Have an Oversized OST File? Know How to Reduce an Oversized OST file in Outlook

You might be wondering why your computer is limping to process a large OST file while it can smoothly run a 100 GB game at a full HD resolution. It is because a Large OST file or an oversized OST file might require a lot of bandwidth to sync with the server that holds your OST file. So, if you ever encounter any error related to an oversized OST file you find all the information on how to fix it in this article.

Why is My OST File so Big?

OST file stores all the Outlook data that is accumulated from the point you created your account. Additionally, it also stores all the attachments, contacts. Generally, it might not sound like a lot but, in the long run, slowly and steadily, OST file grows size.

Why are Large OST files are Such a Huge Problem?

Firstly, an OST or Offline storage table file works on the principle of synchronization. That means you can work on your Outlook even while you are offline and later sync with the server when the internet connection is available.

Since the working of an OST file is entirely dependent on synchronization, the larger the OST file, the longer it takes to sync with the online server and result in Outlook sync errors.

It doesn’t stop there, Outlook keeps performing regular data integrity checks so once again, the larger the OST longer it takes to check, sometimes it might just get stuck in verifying the integrity otherwise known as Outlook stuck at verifying data integrity error. This is why a Large OST file which sounds mild can become highly complicated to handle.

How Big can an OST File Be?

Version OST File Size Limit
Outlook 2007, 2003 20 GB
Outlook 2010, 2013, 2016 50GB


Note: Whenever you are fixing any error, a smart step you can take is to archive the OST file contents. Because, when you are working to fix any Outlook related errors, might damage the OST file. The instant and effortless way to backup the OST file is by using Remo Outlook Backup and Migrate, a one-stop tool for all your Outlook related issues. With the help of Remo, you can Backup, Restore, and even Migrate the OST file.

Fortunately, you do not need to stress yourself on how to fix the oversized OST file, Outlook has an inbuilt utility that will help you on reducing the size of the OST file which is known as the “compact now” option.

How do I reduce the size of my OST file?

In order to reduce an oversized .ost file one can follow the below mentioned steps:

Step 1 : Go to the File tab. 

Step 2 : Then click on Account Settings.

Step 3 : Select  Data File option.

reduce .ost file size

Step 4 : Double click on the Data File option which opens Outlook Data file settings dialogue box with the option Compact Now. 

Step 5 : Click on compact Now to reduce the size of the OST file.

Why is My Mailbox Larger than an OST file?

Mailbox is larger than OST files due to inconsistencies in the calculation. In Outlook 2007 and earlier versions of Outlook there were inconsistencies in size calculations and file structure which made Mailbox 20% to 30% larger in size than corresponding OST files. In case of  Outlook versions above 2010, Outlook size calculation is more accounted for. The latest Outlook uses compression technology which auto-compresses text. Which again made the OST file smaller than the corresponding mailbox.

How do I fix OST has reached maximum size?

Looking for ways to compress .ost file in Outlook or reduce .oversized .ost file size. One can follow the below-mentioned information to shrink .ost file size that has reached the maximum limit.

Extra Info: If you are facing an error in Outlook saying "The Outlook data file has reached the maximum size". It is because of an oversized PST file, know how to deal with a large PST file that has reached its maximum size.

Does Archiving Reduce OST Size?

Yes, regularly archiving the OST or Outlook data file will create a backup that relatively reduces the OST file size on the server. Which is also going to regulate the OST file size from reaching the limit. You can archive the file using the Cleanup option to avoid oversized .ost files. 

Using Clean up Option to Avoid Oversized OST files.

Outlook also has an option that will allow you to clean all the unnecessary files from the trash in a single click. Since, the OST file also stores the deleted items, not cleaning the deleted items pile up and result in a large OST file. To archive the file follow the below mentioned steps:

Step1 : Go to the File tab.

Step2 : Click on Cleanup option and then select Mailbox cleanup 

shrink .ost using Cleanup option

Step3 : Then select Empty and delete the items from the deleted items folder to clear all the deleted items on a single click.

Alternate Method to Avoid Oversized OST files

Step1 : Go to Cleanup options.

Step2 : Then click on Mailbox cleanup wizard which is a dedicated wizard that has all the options that will help in managing redundant data existing in your mailbox.

However, these are temporary remedies, they cannot stop the large OST error to repeat. Fortunately, there are a few adjustments you can implement on Outlook that will avoid the same error happening again in the future.

How to Use Auto Archive to Avoid Oversized OST file

The auto-archive is a comprehensive tool that can be really handy to extensive Outlook users. It will streamline the regular archiving without any intervention. The following are the instructions on how to use Auto archive to avoid oversized .ost file:

Step1 : Go to Cleanup option.

Step2 : Click on Cleanup option and then select Mailbox clean up 

Step3 : You will get an option AutoArchive. Select this option to move older items in the mailbox to the archive folder.

How do I compress an OST file in Outlook?

You can compress .ost file in outlook in order to shrink the Outlook ost file. Follow the below-mentioned steps to compact Outlook .ost file:

Step 1 : Delete items in your Mailbox which is not required and then select the folder which is to be compacted and click on the list view option in the navigation pane.

Step 2 : Right Click on the folder and then click on Delete items.
Step 3 : Now, click on Empty folder option

Step 4 : Click on File tab and click on Account Settings.

Step 5 : Select DataFiles tab and select the file to be compacted.

Step 6 :Now click on settings and then go to Advanced tab.

Step 7 : Then click on Outlook Data File Settings and click on Compact Now option in order to compact the file.

Note: If you are looking for ways to increase Outlook file size you can refer to the above mentioned link.


Above mentioned are some methods which can help you to deal with an oversized .ost file. This article will be further updated as and when we encounter any new updates with regards to compact Outlook .ost file. Suppose if you are encountering problems that are not mentioned above do let us know in the comment section below and let us know if this article was of any help to you.

Have an Oversized OST File? Know How to Reduce an Oversized OST file in Outlook was last modified: June 21st, 2021 by Tony Landry

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