How to Reduce The Size of an Outlook Data File (.pst)

Posted on by Summa Lai

The more email you send and receive, the larger your Outlook mailbox can grow. Large mailboxes can take longer to open and search, and many email providers, including Microsoft 365, limit the maximum size of your mailbox. If you receive a message that your mailbox is over its size limit, deleting messages and folders can help, but if you store your information in an Outlook Data File (.pst) or Offline Outlook Data File (.ost), that data file may not shrink in size automatically.

Note: If you use a POP account, all of your email account information, including your folders, email messages, contacts, calendar, and task information is stored in an Outlook Data File (.pst). If you use an Microsoft 365, Exchange, IMAP, or Outlook.com account, a copy of your mailbox is stored in an Offline Outlook Data File (.ost).

To reduce the size of your Outlook Data File (.pst) or Offline Outlook Data File (.ost), you can manually compact the data file. Select your version of Outlook below to see instructions on how to reduce the size of a .pst or .ost file. 

For Outlook 2013, 2016, 2019 and 365

If you delete items from an Outlook Data File (.pst) or Offline Outlook Data File (.ost), Outlook automatically compacts the file in the background during idle time, when you’re not using your computer, but Outlook is still running.

Note:Outlook only compacts the file if there is more than 20% of ‘white space’ (unused space) in the file.

You can also use the following steps to manually start the compact process immediately, which may take several minutes to complete.

  1. Delete any items that you do not want to keep and empty the Deleted Items folder.
  2. Click File >  Account Settings > Account Settings.
  3. On the Data Files tab, click the data file that you want to compact, and then click Settings.

    Note: If you are using an Exchange or Outlook.com email account, you need to additionally click the Advanced tab > Outlook Data File Settings.
  4. In the Outlook Data File dialog, click Compact Now and then click OK.

Outlook 2010

  1. Click the File tab.
  2. Click Account Settings, and then click Account Settings.
  3. On the Data Files tab, click the data file that you want to compact, and then click Settings.
  4. Click Compact Now.

Outlook 2007

On the File menu, click Data File Management.

Click the data file that you want to compact, and then click Settings.

Click Compact Now.

Ref: https://support.microsoft.com/en-us/office/reduce-the-size-of-your-mailbox-and-outlook-data-files-pst-and-ost-e4c6a4f1-d39c-47dc-a4fa-abe96dc8c7ef#ID0EAABAAA=Office_2007