Managing Outlook OST File Size: Solutions for Dealing with Large Files

Updated On - January 17, 2024 by Armaan | Reading Time: 7 minutes

Outlook’s OST file is an offline storage archive replicating Exchange Server mailboxes. It stores data locally and automatically updates itself anytime whenever the user is connected to the internet. Utilizing this data file allows messages to be viewed and deleted even when one is not connected to the internet. All time Outlook is connected to the internet, any changes made to the local storage file will be synchronized with the server. People frequently continue communicating for years without ever upgrading their inboxes. Data sizes in offline storage eventually exceed available space. This results in malfunctions and freezes the Outlook email client and may cause OST file corruption. Data saved in a storage file requires prompt action to reduce the likelihood of failure. As a result, this article covered a few methods to reduce OST file size in Outlook.

Regular Maintenance to Reduce OST file size in Outlook

Emails, contacts, calendars, notes, journals, tasks, and so on can all be store in one convenient place by using an OST file. All of the mailbox information makes up the bulk of the OST file. By keeping up with mailbox cleaning, you can prevent the offline storage quota from being exceeded. Thus, the potential for corruption is minimize. Checking OST file size can be done in a few different ways.

  • Frequently deleting old messages and other junk from the inbox.
  • Clear up your trash and spam folders, so your offline folder file doesn’t get too big (.ost).
  • When erasing mail from a mailbox, you should always choose the “hard delete” option.
  • It’s important to condense your mailbox folders regularly.

Reasons for the Outlook OST File Being Too Big

  • Large Attachments: One of the most common reasons for the Outlook OST file size being too big is large attachments. Whenever a user sends or receives an email with large attachments, it gets store in the OST file. Over time, these attachments can accumulate and result in a large OST file.
  • Email Size: Another reason for a large OST file size is the size of emails. When a user receives an email with a large file size, it gets stored in the OST file. If the user receives a lot of such emails, the OST file size can quickly become very large.
  • Deleted Items: Deleted items in Outlook are not immediately removed from the OST file. Instead, they are moved to a special folder called the Deleted Items folder. If this folder is not emptied regularly, it can result in a large OST file size.

Manual Solutions to Reduce OST file Size in Outlook

Overly large OST files cause problems in Outlook, including:

  • The inability to open or load the associated.ost document.
  • Cannot append new information to an already-created Exchange. Ost file.
  • Interruptions in Microsoft Outlook.
  • Messages of failure during the synchronization procedure.
  • Inability to resolve problems with OST file synchronization with Microsoft Exchange Server.

Solution Procedures for “Oversized” OST File Errors

Simple Tasks: To manually remove unnecessary or unwanted information from the OST file, follow these key actions or methods.

  • Clear up your OST inboxes of outdated messages for good!
  • Make sure the Deleted Items folder in the OST mailbox is empty
  • Move the OST file from its default location
  • Rename the file; let it be generated again if it contains important or huge attachments.

Inactivating all plug-ins:

Several Outlook add-ons have been shown to impede the program’s efficiency. Disabling Add-Ons is a good choice if you want to speed up Outlook. If you want to do this, here are the steps you need to take:

  • First, open Outlook and navigate to the File menu, then Options, and finally, Add-Ins.

Steps to Manage OST File Size

  • Click the Go button next to COM Add-Ins in the Manage section.

Step 2

  • Click the OK button, then deselect all the plug-ins that aren’t being used right now.

Step 3

Archive OST file data

The data in an OST file could be reduced in size and server space freed up by archiving the file’s contents. The location of the archive OST file can be moved to a new directory. By archiving your OST file, you can keep it from growing above a certain size. This document contains instructions on how to archive it.

  • Open your inbox by going to the File menu, clicking Cleanup Tools, and finally choosing Mailbox Cleanup.

Archive OST file data

  • Next, you can choose between two alternatives:
      • With the help of the AutoArchive feature, older mail can be automatically move to a separate archive folder.
      • Empty the trash by erasing anything in the Trash folder.

Use Outlook’s built-in compression feature:

You can shrink your OST file and make it more manageable. A smaller file size may improve Outlook’s speed if the user has deleted numerous items. Use the provided procedures to compact your OST file.

  • First, get rid of anything that is no longer needed.
  • Then, in the Navigation pane, open the folder’s list view, right-click on the folder, select Deleted items, and finally select Empty folder.
  • Click the File tab, then pick Account Settings from the drop-down menu.
  • Select the data file you want to condense under the Data Files tab, then click the Settings button.

Step 5

  • The Outlook Options dialog box will open; navigate to the Advanced tab and select Outlook Data File Settings.

Step 6

  • The last step is to click Compact Now in the Outlook Data File Settings dialogue box.

Step 7

Limit the Size of OST Files using a Registry Key

Suppose you don’t want to delete older emails or remove attachments from emails. In that case, there’s an alternative way where you can use the registry to reconfigure the OST file size limits by changing the registry key’s entries and values. Limits on OST expansion can be specified by modifying four separate registry items.

Move OST file data into PST format using Regain OST to PST converter

Apart from above mentioned manual solutions, several Outlook users and professionals suggest using Regain OST Converter tool. The tool is coded by professionals and eases the process of OST to PST conversion.

Free Download100% Secure Buy NowPrice: $39

There is no requirement for technical assistance and expertise to use the tool, and even a newbie can use this tool to convert OST files into PST format and get rid of large-size OST files. The good thing about this tool is it also has a PST split option. Hence, you can pick a desired file size for the resultant PST file. Let me show you how you can convert OST to PST using Regain software.

Words for attention: Before diving into the process, make sure to download the free OST to PST converter tool.

  • Firstly, open the OST to PST converter in your system.
  • Secondly, click on the Open option from the ribbon bar.
  • Now add large-size OST files to convert & split into PST.
  • Following, click on the Export option from the ribbon bar.
  • Choose the PST file format option in the next wizard.
  • Select PST size and click on the convert now option.

Voila, you’ll have a PST file of your OST file and can access it in any Outlook version. Also, you can do the process in any Windows operating system.


Managing the Outlook OST file size is essential for optimal performance and preventing corruption of the file. Large attachments, email size, and deleted items are the primary reasons for a large OST file size. However, by regularly deleting unnecessary emails, emptying the Deleted Items folder, using archiving, and using the online mode, users can effectively manage the OST file size and improve their Outlook experience. A reliable OST to PST converter tool is also available to shift OST file data into PST format as a backup.

Frequently Asked Questions

Ques1. Are Outlook Add-ins also a reason behind the oversized OST file?
Ans. If you have multiple Add-ins in Outlook that are too unused, then it can be a reason for the oversized OST file and inversely impact Outlook’s working.

Ques2. I guess I can change the storage variable by using Registry Key. Isn’t that a good way to expand the OST file size limit?
Ans. Yes, it is correct that you can change/ re-configure the OST file size limit by entering the Registry key values and entries. But, it is not a recommended way as it will load on your Outlook, and after a period you face the same issues arising due to oversized PST files.

Ques3. If I use Regain OST to PST Converter, then how can I limit the PST file from being oversized?
Ans. Converting into a PST file can help you in two ways. First, after saving data in the PST file, you can free up the OST files. Using the Split PST option given in the software, you can control the resultant PST size and limit PST files from being oversized.

Ques4. Can I use Outlook’s mailbox cleanup tool to archive the OST items purposed to restrict OST files from being oversized?
Ans. Yes. Using Outook’s Cleanup Tool and the Auto Archive feature, you can archive in a specific folder. After this, you can empty the storage and reduce the size.

Author: Armaan

Hello I have been working in Regain Software for the last 6 months. I have been in the field of writing for many years and worked with many Data recovery companies. Here I am writing technical blogs, Software User Guides and manual methods for email conversion and cloud migration. Now, working on all products of our company. Thanks you

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