Skip to content

Outlook PST - One Name - Two file formats

Why there are two Outlook file formats, both called .PST and how convert to the better one.

Outlook stores all your email, appointments etc in one big file with the extension .PST with the exception of those connected to Exchange Server machines when it’s called an .OST file.

With Outlook 2003 Microsoft updated the PST file to cover two major limitations that have been around since Outlook was created.

The original PST format used the ANSI limited character set which is fine for those of use English-only speakers but not good for a program with supposedly global reach. The new PST format available in Outlook 2003 changed this to Unicode character sets. That’s a side note in this context.

The important change was a vast expansion of the maximum possible size of a PST file. The old format choked just shy of 2GB, which may seem like a lot to some people but was being pushed by a significant number of heavy duty users (like many on the Office Watch team).

The new PST format has a theoretical maximum size of 33TB – that’s 33 TERAbytes or over 33,700 gigabytes. It’s theoretical because there are no hard drives that large … yet. Suffice it to say, we won’t have to worry about the maximum size of a PST/OST file for a while.

If you create a new profile in Outlook the PST file will be in the new format but if you migrate from an earlier version of Outlook it’ll continue to use the old PST file with the same size limitation.


WHAT TYPE OF PST YOU HAVE

To work out which type of PST file you’re running with Outlook 2003 go to Control Panel | Mail | Data Files to see a list. Click on the PST file for your main account/profile then Settings. You’ll see one of two types:



  • “Personal Folders File (97-2002) ” these are the older 2GB maximum files
  • “Personal Folders File ” are the newer mammoth version PST files.

If you’re connected to an Exchange Server system you’ll have an OST file. To work out which type you have go to Tools | Email Accounts | View or change | select the Exchange Server option then Change | More Settings | Advanced and look at the Mailbox Mode. If it says ‘Outlook is running in Unicode mode’ then you have a new style OST file.

There’s nothing wrong with the older PST/OST files but sooner or later you’ll probably get near the 2GB limit and it’s better to switch over before you have problems.

Of course it would have been nice if Microsoft had made this more obvious with a different file extension and a conversion utility but instead they chose to slip-stream this change in hoping no-one notices the problems … which brings us to …

CAN’T READ ‘NEW’ PST’S IN THE OLDER VERSIONS OF OUTLOOK.

If you have trouble reading a PST file with a pre-2003 version of Outlook it’s still most likely a corrupted PST file.

But consider the possibility that the PST has been created on Outlook 2003 and uses the newer format. Despite having the same file extension they are different formats and earlier versions of Outlook don’t know what to do with them.


HOW LARGE ARE YOUR CURRENT PST / FOLDERS?

So how big is your current PST file?

PST/OST files can be anywhere but are normally stored in Documents and Settings//Application Data/Microsoft/Outlook . Application Data is a hidden folder so you may have to tweak Explorer to make them visible.

Alternatively use Windows Search to find *.?ST files including in your search the hidden and system folders.

You can also right-click on any folder and choose Properties then Folder Size to see the totals for the folders and sub-folders.

MIGRATING FROM NEW TO OLD PST FILES.

There’s no elegant conversion option from old to new PST formats. Your only choice is to make a new data file or profile then move all your data from the old folders to the new ones.

Before you start make sure you have plenty of disk space. You should have at the very least free disk space equal to the size of the current PST file. In practice you should never let your hard drive get too full, so make sure you have free disk space that is at least 110% greater than the size of the PST. Other words of wisdom include ensuring that your hard drives never exceed 90% used space.

Go to File | Data File Management | Add then make sure you choose ‘Office Outlook Personal Folders File’ not the ‘Outlook 97-2002’ option. Give the PST an obvious name, a password (optional) and remember where you save it.

The new PST should appear in your All Folders list with no sub-folders.

You can then select folders from your current PST and copy them to the new location. I suggest copying, not moving, so you can recover if something goes wrong. Copying will take a long time and take up a lot of the computer’s resources so don’t expect to get much else done on that machine in the meantime.

You’d use a similar process to migrate an old AutoArchive store to one using the new format PST file. That would allow you to expand your AutoArchive size into the foreseeable future without worrying about hitting the 2GB limit.

Another option is to create a new Outlook profile and recreate your email settings, making sure that you create a new PST file in the new style. Then open Outlook with the new profile, then open the old PST file and copy your old data across.

Either way it’s a long and messy process that is shouting out for a tool from Microsoft – but those shouts have met with no response. Most people are probably running with old style PST files and unaware that they’ll hit a size limit at some stage.

About this author