What is the ICALENDAR file format?
.icalendar is a plain-text calendar data file conforming to the iCalendar standard defined in RFC 5545 (September 2009), which superseded the original RFC 2445 from 1998. Every valid file begins with the line BEGIN:VCALENDAR and ends with END:VCALENDAR, wrapping one or more calendar components.
Supported components include:
VEVENT- calendar events such as meetings and appointments,VTODO- task or to-do items,VJOURNAL- journal entries,VFREEBUSY- free/busy time blocks for scheduling.
Each component contains property:value lines in a MIME-like structure, with long lines folded at 75 octets. The file encoding is UTF-8 and the registered MIME type is text/calendar.
.icalendar is one of four equivalent extensions defined by RFC 5545 alongside .ics (by far the most common in practice), .ical, and .ifb. All four extensions share exactly the same format and MIME type. The .icalendar extension is the least common; most calendar applications default to .ics on export. The iCalendar standard is supported natively by Microsoft Outlook, Apple Calendar, Google Calendar, Mozilla Thunderbird, iOS, and Android.
Security & safety
RISK: LOWPlain-text format; cannot execute code. Real risk is social engineering: unexpected calendar invites can add spam events with phishing links (calendar spam). Treat unexpected .icalendar attachments like unexpected emails - do not click embedded URLs.
Format details
in a nutshellPrograms that open ICALENDAR files
Windows3 apps
Microsoft Outlook
Paid Rename .icalendar to .ics, then double-click, or use File > Open & Export > Import an iCalendar (.ics) file. 
Mozilla Thunderbird
Open-source Rename to .ics; open with Thunderbird's built-in calendar (Events and Tasks > Import). 
Windows Calendar (built-in)
Built-in Rename .icalendar to .ics, then double-click; Windows Calendar offers to add the event. 
macOS1 app
Apple Calendar
Built-in Rename .icalendar to .ics, then double-click; Calendar asks which calendar to add events to. 
Linux1 app
Android1 app
Google Calendar (via web import)
Free Rename to .ics; import through Google Calendar on the web, then it syncs to the Android app. 
Technical details
deep spec| Format type | Plain-text calendar data file (open IETF standard) |
| Governing standard | RFC 5545 (September 2009), superseding RFC 2445 (1998) |
| MIME type | text/calendar |
| Encoding | UTF-8 (legacy files may use ISO-8859-1) |
| File signature | Starts with BEGIN:VCALENDAR; ends with END:VCALENDAR |
| Supported components | VEVENT, VTODO, VJOURNAL, VFREEBUSY |
| Line folding | Long property lines folded at 75 octets per RFC 5545 |
| Version declaration | VERSION:2.0 property inside file body |
| Typical file size | Under 5 KB for a single invite; up to several MB for full calendar exports |
| Equivalent extensions | .ics (most common), .ical, .ifb - all identical format |
| Recurrence support | RRULE property for repeating events; EXDATE for exceptions |
| Platform | Cross-platform: Windows, macOS, Linux, iOS, Android, web |
| Time zone handling | VTIMEZONE component or TZID reference for unambiguous event times |
| Released | 1998 (RFC 2445) |
| Latest version | RFC 5545 (September 2009) |
| Open standard | Yes · royalty-free |
| Specification | datatracker.ietf.org |
ICALENDAR conversions
Community Q&A
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