Office Settings & Configuration

How to Check Current and Maximum Outlook OST File Size

Adam Davis Oct 1, 2026 868 views

Question details

The user needs to verify their current OST file size in Microsoft Outlook and understand why the local file size is smaller than the maximum configured capacity of 50 GB or 100 GB.

How to Check the Current and Maximum Outlook OST File Size

Product: Microsoft Outlook
Device & OS: Windows
Scenario: Checking local OST file size and comparing it to Microsoft 365 mailbox storage limits.
Observed behavior: The OST file size shown in File Explorer is much smaller than the maximum permitted size because it only reflects the current local cache, not the total online mailbox capacity.

Before you start

Ensure Microsoft Outlook is fully closed before navigating to your local AppData folder to accurately check or modify OST file properties.

Solution 1Recommended

Check Current OST Size via File Explorer

Use Outlook's built-in settings to locate your active OST file and verify its actual disk usage on your PC.

The OST file size shown in File Explorer represents your current local mailbox cache. A file size of approximately 0.5 GB is completely normal if that is the amount of data currently synchronized.

Open Outlook Account Settings

Launch Microsoft Outlook, click on the 'File' tab in the top left corner, select 'Account Settings', and then click 'Account Settings' from the drop-down menu.

Locate the Data File

Switch to the 'Data Files' tab. Select your primary email account from the list and click the 'Open File Location...' button. This will open File Explorer directly to where your OST file is stored.

Check File Properties

Right-click the highlighted OST file and select 'Properties'. The 'Size on disk' value indicates your current OST consumption.

Check Current OST Size via File Explorer

Multiple OST Files: If you see multiple OST files in this folder, it is likely due to creating or reconfiguring a profile. Only the file actively linked in your Account Settings is in use.

Solution 2

Verify Maximum Size Limits in the Registry

Check the Windows Registry to see the configured maximum allowed size for your Outlook OST files.

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Frequently Asked Questions

Why does my OST file size not match my online mailbox storage value?

The OST file is a local cache of your mailbox. It only stores emails that have been synchronized to your computer, whereas the online Outlook storage value represents all mailbox data stored on the service servers.

Will changing the registry limits instantly increase my OST file size?

No, registry limits like MaxLargeFileSize only define the maximum allowable size for the cache. The actual OST file size will only grow naturally as more data is downloaded from the server.

What should I do if I have multiple OST files in my Outlook folder?

Multiple OST files often appear after reconfiguring a profile or adding new accounts. Only the one currently linked in your Account Settings is actively used by Outlook. You can safely ignore or backup and delete the older, unused ones.

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