What is the ICS file format?
Files with the .ics extension are calendars, used by calendar programs and email client software. .ics files enable users to publish and share calendar data with users operating on compatible software over the internet. The format follows RFC 5545 (iCalendar) and uses the MIME type text/calendar.
Data contained in .ics files is plain text, encoded in UTF-8. Every valid .ics file begins with the line BEGIN:VCALENDAR and ends with END:VCALENDAR, enclosing one or more components. Data contained within the file includes, for instance, the start and finish time of a meeting, a title, or additional notes. .ics files are often used to send invitations for events.
Each calendar entry is represented by a component:
VEVENT- a scheduled event (meeting, appointment, birthday)VTODO- a task or to-do item with an optional due dateVJOURNAL- a journal entry attached to a specific date
.ics files can be easily inspected in a plain text editor such as Notepad. By importing an .ics file into Google Calendar, Apple Calendar, or Microsoft Outlook you can access events immediately. You may also share a subscription link with others, which allows them to see scheduled events and receive updates in real time. Related calendar formats include .vcs (the older vCalendar predecessor) and .ical (an alternate extension for the same iCalendar format).
Security & safety
RISK: LOWICS is plain text and cannot itself execute code, so the file format is low risk. The real risks are social: a malicious invite can auto-add spam events with links or fake reminders to a calendar (calendar-spam/phishing), and an .ics can carry a URL or attachment reference that leads to a phishing site. Treat unexpected invitations the same as unexpected email - do not click links inside an event you did not expect.
Format details
in a nutshellPrograms that open ICS files
Windows3 apps
Microsoft Outlook
Paid Double-click the .ics, or in Outlook use File > Open & Export > Import an iCalendar (.ics) file. 
Mozilla Thunderbird
Open-source Open the .ics with Thunderbird (built-in calendar, formerly the Lightning add-on) to import the event. 
Windows Calendar (Mail and Calendar / new Outlook)
Built-in Double-click the file; the built-in Calendar app offers to add the event. 
macOS1 app
Linux1 app
GNOME Calendar / Evolution
Open-source Open the .ics in Evolution or GNOME Calendar to import events. 
Android1 app
Google Calendar (via import)
Free ICS import on Android is done through the web Google Calendar; the app then syncs the events. 
Google Calendar
Free Settings (gear) > Import & export > Import, choose the .ics and a target calendar. Technical details
deep spec| MIME type | text/calendar |
| File identification | Plain-text; every valid file opens with BEGIN:VCALENDAR and closes with END:VCALENDAR |
| Character encoding | UTF-8; a UTF-8 BOM (EF BB BF) may optionally precede the first line |
| Line structure | CRLF (\r\n) line endings required by RFC 5545; lines longer than 75 octets must be folded with a CRLF followed by a single space |
| Primary component | VEVENT - encodes a single calendar event with DTSTART, DTEND, SUMMARY, UID, and optional DESCRIPTION |
| Task component | VTODO - represents a to-do item; supports DUE date, PERCENT-COMPLETE, and STATUS properties |
| Time zone handling | VTIMEZONE component embeds daylight-saving rules inline; alternatively, TZID references an IANA time zone database identifier |
| Recurrence rules | RRULE property supports FREQ, BYDAY, COUNT, UNTIL and other parameters (e.g., RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR) |
| Alarm support | VALARM sub-component triggers DISPLAY or EMAIL notifications relative to event start or end time |
| Organizer and attendees | ORGANIZER and ATTENDEE properties carry mailto: URIs; PARTSTAT tracks acceptance or decline status |
| Calendar subscription | Servers can publish .ics URLs using the webcal:// scheme for live, auto-updating calendar subscriptions |
| Alternate extensions | .ical and .ifb are alternate file extensions for the same format; .ifb is used for free/busy-only data |
| Predecessor format | Supersedes vCalendar (.vcs), the older 1.0 standard; .ics files are not backward-compatible with all vCalendar consumers |
| Binary data encoding | Inline binary attachments (ATTACH property) use BASE64 encoding within the plain-text file |
| Meeting request protocol | iTIP (RFC 5546) defines how .ics files are exchanged as meeting invitations via email (METHOD:REQUEST, REPLY, CANCEL) |
| Released | 1998 (RFC 2445), updated 2009 (RFC 5545) |
| Open standard | Yes · royalty-free |
| Specification | datatracker.ietf.org |
ICS conversions
Community Q&A
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