What Is an ICS File? (iCal / iCalendar .ics Explained)
An ICS file (also called iCal or iCalendar) is a standard calendar file format used to share events between apps like Google Calendar, Microsoft Outlook, and Apple Calendar.
If you have an .ics attachment that won’t import—or imports with the wrong time—start here:
- Validate your ICS file (what’s wrong?)
- Fix your ICS file (get a repaired download when available)
- ICS validator guide: validate ICS (what validators check)
- iCal validator (iCalendar / .ics) (same format, different wording)
What does an ICS file contain?
Most ICS files have:
- a
VCALENDARwrapper - one or more
VEVENTblocks (each event) - required properties like
DTSTART,UID, andVERSION - optional (but common) time zone rules (
TZID/VTIMEZONE) - optional recurrence rules (
RRULE)
Even small formatting problems can make imports fail silently in Google/Outlook/Apple.
Why ICS files fail to import (common reasons)
If you see “invalid iCalendar file” or nothing happens on import, it’s usually one of these:
- missing
BEGIN:VCALENDAR/END:VCALENDAR - missing
DTSTARTor invalid date formats - invalid or ambiguous
TZID(e.g.,TZID=EST) - missing/broken
VTIMEZONEblocks (especially for Outlook Desktop/Exchange) - broken line folding (long lines split incorrectly)
- malformed
RRULErecurrence rules
Next steps:
- ICS file won’t import (Google Calendar + Outlook)
- Fix “Invalid ICS File” errors
- Fix ICS timezone errors (events at the wrong time)
Which apps support ICS?
ICS is widely supported, but import behavior differs:
- Google Calendar: strict about parsing; can fail silently on invalid files.
- Outlook: desktop + Exchange are strict about time zones and VTIMEZONE.
- Apple Calendar: sometimes more tolerant, but can still misread time zones and recurrence.
If you want app-by-app differences and a checklist, see: Common ICS import errors by calendar app.
Fix your .ics file in seconds
Upload an iCalendar file and get a clean, import-ready version for Google Calendar, Outlook, Apple Calendar, and Teams.