You have calendars in more than one place and you want your busy time to show up in all of them. Kalender Sync, a Munich product, and Keeper.sh, which is open source and yours to run, both do that job.
They wrote a comparison of us first, and it is accurate about most of what we ship. Here is ours, with a link behind every figure.
What if your employer needs paperwork signed?
Kalender Sync is the shorter route when approval hinges on a signed German data processing agreement.
They publish an AVV, the agreement required under Article 28 of the GDPR. Their page for IT administrators offers "einen signierten AVV nach Art. 28 DSGVO" as a pre-filled PDF, included with the Business plan and available on request otherwise.
The contracting party is named and European. Their Impressum lists PPJ Venture Labs UG (haftungsbeschränkt), Hohenzollernstraße 30, Munich, registered as HRB 308771 at the Amtsgericht München.
They also state where the data sits: "Alle Datenbanken laufen bei Hetzner Online GmbH in Falkenstein (Sachsen)", with tokens encrypted using ChaCha20-Poly1305. Their marketing site runs on Netlify in the United States, which they disclose rather than gloss over.
Keeper.sh publishes no AVV or DPA for the hosted service. The answer here is a different shape: run Keeper.sh on your own server, in whichever country you like. Nobody else is in the path to sign an agreement with.
Can you share your busy time into someone else's calendar?
Kalender Sync places it there for you. They call it Sharings.
You create one for another person, and their site describes it as: "erstelle ein Sharing für jemand anderen. Die Person wird eingeladen, fügt es ihrem Kalender hinzu, und ab da läuft alles automatisch synchron."
So the other person is invited, adds it, and your blocks keep appearing in their calendar. You pick from three visibility levels, from availability only, through title and time, up to full details. Their free plan includes one Sharing.
Keeper.sh connects calendars you own. To let your assistant or your partner see when you are busy, you hand them your private calendar link, which they can subscribe to and read. You choose which of your calendars go into that link, and it lands in their calendar app as a separate calendar rather than merging into their own.
Which calendars does each one connect?
Kalender Sync's integrations page lists Google Calendar, Microsoft Outlook, Apple iCloud, Infomaniak kSuite, GMX, WEB.DE, mailbox.org, and read-only iCal feeds. Fastmail, Proton Calendar, Yahoo, Nextcloud and Posteo are listed as planned rather than live. That GMX, WEB.DE and mailbox.org coverage is deliberate and useful, because those are common German consumer mailboxes that most sync tools ignore.
Keeper.sh supports Google, Outlook, iCloud, Fastmail, and any CalDAV server you can give us an address for. Since GMX, WEB.DE and mailbox.org all speak CalDAV, you can connect them here too, by entering the server details yourself rather than picking a logo. We also read iCal and ICS links, and like them, cannot write back to those.
So they have Fastmail on a roadmap where we have it shipped, and we have an open CalDAV field where they have a curated list. Curated is friendlier. Open covers more.
How fast does a change reach your other calendar?
On paid plans Kalender Sync markets Google and Microsoft updates as "Echtzeit per Webhook". Their documentation is more precise, describing those two as "nahezu sofort", nearly instant.
For their other calendars, including iCloud, GMX, WEB.DE and mailbox.org, the same documentation says they are checked "alle 5 Minuten". Their free plan runs on a 30-minute interval and covers Google and Outlook only.
Keeper.sh reads every calendar every minute, on every plan including Free, and writes copies out every minute on Pro or every 30 minutes on Free.
So a change reaches your other calendar in about a minute on Pro, two at the outside. On Free it is about fifteen minutes, thirty-one at the outside — and reads still happen every minute, on both plans.
Nobody is instant on iCloud, GMX or mailbox.org. Those calendars offer no way for a tool to be told an event changed, so both products check them on a timer: every minute here, every five on theirs.
What does each one cost?
| Keeper.sh | Kalender Sync | |
|---|---|---|
| Free plan | 2 linked accounts, 3 connections, per-minute reads | 2 accounts, 1 Sharing, Google and Outlook, 30-minute interval |
| Entry paid plan | $5/month, unlimited connections | €4/month, 5 accounts, 5 syncs, 5 Sharings |
| Higher paid plan | None | €10/month, unlimited accounts, syncs and Sharings |
| Team plan | None | Custom, from 5 users |
| Yearly option | Not offered | €40 and €100 per year |
All their plans come with a 14-day trial of Business features and no card.
At the entry tier the prices are close enough not to matter. Above five connections Keeper.sh Pro is cheaper than their Business plan, because $5 covers as many as you like. Below that they are competitive, and their tiers include Sharings.
What if you are buying for a company?
Kalender Sync sells to the person who administers everyone else's accounts.
Their pricing page lists central management of employee accounts, "SSO mit Entra ID & Google Workspace", organisation-wide policies and immediate offboarding. It adds invoiced billing and "AVV, Audit-Log, Priority-Support & persönliches Onboarding", quote-based from five users.
Keeper.sh is priced and built for one person: $5 a month, however many calendars you connect. Companies that need central control of everyone's accounts today should take their Team plan.
Where is Keeper.sh genuinely better?
It is open source. Keeper.sh is AGPL-3.0 and the whole sync engine is on GitHub. Their comparison page notes this favourably.
Every CalDAV server, not a list. Covered above.
An API and an agent connector on the free plan. Keeper.sh has a REST API, and an MCP server with 14 tools, so an assistant can list calendars, read events, and also create, edit, delete and RSVP. Both are free, capped at 25 requests a day, and uncapped on Pro.
One feed for every calendar. Keeper.sh gives you a single private iCal link merging the calendars you pick, readable by anything that takes a calendar feed.
Your event details stay put by default. A new connection copies a busy block titled after the calendar it came from, leaving the title, description and location behind. Showing the real detail is a choice you make on Pro.
Per-minute reads on the free plan. Their free tier checks every 30 minutes and covers two providers.
One correction
Their comparison describes Keeper.sh Free as a 30-minute sync, which is right about half of the job. We read your calendars every minute on every plan, including Free, and the 30 minutes is how often the free plan writes the copies out. On Pro, both halves run every minute.
Everything else on their page matches what we ship, including their description of self-hosting.
Running Keeper.sh yourself
Every Pro feature turns on automatically on your own instance, including per-minute writes and the uncapped API. The calendar data never leaves the server you control, which is the version of the paperwork question some organisations prefer.
It is real work, and their comparison page describes the effort accurately. You need Docker and Docker Compose, Postgres, Redis and a domain, plus your own Google and Microsoft applications, which is the fiddliest step.
Most people should use hosted Keeper.sh at $5 a month. Self-hosting is there when you need it, and the repository has the full guide.
FAQ
Which should I choose?
Choose Kalender Sync if you need a signed AVV or an EU contracting party. It also wins if you share availability with other people, buy for a team, or want the product in German. Choose Keeper.sh if you want open source and self-hosting, need CalDAV or Fastmail, or want an API and agent access.
Can I try both?
Yes. Keeper.sh has a free plan that does not expire. Kalender Sync has a free plan and a 14-day trial of Business features without a card.
Sources
Everything above about Kalender Sync comes from their own public pages, checked on the date this post was published. Quotes are in the original German:
- kalender-sync.de/preise — plans, prices and the Team tier
- kalender-sync.de/integrationen — supported and planned providers
- kalender-sync.de/it-admin — the AVV, hosting and encryption
- kalender-sync.de/impressum — company details
- docs.kalender-sync.de/sync/how-it-works — update intervals
- docs.kalender-sync.de/sharings/self-sync — Sharings and Self-Sync
- kalender-sync.de/vergleich/keeper — their comparison with us
Prices and features change. If something here is out of date, their page wins, and we would appreciate you telling us.