Which apps can open an .ics file?

All major calendar apps, including Google Calendar, Apple Calendar, and Microsoft Outlook, can import iCalendar files because the format is a shared standard.

How are recurring events stored?

A recurring series is described by an RRULE on the event, rather than one entry per occurrence. The calendar app expands the rule into individual dates when it displays them.

Does the calendar merger upload my files?

No. Calendar files are merged locally in your browser tab and are never uploaded.

Explainer1 min readUpdated June 26, 2026

What is an .ics calendar file?

Short answer

An .ics file is a calendar in the iCalendar format (RFC 5545), a plain-text standard that apps like Google Calendar, Apple Calendar, and Outlook all understand. It holds one or more events, each with a start, end, summary, and a unique id.

What is inside

Despite the calendar appearance in your app, an .ics file is just structured text. A VCALENDAR wrapper contains one or more VEVENT blocks, each describing a single event with fields for its time, title, and identity.

BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
UID:9f1c-2026-team-sync@example.com
DTSTART:20260629T090000Z
DTEND:20260629T093000Z
SUMMARY:Team sync
END:VEVENT
END:VCALENDAR

A single event in an .ics file

Key fields

DTSTART and DTEND — when the event begins and ends
SUMMARY — the event title
UID — a unique identifier that lets apps recognize the same event across files
RRULE — an optional rule that defines a repeating series

The UID prevents duplicatesBecause each event carries a stable UID, importing the same calendar twice should update the existing events rather than create copies. Merging tools rely on the UID to spot duplicates.

Try it: ICS Calendar MergerCombine several .ics files into one calendar locally, with duplicate events detected by UID.Open tool