Gmail – How to ensure sending a .ICS file on Gmail will result in an “add to calendar”

According to this thread on Google Forums, this is a known limitation and they have it on the slate for future implementation. I expect that the more people tell Google they want this, the more likely Google will implement it and sooner.

EDIT

I've noticed that Google Inbox has started automatically adding events to your Calendar when you accept the invitation. This isn't identical to the functionality you're requesting, but it addresses most cases.

Update:

Normally you can just append your calendar id to the url:

• https://calendar.google.com/calendar?cid=ht3jlfaac5lfd6263ulfh4tql8@group.calendar.google.com

However, there are times when this doesn't work and you're required to use base64 encoding:

• https://calendar.google.com/calendar?cid=ZW4udXNhI2hvbGlkYXlAZ3JvdXAudi5jYWxlbmRhci5nb29nbGUuY29t

The exact rules for when one will work and the other won't seem fuzzy to me. This is a few things I found:

• If the id contains a #, unencoded id won't work, base64 will.
• If the id is a resource (e.g. @resource.calendar.google.com), unencoded id works, base64 doesn't work.
• If the id is a standard group (e.g. @group.calendar.google.com), both unencoded and base64 work.
• If the id is a user's email address (e.g. @example.com), both unencoded and base64 work.
• If the id is a group.v (e.g. @group.v.calendar.google.com), not sure which one will work as I've only seen these with the # symbol which forces you to use base64.

To create a single-click add to calendar link (if you are the calendar owner):

1. Go to Calendar Settings.
2. Under "Access permissions," click the "Get Shareable Link" button.
3. It will be of the form https://calendar.google.com/calendar?cid=bWFya2V0aW5nQHbWFya2V0aW5nQH.
4. Tapping on this link will prompt the user to add it to their Google Calendar.

To create a single-click add to calendar link (if you are not the calendar owner):

1. Get the "Calendar ID."
2. Encode it as base64 (e.g. by using a website such as base64encode.org).
3. You now have the cid in a format that is expected by Google Calendar.

4. Prepend https://calendar.google.com/calendar?cid= to the cid, and voila, you have a single-click add to calendar link.

   (Interestingly enough, the cid url will only work for a base64 encoded parameter even though the UI will show the unencoded email address in the error message.)

To verify that the link works, assuming you don't want to remove the calendar:

1. Hide the calendar in Google Calendar by unchecking it on the left sidebar.
2. Navigate to the link you just created.
3. It should now be checked on the left sidebar.

Note: an alternative to figuring out the encoded cid, is to figure out where the +Google Calendar button (as part of option 1 in the question) takes you (e.g., via monitoring the network, or turning off your network and clicking the link).