Compact PST File Using Outlook “Compact Now” Feature

Have you ever faced this Outlook error, “the outlook data file has reached its maximum size”? Keep in mind that the more email messages you will send and receive, the larger your Outlook data file will grow. And once the file reaches its maximum size, it may cause serious troubles for Outlook users. In this article, we will discuss the most common problems that may arise due to an oversized PST file. In addition, I would like to share an easy solution to compact PST file.

Outlook Data File Size Limit

Microsoft Outlook is one of the most popular email clients around the world. It has almost everything you might be looking for, from a highly interactive user interface to a wide range of smart features. You may already be aware of the Outlook data file size limit. But just in case you do not know, Outlook data file (.PST) has a certain size limit. There are two types of PST created by Microsoft Outlook: ANSI and Unicode.


  • ANSI: ANSI is an old PST file format created by Microsoft Outlook 2002 and earlier versions. For backward compatibility reasons, ANSI format can also be supported by Outlook 2003 and later versions. And if we talk about the size limit, it has the maximum size limit of 2 GB. This means you cannot store the mailbox data more than 2 GB in an ANSI PST file.
  • Unicode: Unicode is the new PST file format created by Microsoft Outlook 2003 and later versions. The maximum size limit of a Unicode PST file (by default) is 20 GB. You can increase the size limit up to 50 GB.

Being a smart Outlook user, it is your responsibility to make sure that the Outlook data file never reaches its maximum size. Because if it does, you are likely to face some serious problems while using Microsoft Outlook. To overcome such a problem, you should reduce PST file size if it is about to reach its maximum limit.

Why Should You Reduce PST File Size?

If you have an oversized PST file, you have to reduce its size. It is strongly recommended not to keep PST files’ size large. An oversized PST file can cause serious problems that include performance issues and file corruption. Microsoft Outlook becomes very slow while trying to open an oversized PST file. You will also face performance issues, such as Microsoft Outlook has stopped working. It will be so annoying for any Outlook user.


Sometimes results may become even worse. Just in case you do not know, an oversized PST file is always at a high risk of corruption. Whenever a PST file crosses its maximum size limit, it is likely to become inaccessible due to corruption. It is one of the most important reasons for PST file corruption. You face all these problems due to an oversized PST file. That is why Outlook experts and other professional users suggest compressing PST file size.


Outlook users might also be interested in – Split PST File into Smaller Parts Using Manual Solutions

How to Compact PST File?

Whenever you receive an error message something like this – “the outlook data file has reached its maximum size”, what would you do? You will probably delete all the unwanted emails and other items from the PST file. But this will not help you out. Although you can delete something from the PST file, it will not reduce the file size. There is one more thing you have to do here, compact PST file using the “Compact Now” feature of Microsoft Outlook. Follow the steps given below to know how to compact PST file:


  • First, open Microsoft Outlook on your system. Now select the emails and other items or a complete folder that want to delete.
  • After deleting that, go to the Deleted Items folder. Do not forget to empty this folder. Once you have emptied the folder, you can now compact PST file.
  • Now click the File tab on the taskbar, select Account Settings > Account Settings.
  • On the Account Settings dialog box, click the Data Files tab. Select the PST file you want to compact, and click Settings. Or, simply double-click it.
  • On the Outlook Data File dialog box, click the Compact Now button to start compressing PST file size.compact PST file

Within a couple of minutes, the selected PST file will be compressed.

Other Ways to Reduce Mailbox Size

Apart from the “Compact Now” option, there are several ways to reduce your mailbox size. You can use Mailbox Cleanup, a built-in tool, that will help you view the size of your mailbox and other individual folders. You can also use this tool to


  • Search for the older or very large email messages that you want to delete or move into a PST file;
  • View the size of your Deleted Items folder and empty it;
  • Delete the conflicted items from the Conflicts folder.

FYI, whenever you modify two or more copies of the same item independently in Outlook, conflicts may occur. Microsoft Outlook detects conflicts during the synchronization process. It creates a separate folder called Conflicts, to store the conflict items.


Follow the steps given below to run the Mailbox Cleanup tool:


  • First, open Microsoft Outlook on your system.
  • Click the File tab on the taskbar, point to Info.
  • Now click Cleanup Tools and select Mailbox Cleanup.
  • On the Mailbox Cleanup dialog box, you will see various options, such as View Mailbox Size, Find items older than, Find items larger than, AutoArchive, View Deleted Items Size, Empty Deleted Items folder, View Conflicts Size, or Delete Conflicts.Outlook Mailbox cleanup tool
  • You can now perform your desired task. You should empty the Deleted Items folder and delete the Conflict items.
  • Finally, click Close to close the Mailbox Cleanup tool.

You can also use the “Conversation Clean Up” option of Microsoft Outlook to remove the redundant email messages automatically. Just in case you do not know, there are numerous messages containing the same information as prior messages in a long email thread. Those are called redundant messages. You can run the Conversation Clean Up tool to remove those redundant messages from the mailbox.

Professional Solution to Compact PST File

Although the above solutions work so nicely, it has one major drawback – time-consuming. It will take a lot of time and manual efforts to compact PST file manually. Generally, the manual solution is not recommended for very big PST files. 5 to 10 GB of PST file is fine. But if you are trying to compact a 20 GB PST file, it would be a challenge for you. In such a situation, you should consider using a professional PST Compact tool that can instantly reduce PST file size without causing any changes or alterations to the original data. There are many software vendors in the online marketplace that provide such solutions, and Bit Data Conversion is one of them.


The PST Compact tool provided by Bit Data Conversion will help you reduce PST file size in the minimum possible time. No matter how large your PST file is, it will successfully compress its size using an advanced algorithm. The best part of this software is that it requires no prior technical knowledge. Even a novice user can use this software with ease. Below are the key features of this software:


  • There is no size limitation. You can select any size of PST file and compress its size within a few mouse clicks.
  • Using this tool, you can select single as well as multiple PST files. For instance, if you have 10 PST files whose size you want to reduce, you can select all of them at a time and compress them in one go.
  • This software can save all the email attachments in a separate folder at the location you specify for the output PST files.
  • While compressing PST file size using this software, you can remove the duplicate items.

Download Demo Version

Before purchasing this PST Compact tool, you must download the free demo version for a free evaluation. By using the demo version, you can compress the first 25 email items of each mailbox folder of the selected PST file(s).


The article has explained the Outlook data file size limit and the problems that may occur due to an oversized PST file. To reduce PST file size, we have gone through an easy manual way that requires no professional software. However, a third-party PST Compact tool is considered the best solution when it comes to compressing PST file size.