opencompany · Nov 1, 2026 · 19m · 12m saved
With Peer Richelsen, Co-Founder of Cal.com
Cal.com, the scheduling company, has passed about 1.3 million signups and grows 2.5 to 3 times a year, mostly from customers who sign themselves up. Peer Richelsen, co-founder of Cal.com, told opencompany that the company is now moving away from the open-source model it was built on.
Cal.com started as an open-source alternative to Calendly. In April 2026 it stopped publishing its full application and kept only a do-it-yourself version open, because Peer and his team concluded AI has made open code far easier to attack.
"The whole way we've done open source in the past 20 years is not going to be the next 20 years. Zero chance." — Peer Richelsen
Peer built the first version over a weekend while running a different startup, and handed it to the stranger who became his co-founder. Community members who invested in the seed and Series A rounds can check the company's numbers for themselves.
The full interview is covered here so you can skip it. 19 minutes of audio, 7 minutes of reading.
Here are the 9 takeaways that matter.
Key Takeaways
Cal.com grows 2.5-3x a year, mostly through self-serve signups rather than sales
About 1.3 to 1.4 million people have signed up
Cal.com closed its full app's source code in April 2026, citing AI-driven security risk
It handles financial, healthcare and meeting data
AI favors the engineer who decides what to build, not the one who only carries out orders
There is no single finish line like an IPO, Peer says, because the running is the part
The missing piece for most AI apps is the network and data behind them, not the interface
Cal.com began as a weekend project after a search for an open-source Calendly found nothing
1. A Weekend Calendly Clone
Peer's first company traded Magic and Pokémon cards and collapsed when COVID hit that market in 2020. He moved on to LeanHire.com, a marketplace for hiring contractors, and needed a scheduling tool for it.
Peer built the first Cal.com in a weekend because no open-source Calendly existed
"I did just some basic research and literally went on Google, like 'Calendly open source'... and was just met with no results. And I think that weekend I just started building it." — Peer Richelsen
The first version worked only with Google Calendar and sat on GitHub as a side project.
