Why We Built Caltsu

I have four calendars: three for clients on Outlook, and a personal one on Google Calendar.

For about two years, my "system" was to check all four before accepting any meeting. It worked, until it didn't. Around the third time I showed up to a client call only to realize I'd double-booked it with a dentist appointment, I decided something had to change.

The options weren't great

I started looking at what was out there. The market pretty much falls into two camps:

Camp 1: AI scheduling assistants. Tools like Reclaim.ai and Morgen want to manage your entire day. They'll find focus time, move your tasks around, and optimize your schedule. They're good at it, if that's what you want. But I don't want an AI managing my day. I just want my calendars to talk to each other.

Camp 2: Calendar sync tools. These were closer. But most of them work by syncing your actual event details between accounts. Your "Therapy — Dr. Korhonen" appointment on Google Calendar gets copied, title and all, to your work Outlook. Now your client can see it. Maybe even the description.

For freelancers and consultants, that's a dealbreaker. Client A should never see that you have a "Project kickoff with Client B" on your calendar. Your team's shared calendar doesn't need to know about your personal appointments.

Some tools let you rename events to "Busy," but it's a workaround. The tool still reads and processes your full event data. It just puts a mask on it at the other end.

What I actually needed

The requirements were simple. I needed a tool that would:

1. Sync my busy/free status across Google, Outlook, and Apple calendars.
2. Never, ever copy event details between accounts.
3. Sync fast enough that a new meeting blocks off the time on my other calendars before someone else can grab it.
4. Work in the background without me having to think about it.

That third point is more important than it sounds. Most sync tools poll for changes every 15 or 30 minutes. That gap is a window where double-bookings are waiting to happen. If you've ever had two different people book back-to-back meetings with you within minutes of each other, you know the feeling.

So I built it

Caltsu connects to your Google, Outlook, and Apple accounts. When a new event appears on one calendar, it creates a simple, private block on the others. The block just says "Busy." That's it. No title, no description, no attendees, no meeting link.

For Pro users, we use webhooks instead of polling. Google and Outlook send a push notification the instant an event changes. That means the sync happens in seconds, not minutes.

The free tier gets you two accounts and one sync rule, polling every 15 minutes. Pro is €7 per month for up to 10 accounts, unlimited rules, and real-time, webhook-powered sync.

Privacy is the default, not a feature

A lot of tools treat privacy as a premium feature or a toggle in the settings. We think that's backwards.

Caltsu never reads your event details. We don't have access to titles, descriptions, attendees, or locations. We only see the time range and its busy/free status. This isn't a setting you turn on — it's how the sync works at a fundamental level.

We can't leak data we don't have.

Who it's for

Caltsu is for the freelancers juggling client accounts. The consultants who need to keep work and personal separate. It's for anyone with more than one calendar who's tired of checking all of them before saying "yes" to a meeting.

If you want an AI assistant to optimize your day, there are great tools for that. Caltsu isn't one of them. We do one thing: make sure your calendars agree on when you're busy.

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Caltsu is launching on Product Hunt soon. Try it free at caltsu.com.