How to Split OST File?

Summary: In this blog, we will discuss the need to split an OST file. Plus, the methods to split the OST file. You can split the file easily and quickly by using Stellar Converter for OST Technician software.

Free download Stellar Converter for OST

An oversized Offline Outlook data file (.ost) may cause several problems in Outlook. And so, you need to ensure that the OST file do not exceed the defined size limit.

Table 1 – Outlook Data File Size Limits

Issues with Oversized OST File

You may experience the following issues as the Outlook data file size grows beyond the permissible limit:

  • Outlook performance degradation
  • The frequent occurrence of Outlook freezing/not responding issue
  • Synchronization errors
  • Delay in sending and receiving new mails

Why You Should Split OST File?

One of the most effective ways to reduce the size of Offline Outlook data file (.ost) is to archive OST into PST. Be wary, though. Moving data from a large-sized OST file to PST increases its size and makes it susceptible to corruption. A better alternative is to split OST file into multiple PSTs.

How You Should Split OST File?

There is no built-in utility for splitting large-sized Outlook data files (.ost). An alternate approach is to backup individual mailbox items from an Offline data file to a PST file one at a time using Outlook Import/Export wizard. But, this can be a tedious and time-consuming process, particularly when you need to split multiple OST files. Moreover, you cannot back up an inaccessible or orphaned OST file. This is where an OST to PST Converter tool such as Stellar Converter for OST Technician comes in handy.

The software not only helps convert inaccessible/orphaned OST into a PST file, but also allows splitting the converted file into multiple PSTs.

Splitting OST File with Stellar Converter for OST Technician

Following are the steps you need to perform to split the converted file before saving it as a PST file:

Step 1: Download, install and run Stellar Converter for OST software.

Step 2: From the software main interface window, click Browse or Find to select the OST file(s) you want to convert and split.

Select OST File for Splitting
Figure 1 – Select OST File

Step 3: Click Convert to proceed with the conversion process.

Convert OST File
Figure 2 – Convert OST File to PST

Step 4: The software will show preview of the converted mailbox data. Select the mailbox data that you want to convert and split into PST, and click Save Converted File from main menu.

Step 5: From the Save As dialog box that opens, choose Split and Save PST under the Advance Options section.

File saving options
Figure 3 – Converted File Saving Options

Step 6: From Split PST File box, you can choose any of these options for splitting the resultant PST file:

  1. Split by Mail ID

To split the file according to email ids, from the ‘Mail ID’ section choose mail ids from the drop-down list. Alternatively, use the Select All/Unselect All buttons for selecting/unselecting all the email ids.

  • Split by Date Range

Select Split By Date Range checkbox under ‘By Date Range’ section. Next, select the in the last checkbox for splitting the PST file by moths or choose start and end date to split it according to a specific period.

  • Split by Size

Make sure the By Size checkbox is checked in the ‘By Size’ section, and then select a size from the dropdown list to split the PST file according to predetermined sizes: 5 GB/10 GB/20 GB/30 GB or 50 GB.

Options to split OST file
Figure 4 – OST File Splitting Options


  • The software automatically splits the new PST file in small sizes depending the Outlook version installed on your computer. For Outlook 2007, the new PST gets split to approximately 18 GB size. For Outlook 2010 or above, the new PST can be split at approximately 45 GB–48 GB of size.
  • You can also select all the three split option checkboxes (By Mail Id, By Date Range, and By Size), the application will queue them and will split PST files using all the three methods.

Step 7: Click the Apply Settings buttons to return to the Save As dialog box.

Step 8. Click Browse to select the destination where you want to save the PST file, and then click OK. Step Step 9: Click OK when the split process gets completed.

The converted file will now get saved as multiple PSTs in the selected location.


The large size of the Offline Outlook data file (.ost) can cause several problems in Outlook. The best approach to reduce OST file size is to split it into multiple PSTs. But, there is no built-in utility that helps in splitting the Outlook data file. You can, however, choose to save individual mail items from an OST file to PST using the ‘Import and Export Wizard’; this can be a time-consuming process as you can save a mail item one at a time to PST. A better alternative is to use Stellar Converter for OST Technician software that converts the OST file and provides several options to split the converted file into multiple PSTs. 

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