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! 

    • Related Articles

    • Migrate contacts out of Microsoft Outlook

      To migrate contacts out of outlook as as a .csv file, follow these instructions. The file can then be uploaded to other platforms. If you are exporting from Office365 on the web, you can do so using the following steps: 1) To start, login to Office ...

    • How to access a shared mailbox in Microsoft

      Shared mailboxes are a great way to create a shared space to receive emails without taking up additional user licences. Steps to Access the Shared Mailbox from your individual email: Log in to your primary Outlook Web Access (OWA) email account by ...

    • Add a calendar in Outlook 365

      To add a calendar in Outlook 365, follow these steps: Open Outlook 365 and sign in to your account. Click on the "Calendar" button in the bottom-left corner of the window. This will open your calendar view. Click on the "New Calendar" button in the ...

    • Migrating from Chrome to Edge

      You might be in a situation where you'd just like to have all of you favorites or auto-fills moved over from one browser to another. Some reasons may be that you're more interested in using Edge now because of that handy little AI that all the cool ...

    • Should I buy older PCs for my business from a large chain?

      The short answer is, "NO." You may be tempted to buy clearance PCs from large stores like Harvey Norman, JB Hi-Fi, or The Good Guys whilst they're on sale or for the peace of mind that comes with having a warranty backed by a large established chain. ...

    • Popular Articles

    • Email deliverability

      If emails are not arriving in your inbox from your bulk email platform you need to consider making a few changes. Over the years email has become a widely used marketing tool. It has one of the lowest costs of all marketing channels. Unfortunately, ...

    • Migrating from Chrome to Edge

      You might be in a situation where you'd just like to have all of you favorites or auto-fills moved over from one browser to another. Some reasons may be that you're more interested in using Edge now because of that handy little AI that all the cool ...

    • Dev as a Service

      CRM Development, or any software development, can be a large project. Often, when creating software or implementing projects, the underlying business case changes over time. We find that an Agile development process is the best way to deliver real ...

    • Tag your Zoho Partner

      To allow us to provide you with support we need to add your account to our partner portal. If you are reading this, it is likely that you would like to tag us as your Zoho partner. Tagging us as your partner allows us to complete support requests on ...

    • Relevate People Recruitment FAQs

      Welcome to Relevate People, This article will guide you through some common questions about working with us: FAQs Am I working for Relevate People? Relevate People is a Technical Virtual Staffing organisation. We provide the facilities for our ...