Home Technology How Long Does It Take To Compact An OST File

How Long Does It Take To Compact An OST File

How long does it take to compact ost file

“Compacting PST manually, it can’t be predicted how much time will it take, while if compressed with automated tool, it takes 7 to 11 seconds!

An OST file (.ost) is an offline Storage Table file in Microsoft Outlook. The Offline folders make it possible to work offline and then to synchronize the changes with Exchange server while the next time they connect. The ability to work offline is useful in the environments with limited or unreliable connectivity. This article tells about compact an OST file. These files are used in two scenarios: first with Microsoft Exchange Server’s Cached Exchange Mode and the second one is the Outlook Connector for Windows Live Hotmail.

Need To Compact An OST File

One can delete unwanted items from the various OST file, and then compact an OST file to make them smaller to conserve the space on the hard disk. When deleting the unwanted items from each folder, it moves them to the Deleted Items folder, which is present in the same file. To permanently delete Must delete the items from Deleted Items folder to permanently delete them so that Outlook can compact the file. Does not remove all the free spaces during compacting process. It leaves either 16 kilobytes (KB) or 4 percent (%) of the file size (before compacting) whichever is greater.
For compacting OST file outlook 2003, 2007, 2010, 2013, 2016 it must meet the requirements:

  • The file must be 16 KB or larger.
  • The file must have at least 16 KB free space since the last time compacted it.

Manual Method To Compact An OST File:

The Size of OST file can be reduced with the help of Mailbox cleanup tool which is a feature of MS Outlook 2003, 2007, 2010, 2013, 2016. The following steps help to compact an OST file:

Step 1: Open MS Outlook then click on the
Tools” ->“Mailbox cleanup”->“View Mailbox size”.
A screen appears which shows you different folders and their sizes.

Step 2: To delete any folder click on the folder, press the “Delete”. To delete files within a folder, open the folder and select the files you want to delete and finally press the “Delete” button.

Step 3: Again click on the “Tools” -> “Account settings”. From the list that appears, click on the option “Microsoft Exchange Server”. A new menu appears.

Step 4: In the new menu click on “Change” -> “More settings” ->“Advanced”-> “Offline Folder File Settings”.

Step 5: After clicking on the “Offline Folder File Settings” -> “Compact now” to compact an OST file. Files start to compress and the size of the file is reduced.

How long will the Compacting process take?

How long does it take to compact an OST file depends upon the total size of your data file and how much “white space” needs to be cleaned up. For my smaller OST data files, it completes in just a few minutes. But on my larger OST mailbox files that are over a Gig in size, it typically takes 20 to 30 minutes to complete. I have even seen situations when the compact can take up to an hour to finish on huge data files, and sometimes even longer.
when running the compact outlook offline storage file process, you will not be able to do anything else in Outlook, and the computer may even be somewhat sluggish. However, one can still switch to other programs and perform other activities while the compact process is underway.

Automatic Method To Compact An OST File:

The automatic process is carried out with the help of SysTools PST Compress Tool, but before that, you will have to convert OST file into PST format and then, you can easily compress converted or resultant PST file using the PST Compress application. The tool easily reduce Outlook PST file size without getting the issue of slow compacting.


  • Tool can shrink Multiple PST files at once.
  • Supports Compaction of data files of both ANSI and UNICODE types.
  • Provides PST compression by Removing, Extracting and Saving attachments.
  • Reduces Outlook data file to minimum size without any data modifications.
  • Displays log report of complete PST compaction.
  • Runs on MS Outlook 2016 and all below versions.
  • Supports all Windows versions.


There are no hard and fast rules for how often to compact Outlook OST data file. It depends on how many Outlook items you process, and their size. If having issues, such as Outlook running slowly or need more disk space, then you should run the compact process right away. Luckily, there is a simple way to compact an OST file by using OST to PST Converter.

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