How to Increase ost file size in Microsoft 365

Microsoft 365, formerly known as Office 365, is a powerful suite of productivity tools that millions of individuals and businesses rely on every day. One of the main features of Microsoft 365 is the use of offline Outlook Data Files (OST files), which allows users to access their mailbox data even without an active Internet connection. However, these OST files have size limits, and users may face problems if they exceed these limits. In this blog post, we will explore the effective ways to increase OST file size in Microsoft 365.

Understanding OST Files and Their Limitations

Before we get into the solutions, let’s understand OST files and their limitations. An OST file is a synchronized copy of a user’s mailbox stored on their local computer when using Microsoft Outlook with Microsoft 365. These files facilitate access to email, calendar items, contacts, and other mailbox data even when you are offline.

However, OST files are subject to size restrictions, which can lead to potential problems if a mailbox grows beyond a specified limit. The default maximum size for OST files in Outlook 2019 and Microsoft 365 is around 50 GB. When an OST file approaches this limit, users may encounter issues such as slow performance, synchronization issues, or complete failure to update.

Method 1. Over by Registry

Changing the necessary Windows registry entries, by following these steps.

  1. Click on the Windows Button, type regedit, and click on Registry Editor to open it.
ost file size increase
  1. Route to HKEY_Current_User\Software\Microsoft\Office\X.0\Outlook\PST
    Note: (‘X.0’ matches the Outlook Version ‘16.0‘ – 2016, 2019 and MS 365, ‘15.0‘ – 2013, ‘14.0‘ – 2010)

3. On the PST, Right-Click on the empty space in the right panel ” New > DWORD (32-bit) Value ” > type in MaxLargeFileSize > press Enter.

4. Click on MaxLargeFileSize, select Decimal and type value data: (102400 for 100 GB) in the Value Data and click OK.

Above Method will help to resolve your concern.

Method 2. Some more solutions for reduce the OST file size

Solution 1: Archiving Old Items

The simplest way to manage OST file size is by storing older and less frequently accessed items. Microsoft Outlook provides an archive feature that allows users to move old email, calendar items, and other data into a separate file called a PST (Personal Storage Table) file. By doing so, you can significantly reduce the OST file size and maintain better performance.

To archive items in Outlook:

  1. Open Windows MS Outlook and go to the “File” tab.
  2. Select “Options” and then click on “Advanced”.
  3. Under “AutoArchive”, click “AutoArchive Settings”.
  4. Set the desired archiving options and choose a location to save the archived PST file.

Solution 2: Compact OST Files

Compressing OST files is another effective way to free up space and reduce their size. The OST file may not automatically shrink when users delete items in their mailboxes or modify existing items. Compacting helps to reclaim unused space within the OST file, optimizes its size and performance.

To compress OST files:

  1. Quit Microsoft Outlook.
  2. Locate the OST file on your computer. The default location is usually: C:\Users\AppData\Local\Microsoft\Outlook.
  3. On the OST file Right-Click and select “Properties”.
  4. And in the “General” tab, click on the “Advanced” button.
  5. In the “Advanced Properties” window, click “Shrink Now”.

Solution 3: Disable or reduce Cached Exchange Mode

Cached Exchange Mode allows Outlook to store a local copy of a user’s mailbox on their computer, increasing performance and enabling offline access. However, this feature may contribute to the larger OST file size.

To disable or reduce Cached Exchange Mode:

  1. Open Windows MS Outlook and go to the “File” tab.
  2. Select “Account Settings” and select “Account Settings” again.
  3. In the “Email” tab, select your Microsoft 365 account and click “Change”.
  4. In the “Change Account” window, uncheck the “Use Cached Exchange Mode” option or adjust the “Keep mail offline” slider to a shorter time frame.

Check out the source Microsoft for more details about that.


Optimizing OST file size in Microsoft 365 is important to ensure smooth and efficient mailbox management. By adopting the methods mentioned above, you can increase OST file size, prevent performance issues and maintain a smooth user experience in Outlook.

Remember to regularly archive old items, compact OST files, and fine-tune the Cached Exchange Mode settings to keep your Microsoft 365 mailbox and OST files running efficiently. These steps will help you get the most out of the powerful features Microsoft 365 has to offer and ensure a productive work experience.

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