27 Calendar APIs to Integrate With: Google Calendar, Outlook, Calendly and Unified Scheduling APIs
March 25, 2026
Calendar and scheduling integrations sit at the center of many SaaS products.
From booking meetings and syncing calendars to powering AI meeting assistants and scheduling workflows, products increasingly rely on APIs from platforms like Google Calendar, Microsoft Outlook, and tools like Calendly.
The challenge is that every calendar provider handles events, availability, meetings, and scheduling logic differently.
In this guide, we'll cover 27+ calendar APIs developers integrate with, the complexity of building scheduling integrations, and how unified calendar APIs simplify real-time event data access.
What is a Calendar API?
A calendar API allows applications to read and write scheduling data across calendar platforms.
Typical capabilities include:
- creating and updating events
- retrieving calendar data
- checking availability (free/busy)
- managing attendees
- generating scheduling links
- accessing meeting recordings or transcripts
- handling recurring events
These APIs power scheduling tools, productivity apps, CRMs, AI assistants, and workflow automation systems.
Why SaaS products integrate with calendar APIs
Calendar integrations are critical across multiple product categories.
Common use cases include:
Scheduling and booking tools
Find availability and create meetings across Google Calendar, Outlook, and scheduling platforms.
AI meeting assistants
Access events, transcripts, and recordings to summarize meetings and extract insights.
CRM and sales workflows
Sync meetings with customer records and sales activity.
Internal productivity tools
Track meetings, time usage, and availability across teams.
Workflow automation
Trigger actions when meetings are created, updated, or canceled.
Resource scheduling
Manage rooms, equipment, or shared resources.
Because calendar data is real-time and user-facing, accuracy and reliability matter.
27+ calendar APIs developers integrate with
Below are the most commonly integrated calendar and scheduling platforms.
Google Calendar API
One of the most widely used calendar systems.
Supports:
- events and calendars
- free/busy availability
- attendees and meeting metadata
Microsoft Outlook / Office 365 API
Microsoft Graph Calendar API
Used across enterprise environments.
Supports:
- events and scheduling
- availability data
- meeting coordination
Calendly API
Scheduling platform for booking meetings.
Used for:
- scheduling links
- booking workflows
- availability management
Cal.com API
Open scheduling infrastructure used by many SaaS products.
Zoom API
Used for meetings, webinars, and recordings.
Google Meet API
Used for meeting scheduling and conferencing.
Microsoft Teams API
Supports meetings and collaboration workflows.
Acuity Scheduling API
Used for appointment booking and scheduling.
Apple iCloud Calendar API
Used for personal calendar integrations.
Webex API
Enterprise meetings and conferencing.
RingCentral API
Scheduling and communications platform.
Zoho Calendar API
Zoho Calendar
Zoho Meeting API
Zoho Meeting
Square Appointments API
Salesforce Calendar API
Additional platforms
Other commonly integrated tools include:
The scheduling ecosystem spans calendars, meeting tools, and booking platforms, making coverage important.
Challenges when integrating calendar APIs
Calendar integrations introduce complexity that most teams underestimate.
Different event models
Each platform represents events differently (fields, attendees, recurrence rules).
Availability inconsistencies
Free/busy data varies by provider and can behave differently across edge cases.
Recurring events
Handling recurrence rules consistently across platforms is non-trivial.
Meeting vs scheduling separation
Some platforms manage events, others manage booking workflows separately.
Real-time expectations
Calendar data must be accurate in real time. Delayed syncs lead to double-booking and broken workflows.
Complex authentication
OAuth flows differ across providers and often require careful scope handling.
As you support more platforms, these differences become a maintenance burden.
The role of unified calendar APIs
A unified calendar API standardizes how you interact with calendar platforms.
Instead of building separate integrations for Google Calendar, Outlook, Calendly, and others, you:
- integrate once
- use consistent event models
- handle availability in a unified way
- manage scheduling logic consistently
- reduce integration maintenance
This is especially important for products that depend on accurate scheduling and availability.
Build once with the Unified Calendar API
The Unified Calendar API enables access to 27+ calendar and scheduling integrations through a single API.
Supported platforms include:
- Google Calendar
- Microsoft Outlook
- Calendly
- Zoom
- Microsoft Teams
- Webex
- Zoho Calendar
- and many more
Unified calendar objects
Unified standardizes scheduling data into core objects:
Calendar
Represents calendar accounts and configurations.
Event
Represents meetings, appointments, and scheduled events.
Busy
Represents availability and time blocks.
Link
Represents scheduling links and booking pages.
Recording
Represents meeting recordings and transcripts.
Webinar
Represents webinar events and registrations.
These objects support consistent methods across all platforms.
Why teams choose Unified.to for calendar integrations
Real-time, pass-through architecture
Every request hits the source platform live. No caching, no stale availability data.
Zero-storage by design
Calendar and meeting data is never stored at rest, reducing risk and compliance overhead.
Unified availability (Free/Busy)
Check availability across multiple calendar providers using a single API.
Scheduling without building infrastructure
Use pre-built components or APIs to handle booking and scheduling flows.
Built for AI workflows
Calendar data can be used for:
- AI meeting assistants
- scheduling copilots
- automated workflows
- transcript analysis
Common use cases for a unified calendar API
Scheduling tools
Build booking systems that work across Google Calendar, Outlook, and scheduling platforms.
AI meeting assistants
Access events, recordings, and transcripts to generate summaries and insights.
Calendar sync tools
Keep multiple calendars in sync across platforms.
Availability-based workflows
Trigger actions based on availability or scheduled events.
Resource scheduling
Manage rooms, equipment, or shared resources across calendars.
Final thoughts
Calendar integrations are deceptively complex.
Even though most platforms deal with events and availability, differences in data models, scheduling logic, and real-time requirements make scaling integrations difficult.
A unified calendar API simplifies this by providing a consistent interface across all platforms.
Instead of managing dozens of integrations, you build once and support all major calendar and scheduling systems.
That allows your team to focus on building scheduling features, not maintaining infrastructure.