iCalendar
An ICS (iCalendar) file stores calendar events, meeting invitations, and scheduling data in a plain-text format defined by RFC 5545. It is universally supported by Google Calendar, Apple Calendar, Outlook, and all major calendar applications.
Calendar data format. Conversion is not applicable.
Common questions
Can I subscribe to an ICS calendar URL instead of importing?
Yes. Most calendar apps support subscribing to an ICS URL, which automatically syncs updates. In Google Calendar, use 'From URL' under Other calendars. This is how public holiday and sports calendars are distributed.
What is the difference between VEVENT and VTODO?
VEVENT represents calendar events with start and end times. VTODO represents tasks or to-do items with optional due dates and completion status. Both are components within a VCALENDAR container.
How do recurring events work in ICS?
ICS uses RRULE (recurrence rule) properties like RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR to define repeating patterns. EXDATE excludes specific dates from the recurrence set.
What makes .ICS special
What is an ICS file?
ICS (iCalendar) is the standard file format for calendar data exchange, defined in RFC 5545. ICS files contain event information including dates, times, locations, descriptions, and attendees. They are universally supported by calendar applications and are the backbone of calendar sharing and meeting invitations.
Continue reading — full technical deep dive
How to open ICS files
- Google Calendar (Web) — Import or subscribe
- Apple Calendar (macOS, iOS) — Double-click to add
- Microsoft Outlook (Windows, macOS) — Import events
- Any text editor — ICS files are plain text
Technical specifications
| Property | Value |
|---|---|
| Standard | RFC 5545 (iCalendar) |
| Encoding | UTF-8 |
| Components | VEVENT, VTODO, VJOURNAL, VFREEBUSY |
| Recurrence | RRULE (RFC 5545 recurrence rules) |
| Time Zones | VTIMEZONE or TZID references |
Common use cases
- Meeting invitations: Email-based calendar invites.
- Calendar subscriptions: Subscribing to shared calendars.
- Event export: Exporting events from calendar apps.
- Scheduling: Booking systems and appointment tools.
.ICS compared to alternatives
| Formats | Criteria | Winner |
|---|---|---|
| .ICS vs .CSV | Calendar data interchange ICS preserves recurrence rules, time zones, attendees, and alarms that CSV cannot represent | ICS wins |
| .ICS vs .JCAL (RFC 7265) | Web API compatibility jCal is a JSON representation of iCalendar, better suited for REST APIs and JavaScript processing | JCAL wins |
| .ICS vs .VCF | PIM data exchange ICS handles calendar/scheduling data while VCF handles contact data; they are complementary standards | ICS wins |
Technical reference
- MIME Type
text/calendar- Developer
- IETF
- Year Introduced
- 1998
- Open Standard
- Yes — View specification
Binary Structure
ICS is a plain-text format using BEGIN:VCALENDAR and END:VCALENDAR delimiters. Events are nested as VEVENT components with DTSTART, DTEND, SUMMARY, and other properties. Supports recurrence via RRULE and time zones via VTIMEZONE.
| Offset | Length | Field | Example | Description |
|---|---|---|---|---|
Line 1 | 16 bytes | BEGIN marker | BEGIN:VCALENDAR | Start of the iCalendar data stream |
Line 2 | variable | VERSION | VERSION:2.0 | iCalendar specification version (always 2.0 for RFC 5545) |
Line 3 | variable | PRODID | PRODID:-//Google Inc//Google Calendar//EN | Product identifier of the application that created the file |
Last line | 14 bytes | END marker | END:VCALENDAR | End of the iCalendar data stream |
Attack Vectors
- Malicious URLs in DESCRIPTION or URL properties leading to phishing sites
- Oversized ICS files with thousands of VEVENT entries causing denial-of-service on import
Mitigation: FileDex does not open, execute, or parse these files. Reference page only.