How Peer Richelsen Built an Open-Source Calendly Alternative with 20K+ GitHub Stars

Peer Richelsen was frustrated with scheduling tools. As a developer, he wanted full control over his scheduling infrastructure — something Calendly and other closed-source tools simply couldn't offer. In early 2021, he launched Calendso, a fully open-source scheduling platform that anyone could self-host, customize, and extend. The idea was radical in its simplicity: take the core value of Calendly and make it transparent, extensible, and free.

The Open-Source Advantage

Calendso's open-source approach immediately resonated with the developer community. Within weeks of launching on GitHub, the project started accumulating stars at a remarkable pace. Developers loved that they could inspect every line of code, contribute improvements, and self-host without vendor lock-in. The project hit 10,000 GitHub stars faster than most open-source tools, eventually surpassing 20,000. Each star represented not just interest but potential advocacy — developers who would recommend Cal.com to their organizations.

The Product Hunt Launch and Rebrand

The Product Hunt launch was a defining moment. Calendso hit #1 Product of the Day, driving thousands of signups in a single day. The visibility attracted both individual users and companies looking for scheduling infrastructure. Shortly after, Peer made a bold move: he purchased the premium domain cal.com and rebranded the entire project. The domain acquisition cost a significant sum, but it signaled ambition and legitimacy. "Cal.com" was short, memorable, and positioned the product as the default scheduling infrastructure — not just another Calendly clone.

Raising $32M and Scaling

The combination of open-source traction, community enthusiasm, and a premium brand attracted serious investor attention. Cal.com raised $32M in funding, which Peer used to build out the team, develop enterprise features, and create a hosted platform alongside the self-hosted option. The freemium model worked elegantly: individual developers could self-host for free, while companies that wanted managed hosting, team features, and support paid for Cal.com's cloud platform. This dual approach — open-source core with commercial cloud — became the engine for sustainable growth.

Building the Scheduling Layer of the Internet

Peer's vision extended beyond a simple scheduling app. He positioned Cal.com as scheduling infrastructure — an API and platform that other companies could build on top of. This infrastructure play attracted enterprise customers who needed white-label scheduling embedded in their own products. By 2022, Cal.com had crossed $100K ARR, had a thriving open-source community contributing code, and was processing millions of bookings. The combination of developer love, open-source distribution, and enterprise revenue created a flywheel that Calendly, despite its massive head start, couldn't easily replicate.