Free online OST to ICS converter

A calendar stored in an Outlook OST file may need to be opened in Google Calendar, Apple Calendar, Thunderbird, or another application that accepts iCalendar files. Converting it to ICS extracts calendar events into a portable format for sharing, editing, or archiving.

What the OST format is

OST is Microsoft Outlook’s offline mailbox database. Outlook creates it for Exchange, Microsoft 365, and some other account types so mail, calendars, contacts, tasks, and folders remain available while disconnected from the server.

The file is usually stored in a hidden Outlook profile folder and is tied to the account and profile that created it. An OST is not a normal document and cannot generally be opened by calendar applications directly. If the account still works in Outlook, the calendar should be exported from the connected mailbox rather than converted from the database file.

What the ICS format is

ICS is the common file extension for iCalendar data, defined by RFC 5545. It stores calendar objects such as events, meetings, recurrence rules, reminders, locations, descriptions, attendees, and time-zone information in a text-based format.

ICS files can be imported into Google Calendar, Apple Calendar, Mozilla Thunderbird, Outlook, and many web-based calendar systems. They are suitable for transferring calendar entries, publishing schedules, and keeping a readable calendar archive, but they do not contain the complete Outlook mailbox.

How to convert OST to ICS

Export from Outlook when the account is available

1. Open Outlook using the profile associated with the OST and allow it to synchronize with the Exchange or Microsoft 365 account.
2. Open the Calendar view and select the calendar to export.
3. Choose File → Save Calendar.
4. Set the date range and the amount of detail to include. Select Full details if descriptions, locations, and attendee information are required.
5. Choose a destination and save the file with the .ics extension.
6. Open the resulting ICS file in the target calendar application, or use that application’s import function.

This method is preferable because Outlook reads the mailbox normally and exports the calendar without requiring direct access to the OST database.

Convert an OST file without the original Outlook profile

If the account was deleted, the OST is orphaned, or Outlook cannot open the profile, use the built-in converter on 101convert.com. Select the OST-to-ICS conversion, upload the OST file, start the conversion, and download the generated .ics file when processing finishes. No software installation is required.

For confidential calendars, check the service’s file-handling and retention terms before uploading. A local OST recovery or conversion utility is more appropriate when the file contains sensitive meeting details or cannot be uploaded.

Quality and compatibility limits

Only calendar data is transferred. Email messages, contacts, tasks, Outlook folders, categories, voting buttons, and other mailbox content are not represented by a standard ICS file.

Outlook-specific features may be simplified or omitted. Verify recurring meetings, exceptions, reminders, attendee responses, attachments, and time zones after import. An ICS export does not preserve the original OST and normally does not change it.

If the OST contains several calendars, export each calendar separately when using Outlook. An orphaned or damaged OST may require mailbox recovery before calendar entries can be extracted, and an encrypted or incomplete file may produce a partial ICS result.