Send Calendar Invites built on the .ics standard
Every calendar invite Let's Calendar sends is a perfectly formatted .ics file — the universal calendar standard used by Google Calendar and Outlook, and the native format behind ical on Apple devices.
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.ics file & ical format — complete guide
Overview
What is an .ics file?
An .ics file is a standard calendar file format used to share event and meeting details across calendar applications like Google Calendar and Microsoft Outlook. It allows users to add events directly to their calendars with a single click.
These files contain important event information such as date, time, location, organizer details, attendees, and recurring schedules. Since the .ics format is supported by most calendar platforms, it is widely used for sending calendar invites and event schedules.
1998
Year the .ics format was standardised (RFC 2445)
100%
Universal, works with every major calendar app
.ics
The file extension for all iCalendar files
RFC 5545
The current internet standard defining the .ics format
Anatomy
What's inside an .ics file?
An .ics file is structured in specific blocks. It wraps event data inside VCALENDAR and VEVENT sections. Each piece of event information is a key-value pair on its own line making the file both human-readable and machine-readable.
Fields inside every .ics file:
SUMMARY
Event Title: The name of the meeting or event shown in the attendee's calendar
DTSTART / DTEND
Date & Time: Start and end timestamps in UTC format, time-zone safe across every country
LOCATION
Location or Meeting Link: Physical address or a Zoom / Google Meet link — clickable directly from the calendar
ORGANIZER
Sender Identity: The email of the person sending the invite with Let's Calendar, this is always your own email
ATTENDEE
Attendee Email: Each recipient is a separate ATTENDEE entry enabling private 1:1 invites at scale
RRULE
Recurrence Rule: Defines how often the event repeats weekly, monthly, daily encoded in the .ics format
ics format — key properties table:
| Property | Required | Let's Calendar |
|---|---|---|
| SUMMARY | Yes | Auto-filled |
| DTSTART / DTEND | Yes | Auto-filled |
| ORGANIZER | Yes | Your email |
| LOCATION | Optional | Auto-filled |
| RRULE | Optional | Supported |
| SEQUENCE | For updates | Auto-managed |
Compatibility
.ics files work everywhere
Because the ics format is a universal open standard, every calendar invite sent by Let's Calendar works across all platforms desktop, mobile, and web.
Compatibility Table:
| Calendar App | ICS Support | 1-Click Accept |
|---|---|---|
| Google Calendar | ✓ Full | ✓ Yes |
| Microsoft Outlook | ✓ Full | ✓ Yes |
| Yahoo Calendar | ✓ Full | ✓ Yes |
| Samsung Calendar | ✓ Full | ✓ Yes |
| Thunderbird | ✓ Full | ✓ Yes |
| Android Calendar | ✓ Full | ✓ Yes |
| iOS Calendar | ✓ Full | ✓ Yes |
Why .ics is more reliable than a plain email
A plain email with meeting details can be missed or forgotten. An .ics-based calendar invite lands directly on the attendee's calendar with automatic reminders dramatically improving attendance rates.
Overview
What is ical?
iCal is commonly known as Apple’s calendar application available on Mac, iPhone, and iPad devices. It is built on the iCalendar (.ics) standard, which allows calendar events and meeting invites to work across different platforms and applications.
Because of this universal format, users can easily receive, open, and manage calendar invites with features like reminders, recurring events, and RSVP options across Apple devices.
2002
Year Apple launched iCal on macOS
2012
Renamed "Calendar" still called ical by most users
3 in 1
Works natively on Mac, iPhone, and iPad
RFC 5545
The ical format standard Apple Calendar reads natively
Integration
How ical works on Apple devices
Apple Mail automatically detects calendar invite files and displays native Accept, Decline, or Maybe options directly inside the email. Once accepted, the event is instantly added to the user’s Apple Calendar with all details including time, location, meeting links, descriptions, and recurring schedules.
How ical reads a Let's Calendar invite:
Apple Mail
Native Invite Recognition: Apple Mail on Mac and iPhone reads the ical format in .ics files and displays Accept / Decline / Maybe buttons automatically
ical App
Event Added to Calendar: On accept, the event lands directly in Apple Calendar (ical) on every Apple device the attendee owns Mac, iPhone, and iPad sync automatically
RRULE
Recurring Event Support: ical reads recurring event rules from the .ics file — so all future event sessions appear in Apple Calendar from a single invite.
SEQUENCE
Update Sync: When an event changes, ical reads the updated SEQUENCE number in the new .ics file and updates the existing calendar entry no duplicate events
Features
Everything Let's Calendar Does with ICS Files for You
You focus on the event. We handle every .ics file detail automatically.
Auto-Generated ICS Files
Let's Calendar automatically creates a perfectly valid .ics file for every attendee, no manual encoding or format errors.
Universal iCal Format
Every calendar invite follows the RFC 5545 standard, guaranteed to work in Google, Outlook, and Apple Calendar.
Personalised Per-Attendee .ics
Each .ics file is uniquely generated per recipient with their name and a unique UID for a 1:1 feel.
Recurring Event .ics Support
Set RRULE for weekly, monthly, or custom recurring events encoded directly in the ics format.
Seamless .ics Updates
Smart SEQUENCE increments ensure attendees' calendars update automatically with no duplicates.
RSVP Tracking via .ics
Native tracking of Accept, Decline, and Tentative responses directly in your live dashboard.
Send Your First Free Calendar Invite
No credit card required · Free plan available · Works with Gmail, Outlook & Apple
FAQ
Frequently Asked Questions
An ICS file (extension .ics) is a plain-text file that stores calendar event data in the iCalendar (iCal) format. It is the universal standard for sharing calendar events between applications. When you open or receive an ICS file, your calendar app Google Calendar, Outlook, Apple Calendar reads the iCal format and displays the event. Every calendar invite sent by Let's Calendar is delivered as a perfectly formatted ICS file.
iCal is short for iCalendar, the internet standard (RFC 5545) that defines how calendar event data should be structured and shared. The iCal format uses a text-based structure of key-value pairs, wrapped in BEGIN/END blocks. Any application that supports the iCal format can read and display events created by any other application that follows the standard. which is why ICS files work across all major calendar platforms.
They refer to the same thing. "iCal" or "iCalendar" is the name of the standard and format. "ICS" is the file extension used for files in that format (.ics). So an ICS file is a file written in iCal format, the terms are often used interchangeably.
When an ICS file is sent via email with the property set in the iCal format, the recipient's email client recognises it as a calendar invite. Gmail, Outlook, and Apple Mail all automatically display Accept / Decline / Tentative buttons. When the recipient accepts, the event is added to their calendar with all details from the ICS file.
No. Let's Calendar handles all ICS file generation automatically. You simply fill in your event details in the campaign editor, and Let's Calendar creates a perfectly valid ICS file in iCal format for every single attendee. You never need to write or edit an ICS file manually.
Get Started
Send your first calendar invite powered by .ics, free
No credit card. No .ics files to write. Let's Calendar generates everything in ical format automatically, so your invites land perfectly on Google Calendar, Outlook, and ical on every Apple device.
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Works with Gmail, Outlook, Apple & more
