Calendar APIs allow businesses to integrate applications with calendars across service providers, such as Microsoft Outlook and Google Calendar. Without calendar APIs, manually building scheduling connectivities within an app or website exhausts resources and time.
Using calendar APIs means development teams can focus on building mission-critical features that maximize adoption and retention. Businesses can save time while increasing scheduling productivity for their end users and easily capture valuable insights.
But determining which calendar API will best optimize your scheduling workflows can be difficult. So today, we’ll cover the best calendar APIs on the market so you can find the right one for your business.
Quick summary: top calendar APIs
Nylas Calendar API – Universal calendar integration connecting to Google, Microsoft, and Apple calendars through a single API. Best for: Developers building production apps needing multi-provider sync.
Cronofy Calendar API – Multi-provider calendar connectivity with real-time sync and embeddable UI elements. Best for: Teams requiring two-way calendar sync with scheduling UI.
Aurinko API – Unified API for email, calendar, contacts, and tasks with pre-built CRM sync logic. Best for: Teams building CRM or ATS products needing bi-directional sync.
Calendly API – Read-only API for embedding scheduling pages and retrieving meeting data. Best for: Businesses embedding simple appointment booking on websites.
Google Calendar API – Native integration for Google Workspace calendars with full CRUD operations. Best for: Apps exclusively serving Google Workspace users.
Cal.com API – Hosted scheduling platform with a free tier; Cal.diy is the MIT-licensed self-hosted alternative. Best for: Teams wanting a Calendly alternative or developers self-hosting via Cal.diy.
Outlook Calendar API – Native Microsoft 365 calendar integration. Best for: Apps exclusively serving Microsoft 365 users.
What is a calendar API?
A calendar API is a programming interface that allows developers to integrate calendar functionality, such as event creation, scheduling, and syncing, into applications.
With the help of calendar APIs, developers can easily centralize scheduling workflows within a platform, enabling users to create and manage calendar events, invite attendees, sync calendars, set reminders, and more.
However, not all calendar APIs are created equal. Calendar solutions for embedding scheduling capabilities in an application or website should offer a wide range of customization options to fit unique business needs.
Many calendar APIs are missing crucial capabilities that can roadblock your integration efforts. For example, while some calendar APIs offer a free/busy resource showing a single user’s availability, you may not be able to compare the availability of multiple individuals easily.
Similarly, if you want to find availability for multiple meetings with several participants, without the right calendar solution, this functionality can require additional development effort and custom code.
To avoid spending invaluable developer time and resources, you’ll want to opt for a calendar API that offers availability endpoints. Doing so will allow users to see shared availability across multiple parties, both internal and external, to an organization. And consecutive availability endpoints provide time slots across participants for multiple, consecutive meetings.
Availability is just one example of features you may be looking for out of a calendar API. When looking for the best calendar solution, it’s important to consider the needs of your users.
Below we’ve outlined the top calendar APIs on the market, highlighting how they work, key features, and pricing structures to help you determine the best fit for your organization.
Calendar API comparison table
API Name
Supported Providers
Key Features
Pricing Tier
Nylas Calendar API
Google, Microsoft, Apple
Availability endpoints, virtual calendars, service accounts
Best for: Developers building production applications that need universal calendar connectivity across Google, Microsoft, and Apple.
Nylas Calendar API is a universal calendar integration platform that connects applications to Google, Microsoft, and Apple calendars through a single API. It provides developers the flexibility to quickly and securely build integrations and customize workflows for your users, offering a full scheduling workflow from availability queries to embedded booking UI. By helping users automate their scheduling workflows, the Nylas Calendar API removes the friction that drops off users mid-flow, which supports retention and CSAT.
The Nylas platform covers email, calendar, and contacts through one integration, so teams building products where scheduling connects to email context or contact data don’t stitch together separate APIs. Teams building AI agent products can extend the platform with Agent Accounts, which give agents their own calendar identities to schedule from.
Key features:
Availability endpoints: Remove the back-and-forth with automated scheduling based on free/busy availability
Customizable workflows: Configure scheduling workflows with custom integrations and build native experiences
Eliminates siloed data: Capture calendar data in applications and unlock valuable insights to build automated user workflows
Automated reminders: Send alerts and notifications for upcoming events across email, Slack, and Microsoft Teams
Virtual calendars: Schedule events for people and objects or resources that aren’t associated with an existing calendar account
Service accounts: Onboard entire organizations instantly through service accounts, without requiring individual user OAuth flows
Customizable scheduling UI: Tailor scheduling tools with a design unique to your business
Best for: Teams requiring real-time two-way calendar sync with embeddable scheduling UI components.
Cronofy is a calendar API platform that connects applications with multiple calendar service providers, including Google Calendar, Microsoft Outlook, and Apple Calendar. The API provides developers with guides and SDKs to assist them in navigating the platform. Cronofy offers real-time availability checking, which helps users avoid scheduling conflicts.
The API also includes UI elements that can be embedded into an application and the ability to send out calendar invites with video conferencing details.
Key features:
Enables users to build scheduling features into calendars
Enables users to build real-time, two-way sync across calendars
Full create, read, update, delete (CRUD) operations that allow Cronofy to read and delete individual events, on behalf of the user, created by the API
Pricing:
Cronofy Calendar API pricing has shifted to a usage-based model in 2026, with a $99/month minimum commitment for standalone use.
For teams that prefer plan-based pricing, Cronofy’s Emerging API plan starts at $819/month annually, or $139/month for the Starter tier.
Best for: Teams building CRM, ATS, or PM platforms that need pre-built email and calendar sync into Salesforce, HubSpot, or SugarCRM.
Aurinko is a unified API platform for email, calendar, contacts, and tasks across Gmail, Outlook, Office 365, iCloud, and Exchange. Its BrightSync product ships pre-built sync logic for CRM and PM platforms, so teams building products like Salesforce or HubSpot integrations get bi-directional activity capture without writing sync code from scratch. Aurinko also offers Gmail and Outlook add-ons that let CRM users log activity from inside their inbox.
Key features:
Unified API access to email, calendar, contacts, and tasks across major providers
Best for: Businesses embedding simple appointment booking functionality on websites without complex integration needs.
Calendly is a scheduling automation platform with a REST-based API that allows businesses to retrieve Calendly links, report on scheduled meetings, and retrieve availability for a specific event type for team members within an organization. The platform also has an Embed API, which lets businesses add a user’s Calendly scheduling page to their websites by enabling inline, pop-up text, or pop-up widget variations.
Calendly’s API is read-only, intended to make it easier for businesses to embed calendar functionality on web pages. For example, it can only read the free/busy status of events in users’ calendars. For organizations looking to extract scheduling data, you’ll want to consider an API that uses event metadata to surface deeper insights.
Key features:
Automates event scheduling, canceling, and rescheduling via webhooks
Enables users to embed Calendly API via OAuth or personal access tokens
Admins can report on events and access information about event types
Pricing:
Calendly is primarily feature-based and offers four pricing models:
Basic: Free
Standard: $10 per user per month (annually) | or $12 per user per month (monthly)
Teams: $16 per user per month (annually) | or $20 per user per month (monthly)
Enterprise: Custom quote basis
5. Google Calendar API
Best for: Applications exclusively serving Google Workspace users who need native calendar integration.
The Google Calendar API provides full CRUD operations for calendar events, calendar settings management, and calendar metadata access. The API handles single or recurring events, calendar management, user preferences, and access control rules.
Google’s calendar API follows RESTful principles and can be accessed via explicit HTTP calls or through the use of Google Client Libraries. The API uses OAuth 2.0 for authentication and supports multiple programming languages.
Key features:
Enables users to add events, including recurring events
Users can specify and manage a working location where others can view before invitation to an event
Push notifications to inform users of changes
Provides extended properties which enables users to store data without an external database
Pricing:
As a support tool, Google Calendar is free to use. However, it doesn’t have as many features or capabilities as other paid options, like Nylas.
6. Cal.com API and Cal.diy
Best for: Teams wanting a Calendly alternative with more customization, and developers who want to self-host a scheduling platform.
Cal.com is a scheduling platform with an API and embeddable booking components. In April 2026, Cal.com moved its commercial product from open source to closed source, citing AI-powered security threats. The company simultaneously launched Cal.diy, an MIT-licensed open-source version aimed at hobbyists and self-hosters. Cal.diy is positioned for personal, non-production use.
For teams evaluating Cal.com in 2026, the choice is between the hosted commercial product (closed source, supported, enterprise features) and Cal.diy (open source, self-hosted, community-maintained).
Key features:
Managed booking pages, event types, and workflows on the hosted Cal.com platform
API access for event and booking CRUD, availability queries, and team management
Native integrations with Google Calendar, Microsoft 365, Apple, and CalDAV
Cal.diy provides a self-hostable open-source alternative for developers who need full code access
Best for: Applications exclusively serving Microsoft 365 users who need native Outlook integration.
The Microsoft Outlook Calendar API is a solution that caters to personal, educational, and business services. It’s linked with Outlook email services and helps users integrate and manage calendar events and appointments.
The Outlook Calendar API is synced with other Microsoft 365 products and enables access to phone and email contacts, as well as OneDrive.
Key features:
Create, manage and respond to events
Track changes to an event and decline or accept a meeting
Share a calendar and delegate a calendar to another user
Book meetings at convenient times by enabling free/busy availability check
Schedule recurring events
Pricing:
Microsoft Outlook calendar API is free for personal use but offers a subscription-based model for the business and education sectors.
Summary of top calendar APIs
Are you looking to further narrow down your options? We’ve categorized the top calendar APIs by integrations below.
Calendar APIs that connect with Microsoft Outlook calendars
Nylas
Cronofy
Aurinko
Calendly
Cal.com
Outlook Calendar API
Use the best calendar API
The best calendar API is going to depend on what your business is looking for. If you need to quickly add scheduling and other calendar capabilities to your business application, while saving your developers thousands of hours of building time, Nylas is the top choice.
Google Calendar API is the best free option for applications serving only Google Workspace users. For multi-provider support, Cal.com offers a free tier with basic functionality, and Aurinko offers a 14-day free trial with no credit card required.
Which calendar APIs support both Google and Outlook?
Nylas, Cronofy, Aurinko, Calendly, and Cal.com all support integration with both Google Calendar and Microsoft Outlook calendars.
What is the difference between a calendar API and a scheduling API?
A calendar API provides access to calendar data (events, availability, settings), while a scheduling API focuses specifically on booking workflows. Many platforms, like Nylas and Cronofy, offer both capabilities.
Can I use a calendar API to sync multiple calendar providers?
Yes. Universal calendar APIs like Nylas, Cronofy, and Aurinko allow you to connect to Google, Microsoft, and Apple calendars through a single integration point.
How much does a calendar API cost?
Pricing varies widely. Google Calendar API is free, Aurinko starts at $1/account/month with data-transfer metering, Calendly starts at $10/user/month, Cronofy has a $99/month minimum, and enterprise solutions like Nylas offer custom pricing based on connected accounts.
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