How to Increase OST (Offline Outlook Data File) Size in Microsoft 365 Outlook

How to Increase OST (Offline Outlook Data File) Size in Microsoft 365 Outlook

Microsoft Outlook has a default limit on the OST file size, but you can increase it by modifying the registry settings. Follow these steps to adjust the OST file size limit.

Step 1: Check Current OST File Size

  1. Open Outlook.
  2. Click on File > Account Settings > Account Settings.
  3. Select your Microsoft 365 email account and click Change.
  4. Click More Settings > Advanced tab.
  5. Check the Cached Exchange Mode Settings to see how much mail is stored offline.

Step 2: Modify Registry to Increase OST Size

Warning: Modifying the Windows Registry incorrectly can cause system issues. Ensure you back up your registry before proceeding.

1. Open Registry Editor

  1. Press Win + R, type regedit, and press Enter.
  2. Navigate to:

HKEY_CURRENT_USER\Software\Microsoft\Office\XX.0\Outlook\PST

Replace XX.0 with your Outlook version:

    • Outlook 2021 / Microsoft 365 → 16.0
    • Outlook 2019 → 16.0
    • Outlook 2016 → 16.0
    • Outlook 2013 → 15.0
    • Outlook 2010 → 14.0

2. Increase OST File Size

  1. In the right pane, look for MaxLargeFileSize and WarnLargeFileSize.
  2. If they don’t exist, create them:
    • Right-click → New → DWORD (32-bit) Value.
    • Name it MaxLargeFileSize and set it to a decimal value (e.g., 102400 for 100GB).
    • Name it WarnLargeFileSize and set it lower than MaxLargeFileSize (e.g., 95232 for 95GB).

Suggested Values:

  • 50GB: MaxLargeFileSize = 51200, WarnLargeFileSize = 48640
  • 100GB: MaxLargeFileSize = 102400, WarnLargeFileSize = 95232
  1. Close Registry Editor and restart your computer.

Step 3: Ensure Cached Mode is Enabled

  1. Open Outlook.
  2. Go to File > Account Settings > Account Settings.
  3. Select your account and click Change.
  4. Check Use Cached Exchange Mode and adjust the slider to download more emails.

Step 4: Compact the OST File (Optional)

If your OST is large and causing issues, try compacting it:

  1. Go to File > Account Settings > Account Settings.
  2. Click Data Files and select your OST file.
  3. Click Settings > Advanced > Outlook Data File Settings > Compact Now.

Step 5: Optimize Outlook Performance

If Outlook is slow, try keeping fewer emails offline:

  1. Go to File > Account Settings > Account Settings.
  2. Select your Microsoft 365 account.
  3. Click More Settings > Advanced.
  4. Adjust the "Mail to keep offline" slider to a shorter period (e.g., 1 year instead of All).

Following these steps should increase the OST file size limit and improve Outlook’s performance. If you need further assistance, feel free to reach out! 

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