ICS too big

Question

Trying to move things from my previous emails set up to a new 365. but I can't add my ics file.

Answer 1

Hi Chilliwack Teachers, 

If you're importing the calendar into Outlook on the web and the .ics file is too large, try splitting the calendar into smaller batches before importing it. Please follow the steps below:

Step 1: Check the file size

1. Locate the .ics file on your computer.
2. Right-click the file and select Properties (Windows) or Get Info (Mac).
3. Review the file size.

Step 2: Split the calendar into smaller date ranges

If the calendar contains many years of events, export smaller date ranges from the original calendar source, for example:

• 2020-2022
• 2023-2024
• 2025-2026

This will create multiple smaller .ics files that are typically easier for Outlook on the web to process.

If your current calendar application does not offer the option to export by date range, you may also consider using a third-party .ics splitting tool to divide the file into smaller calendar files based on your requirements.

Step 3: Import each file separately in Outlook on the web

If you're migrating from another mail service (such as Gmail, Yahoo, or another Exchange account), consider using the migration or synchronization tools provided by that service rather than importing a single large .ics file.

If you have any questions or need further assistance, please feel free to share them in the comments on this post.

---

If the answer is helpful, please click 'Yes' and kindly upvote it.

Note: Please follow the steps in the forum documentation to enable e-mail notifications if you want to receive the related email notification for this thread. 

Answer 2

Split the oversized .ics calendar into several smaller files—preferably one file per year—and import them individually into the Microsoft 365 calendar.

Recommended steps

1. In the old calendar service or application, export separate date ranges, such as:
   • Calendar-2023.ics
   • Calendar-2024.ics
   • Calendar-2025.ics
   Re-exporting by date range is safer than manually editing the file. Before continuing, open or inspect each export and confirm that it contains the expected events. Splitting large calendars into smaller ranges is the practical workaround recommended for this import problem.^1^
2. Sign in to Outlook on the web with the new Microsoft 365 account.
3. Go to Calendar → Add calendar → Upload from file.
4. Select the first smaller .ics file.
5. Under Select a calendar, choose the destination calendar, then select Import and Save.
6. Repeat for every remaining file. These are the documented steps for importing .ics files into a work or school account.^2^

Importing creates a snapshot: later changes made in the old calendar will not update the imported events automatically.^2^

If the old calendar is still online

If it provides a public or authenticated iCal URL, try Calendar → Add calendar → Subscribe from web instead. A subscription receives updates from the source calendar, whereas an uploaded file does not. This is useful for ongoing synchronization, but it is not the best choice when the goal is to retire the old account.^2^

Microsoft’s Outlook on the web import instructions do not publish a specific maximum .ics file size, so avoid relying on an unverified numeric limit. If even a small yearly export fails, export a shorter interval—such as one month—and check the source calendar for a malformed event.

An .ics file contains calendar events only. Email messages must be migrated separately, such as through an Outlook data-file or administrator-managed mailbox migration process.

---

References

1. calandar problem on outlook - Microsoft Q&A
2. Import or subscribe to a calendar in Outlook.com or Outlook on the web | Microsoft Support