What is a .ics file?
iCalendar · Data · IETF Calendaring and Scheduling Working Group · 1998
ICS is the iCalendar format, a plain-text calendar event, or a whole calendar of them, that any calendar app can import. It is what powers 'Add to Calendar' buttons and the calendar invites your email client already knows how to open.
What is a .ics file used for?
An ICS file is a calendar event, or a whole calendar of them, in plain text, holding a title, start and end time, location and other details between BEGIN:VCALENDAR and END:VCALENDAR lines.
It is the format behind 'Add to calendar' buttons and calendar invites everywhere: opening one offers to add the event to Google Calendar, Outlook, Apple Calendar or whichever app is set as your default.
The current standard is IETF RFC 5545, which replaced the original RFC 2445 in 2009 and is what every mainstream calendar app implements today.
An ICS can describe a single event or a recurring series, a weekly meeting, for example, and one file can also bundle multiple events, which is how a whole calendar is exported and shared as a single attachment.
What opens a .ics file?
The .ics format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list.
Windows
- Outlook
- Calendar app
- Google Calendar (import)
macOS
- Calendar
- Outlook
Linux
- GNOME Calendar
- Thunderbird
Web
- Google Calendar, Outlook.com (import)
Open a .ics file step by step
- Confirm the file really is a .ics. Drop it into the WhatFileType identifier, which reads its signature in your browser without uploading anything.
- On Windows, open it with Outlook or Calendar app.
- On Mac, use Calendar or Outlook.
- On Linux, try GNOME Calendar or Thunderbird.
- Still stuck? Convert the .ics to Not convertible, which opens more widely.
Why can't I open my .ics file?
Almost always because no installed app understands ICS. Install one of the apps above, or convert the file to a format your existing software already reads. If double-clicking opens the wrong program instead of nothing at all, the file association is set to the wrong app rather than the file being broken.
The other possibility is that the file is not really a .ics at all. Drop it into the file identifier to check it by its bytes instead of its name. Nothing is uploaded.
How to convert a .ics file
You can convert a .ics file to Not convertible using a conversion tool or the export menu of an app that opens it.
What is the .ics file signature (magic bytes)?
A .ics file starts with the hex bytes 42 45 47 49 4E 3A 56 43 41 4C 45 4E 44 41 52, which read as BEGIN:VCALENDAR in ASCII. Programs identify ICS by those bytes, not by the filename, so a renamed file still gives itself away.
These are the documented signatures for ICS.
| Hex signature | Offset | ASCII | Note |
|---|---|---|---|
| 42 45 47 49 4E 3A 56 43 41 4C 45 4E 44 41 52 | 0 | BEGIN:VCALENDAR | 'BEGIN:VCALENDAR' |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .ics file?
ICS is the iCalendar format, a plain-text calendar event, or a whole calendar of them, that any calendar app can import. It is what powers 'Add to Calendar' buttons and the calendar invites your email client already knows how to open.
How do I open an ICS file?
Double-click it and your default calendar app (Outlook, Apple Calendar, Google Calendar) will offer to add the event or events to your calendar.
Can one ICS file hold more than one event?
Yes. An ICS can bundle any number of events, each its own block inside the file, which is how a full calendar export becomes a single attachment.
Related formats
Sources
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