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.
How to open 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)
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.
.ics file signature (magic bytes)
Programs recognise a .ics file by the bytes at the start of the file, not by its name. These are the signatures for ICS.
| Hex signature | Offset | Note |
|---|---|---|
| 42 45 47 49 4E 3A 56 43 41 4C 45 4E 44 41 52 | 0 | '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
Details on this page were checked against authoritative references: