How to Compact OST File without Opening Outlook? 5 Methods

  

Rollins Duke   
Published: April 22nd, 2025 • 11 Min Read

Overview: This write is going to explain the most searched query on the internet on how to compact OST file to decrease

OST file size. Here we will describe all the possible ways to shrink your OST file of Outlook 2021, 2019, 2016, 2013, 2010, and all previous versions using manual and expert solutions.

We all know that an OST file stands for Offline Storage Table, which is used by Outlook email clients to store an offline copy of your data including emails, contacts, calendars, notes, tasks, journals, etc. But many times, happened when the OST file has reached maximum size, that may occur many errors in Outlook including:

• Outlook performance getting slow
• Outlook OST data may become inaccessible or OST files may be opened.
• Synchronization errors.
• Getting errors while sending and receiving emails
• Outlook not responding

In these kinds of situations, it’s become important to compact OST file to reduce size. Besides saving storage space, it will help you in Outlook performance and prevent errors, that may occur due to large OST files.

Recommended: Suggestions to Fix Outlook Mailbox Full After Archiving

Why Do You Need to Compact OST File?

• Overtime, your Outlook OST can be bloated with unnecessary data, especially if you frequently delete emails, attachments or entries. Even though these items appear deleted in Outlook the actual space available in the OST file isn’t immediately freed.
• A Large OST file can slow down Outlook startup and response time. Compacting files helps to reduce file size, making Outlook runs more smoothly and load faster.
• Compact OST file removes redundant or empty space allows you to reclaim valuable storage on your system, especially useful for limited storage.
• Oversized OST files are always a high possibility of file corruption. Regular compaction will reduce corruption chances and keep your mailbox safe and active.
• A smaller, well-optimized OST file syncs faster with Exchange or Microsoft 365 server, improving overall productivity and reducing sync related errors.
• When preparing for mailbox migration or backup, a compact OST file is easier to manage and quicker for transfer.

Signs Your OST File Needs Compaction

• If you notice delay in sending and receiving emails or sluggishness when switching folders, your OST file may be reached and filled with unnecessary white spaces.
• Frequent application crashes or Outlook becoming unresponsive are warning signs that your OST file may be bloating and reaching its higher limit.
• If your OST file is several gigabytes exceeding 10 GB it could be time to compact the OST file to improve performance and reduce the chance of file corruption.
• Problems during mailbox synchronization such as delayed or incomplete sync, can result from an overgrown OST file affecting communication between Outlook and the server.
• If you’re running out of storage on your device and the storage chunk of storage, it’s a good time to consider compacting it.
• Deleting or archiving emails doesn’t instantly reduce OST file size. If you have removed a lot of data recently, a compact OST file will help to reclaim free up storage.

Benefits of Compacting OST File

• Oversized OST files are more likely to be corrupted. By compacting your OST file, you lower the risk of encountering errors that could lead to inaccessible or lost data.
• Compacting clears out of unused space left behind by deleted items, which helps free storage- especially for users with limited drive capacity.
• It can sync more efficiently with Microsoft Exchange or Outlook.com servers for quicker accessibility and fewer sync errors.
• Compact Outlook data files are easier and faster to backup or migrate to another system.
• Outlook processes larger files with more CPU and memory usage. Compacting reduces system strain and keeps Outlook running lighter and faster.

Manual Methods to Decrease OST File Size

There are 3 manual methods available to compact OST file without any hassle. All the methods are given below one by one in detail:

Method 1: Compact OST File with “Compact Now” Option

This is the default feature in the Outlook email client, that automatically compact OST file when the white or unused space reaches 20% of the file size. Also, you can shrink the OST file manually by following the mentioned steps:

Note: Before using this option, check the deleted items folder and restore the important email that you have deleted by mistake. Because after compacting the OST file, you can not recover OST file data and emails that have been deleted.

1. Open an Outlook account.
2. Click on the File >> Account Settings >> Account Settings options.

   

3. Open the Data Files tab and choose the OST file you want to compact.

   

4. Choose Settings >> Advanced >> and Outlook Data Files Settings options.
5. Hit the Compact Now icon and choose OK.

And your OST file will be compacted successfully.

Method 2: Compact OST File Using Registry Editor (Advanced Users)

1. Open Registry editor by pressing Win+R and type regedit.
2. After pressing enter navigate to Outlook version: HKEY_CURRENT_USER\Software\Microsoft\Office\\Outlook\PST
3. Replace version with your Outlook version such as 2019, 2016 or any other.
4. You can add or update registry keys to influence OST files.
5. MaxFileSize, WarnFileSize, MaxLargeFileSize, WarnLargeFileSize.
6. Right-click on the right pane, choose New> DWord(32-bit Value), name it accordingly, and assign value in bytes.
7. After making changes, close the Registry editor and restart Outlook.

Method 3: Clean Your Mailbox in Outlook

To compact OST files, you can also clean up your Outlook mailbox by deleting unwanted emails and folders to save your space. The steps are mentioned below:

1. In Outlook go to File >> Info >> select Mailbox Cleanup option.
2. Hit the Mailbox Cleanup… from the appearing drop-down.
3. Choose “View Mailbox Size…” to see the actual size of the mailbox.
4. Now select the one option between:
5. Find items older than to delete those emails from a particular number of days that you don’t want to require anymore.
6. Find items large than to delete folders that are larger than a specific size.

If your mailbox contains only important emails, and you don’t want to delete them, then follow the next method.

Method 4: Shrink OST File Archive OST Emails

You can compact OST file without deleting anything in your Outlook account by archiving your emails.

For Outlook 2021, 2019 and 2016

1. In Outlook open File >> Tools >> Clean up old items options.

   

2. Select the required folder that you want to archive.

   

3. And hit the OK button.

In Outlook 2013

1. Open Outlook email client.
2. File > Cleanup Tools > Archive options.

And your all emails will be moved into archive folders and will save on the local computer in PST format. with this method, there is no need to delete folders manually.

Manual methods are straightforward and free, but there are some points that you must remember:

• Outlook must be installed on your system.
• After deleting your emails will not be recovered.

If you have added a large number of items to the archive folder, which will increase the PST file that are stored in the archive folder data. That may cause PST file corruption.

If you have an orphan OST file or you don’t want to use these manual methods, then go for the expert solution, that will compact OST file without losing or deleting any information. Also, there are many benefits of using expert solutions that are mentioned below:

Working Steps to Compact OST Files

1. Download and install the software.

   

2. Add the OST file manually using Select Files and Select Folders option or automatically using the “Convert Configured Outlook Mailboxes Data” option.

   

3. Enable the Use Recovery Mode option to repair corrupted OST file.

   

4. Check the required folders and subfolders from the screen and the Next button to continue.

   

5. Select the desired location using the Browse button.

   

6. Choose the required option to split the resultant file.

   

7. Apply the advanced filter option from the Split Options tab.

   

8. Hit the Split option to start the process of compacting OST files.

   

And your compacted resultant file will be saved at the selected location in PST format.

Tips to Prevent OST File from Growing Too Large

• Always empty your Deleted items and Trash folder to take space in the OST file until permanently removed.
• Use Outlook inbuilt arching feature to move older emails to a separate PST file. This lightens the active OST file and improves performance.
• Instead of saving large files in your mailbox, store them on a shared drive or cloud service and send links this reduces OST file bloat.
• Setup automatic archiving in Outlook to move old emails and attachments out of the OST file on a scheduled basis.
• Outlook offers a mailbox cleanup that helps identify large folders and messages. Regularly using it helps maintain healthy OST size.
• Even after cleaning, Outlook doesn’t immediately reduce file size. Manually compacting OST file once every week to reclaim white space and keep the data file optimized.
• If using Outlook with Exchange or Microsoft 365, you can limit the number of days to sync email. For instance syncing only for the last 6 to 12 months reduces data stored in an OST file.
• Reduce mailbox content by subscribing from a folder you don’t require (e.g., newsletter Or shared mailboxes). This limits the synced data and keeps your OST file size under controls.
• Clearing forms and autocomplete caches can also help in some cases. While minor, it contributes to overall data file health.
• Keep an eye on your OST file size from Outlook settings Or file location. If it’s growing faster than expected, take preventive actions early.

Time to Say Goodbye

Here we have explained the best methods to compact OST file without opening an Outlook email client. We have mentioned both manual methods and expert solutions. Manual methods are straightforward and amazing but they have some limitations, so I have suggested you use manual methods, in case you have certain data. And use an expert solution if you have multiple and large OST files. If you have any doubts or queries related to the tool, then you can download the software for free to analyze the performance.

Recommended: Methods to Reduce PST File Size without Outlook