Outlook (Desktop & OWA) fails to import/subscribe to valid ICS calendar file — all file sizes fail

Question

Outlook (Desktop & OWA) fails to import/subscribe to valid ICS calendar file — all file sizes fail

Raja S 0

We are experiencing a consistent failure when attempting to subscribe to or

import an ICS calendar file in both Outlook Desktop (Microsoft 365) and

Outlook on the Web (OWA). The issue occurs regardless of file size.

ENVIRONMENT:

Outlook Desktop: Microsoft 365 (classic and new Outlook)
Outlook Web Access (OWA)
ICS file generated by app

STEPS TO REPRODUCE:

Obtain a valid ICS file or subscription URL
In OWA: Calendar > Add Calendar > Upload from file / Subscribe from web
In Outlook Desktop: File > Account Settings > Internet Calendars > New
Enter the ICS URL or upload the file
Observe failure

WHAT WAS TESTED:

We generated ICS test files at multiple sizes to isolate the issue:

9.88 MB (45,000 events) — Failed
7.99 MB (36,400 events) — Failed
1.98 MB (9,100 events) — Failed
1.88 MB (8,650 events) — Failed
0.99 MB (4,550 events) — Failed

The ICS file is confirmed valid:

Passes RFC 5545 iCalendar format
Successfully imports into Google Calendar
File opens and displays correctly when renamed to .txt

EXPECTED BEHAVIOR:

ICS file should import/subscribe successfully in Outlook and OWA,

as it does in Google Calendar.

ACTUAL BEHAVIOR:

Import and subscription fail silently or with a generic error in both

Outlook Desktop and OWA at all tested file sizes.

ADDITIONAL NOTES:

Google Calendar subscription with the same ICS URL: SUCCESS
OWA import with same file: FAILED
Outlook Desktop subscription: FAILED
Issue is reproducible across multiple user accounts

0 comments

1 answer

Your answer

Answer 1

Helen Luu 2,700 Independent Advisor

Hello @Raja S

Based on the information you provided, I understand that you're attempting to import an .ics file into your Outlook calendar. However, despite testing with different file sizes and varying numbers of events, the import still fails without displaying any error message.

Could you please confirm whether importing a smaller .ics file containing approximately 100 to 500 events is successful?

Additionally, have you previously been able to import an .ics file with around 5,000 events successfully, or has the issue occurred from the beginning?

As a troubleshooting step, testing with a smaller file can help determine whether the issue is related to the size or content of the .ics file, or if another factor may be affecting the import process.

The additional information will help us narrow down the possible causes and identify the appropriate next steps for troubleshooting.

I look forward to your update.

Raja S 0

Hi @Helen Luu ,

Thank you for the guidance. I have now completed the testing and have some clearer findings to share.

Test Results:

File	Events	File Size	Result
Small file	~300 events	~66 KB	✅ Imported successfully
--------	--------	--------	--------
Small file	~300 events	~66 KB	✅ Imported successfully
Medium file	~450 events	~99 KB	✅ Imported successfully
Medium file	~500 events	~110 KB	✅ Imported successfully
Medium file	~650 events	~143 KB	✅ Imported successfully
Large file	~700 events	~154 KB	❌ Import fails silently

Summary:

Importing .ics files with up to 650 events (~143 KB) completes successfully.
Importing a file with 700 events (~154 KB) fails with the error message of Could not import calendar try again later.

To answer your earlier questions:

Yes, importing smaller files (100–500 events) is successful.
This issue has occurred from the beginning — I have not been able to successfully import a file with more than 700 events at any point.

It appears there may be a size or event count limit in Outlook's import functionality. Could you advise on whether this is a known limitation, and if there is a workaround?

Thank you for your continued support.

Hornblower409 14,425 Reputation points

2026-08-26T14:36:57.7866667+00:00

@Raja S said

I have now completed the testing and have some clearer findings to share

Thank you for the detailed analysis. Very well done.

But I am not finding any official Microsoft docs that state there is a size/events limit on importing an ICS. In fact I see a report of importing (at least partially) much larger ICS files. e.g.

https://learn.microsoft.com/en-us/answers/questions/4682767/why-can-i-only-import-some-of-a-admittedly-very-la

it does start to import it, but stops after around 6200 of the records out of a total of 19,668. I did find those records, earlier in time, all there as they should be, totally readable

Certainly can not rule out some undocumented size/count restriction. This is Outlook. Anything can change without notice.

But, at this point, you might also want to be looking for possible problems with the content of the ICS, not it's size. e.g.

Assuming you did your testing with the same ICS, buy just chopping off the end, then the problem may be in the events between 650 and 700. Try cutting out just that segment of events and test separately?

Run your ICS thru a strict validator? [External Non-Microsoft Site] https://icalendar.org/validator.html

Get an ICS Export of a similar size/count but from a different source?
Helen Luu 2,700 Reputation points Independent Advisor

2026-08-26T15:48:37.16+00:00
@Raja S

Similar to Hornblower409, I have not been able to find any official Microsoft documentation that specifies a limitation on the number of events or the file size supported when importing calendar data into Outlook. Therefore, as a workaround, I would recommend splitting the calendar data into smaller batches and importing the events into Outlook Calendar separately.

To obtain a definitive answer and further investigate, I recommend contacting Microsoft Support. Microsoft Support has access to specialized diagnostic tools and internal resources that are not available through this community forum.

If you are a Microsoft 365 administrator, you can create a support request directly through the Microsoft 365 Admin Center by following the guidance in Get support - Microsoft 365 admin | Microsoft Learn.

If you are an end user, please contact your Microsoft 365 administrator and ask them to submit a support ticket on your behalf.

When submitting the support request, please make sure to:

Disable the AI assistance option so that the Contact Support button becomes available.

Provide detailed information about the issue, including the steps you have taken to troubleshoot it.

Attach a sample .ics file that reproduces the problem, if possible. This will help the support engineers investigate the issue more effectively.

As this is a user-to-user community forum, we do not have access to the tools required to analyze files that contain personal or sensitive information. Only Microsoft Support can perform a detailed investigation of such files and determine whether there are any limitations or issues affecting the import process.

I hope this helps. Please let me know if you have any additional questions or need further assistance.