on your computer's hard drive.
  • PST files are created automatically when you add a local email account to Outlook, and Outlook manages them without you having to think about it.
  • You can export messages or folders from Outlook into a new PST file to back them up or move them to another computer.
  • PST files have a size limit of about 50 GB, so very large mailboxes may need to be split across multiple files or archived.

Where Outlook stores your PST file

On Windows, Outlook typically saves PST files in C:\Users\[Your Username]\AppData\Local\Microsoft\Outlook. On Mac, they go into ~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile or a similar path depending on your Outlook version. You don't usually need to know this location — Outlook finds the file automatically when you open the program.

If you've been using Outlook for years, you might have multiple PST files. Some people create separate ones for different projects, years, or purposes. You can see all the PST files Outlook knows about by going to File > Open & Export > Open Outlook Data File in Outlook on Windows, or File > Open in Outlook on Mac.

The difference between PST files and cloud-based email

If you use Microsoft 365, Gmail, or any other cloud email service, your messages live on the company's servers, not on your computer. Outlook downloads copies to your machine so you can read them offline, but the originals stay in the cloud. With a PST file, the messages live only on your computer — there's no automatic backup on a server somewhere.

This matters for two reasons. First, if your hard drive fails, you lose everything unless you've manually backed up the PST file. Second, you can only access your email from the computer where the PST file lives. If you want to check your mail from your phone or a different computer, you're out of luck unless you've set up cloud sync or moved the file.

How to back up your PST file

The simplest way is to copy the PST file itself to an external drive or cloud storage like OneDrive or Google Drive. Close Outlook first — you can't reliably copy a PST file while Outlook is using it. Then navigate to the folder where your PST file lives and drag it to your backup location.

Outlook also has a built-in export function. Open Outlook, go to File > Open & Export > Import/Export (Windows) or File > Export (Mac). Choose "Export to a file," select PST format, pick the folder or mailbox you want to save, and choose where to put the new file. This creates a copy without touching the original, so it's safe even if Outlook is running.

PST file size limits and when they matter

A PST file can grow to about 50 GB before Outlook starts having trouble searching and opening it. If you've been using the same Outlook account for 10 or 15 years and never delete anything, you might hit this limit. When you do, Outlook slows down noticeably and may refuse to add new messages until you clean things up.

If your PST file is getting large, you have a few options. You can delete old messages you don't need anymore. You can create a new PST file and move older messages into it, keeping your main file smaller and faster. Or you can switch to a cloud-based email account like Microsoft 365, which doesn't have the same size restrictions and lets you access your mail from anywhere.

Opening a PST file on a different computer

You can move a PST file to another computer and open it in Outlook there. Copy the file to the new machine, then in Outlook go to File > Open & Export > Open Outlook Data File (Windows) or File > Open (Mac). Navigate to the PST file and select it. Outlook will add it to your list of data files and you can browse everything inside.

This is useful if you're switching computers or need to access old email archives. The PST file itself doesn't change — it's just a container of data that Outlook knows how to read. You can open the same PST file in Outlook on Windows, Mac, or even older versions of Outlook, though very old PST files sometimes need to be converted to work with newer versions.

Why you might see multiple PST files

Outlook can manage more than one PST file at the same time. Some people create separate files for different purposes — one for work, one for personal mail, one for archived messages from years ago. Each one shows up as a separate folder in your Outlook sidebar, and you can switch between them.

If you've imported email from another program or migrated from an older computer, you might have inherited extra PST files. You can delete the ones you don't need anymore by right-clicking them in Outlook and choosing "Close" or "Remove," though this just removes them from Outlook's view — it doesn't erase the file from your hard drive unless you manually delete it.

Frequently Asked Questions

Can I open a PST file without Outlook?

Not easily. PST files are proprietary to Outlook, and most other email programs don't read them. If you need to access messages in a PST file but don't have Outlook, you'd need to install Outlook or use a third-party tool designed to read PST files. Microsoft also offers a free Outlook Data File Viewer for Windows that lets you browse a PST file without the full Outlook program.

What happens if my PST file gets corrupted?

Outlook may refuse to open it or show an error message. Windows includes a built-in repair tool called Scanpst.exe that can sometimes fix minor corruption. On Mac, Outlook has a similar tool called Rebuild. If the file is severely damaged and repair tools don't work, you may need to restore from a backup or use a professional data recovery service.

Can I convert a PST file to another format?

Yes. Outlook can export a PST file to other formats like CSV (for contacts) or ICS (for calendar events). Go to File > Open & Export > Import/Export and choose the format you want. You can also export to a new PST file with a different name or location, which is useful for splitting large files or creating backups.

Do I need to do anything special if I'm switching from Outlook to Gmail or another email service?

You can export your PST file to CSV or ICS format and import it into the new service, though the process varies depending on what you're switching to. Gmail, for example, can import messages from a PST file if you use a third-party tool or Google's migration service. Contact the new service's support team for specific steps.