You have calendars in more than one place and you want your busy time to show up in all of them. Keeper.sh, which costs $5 a month however many calendars you connect, and Kalender Sync, a Munich product, 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.
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, they accept an invitation, and your blocks keep appearing in their calendar from then on. 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. GMX, WEB.DE and mailbox.org are common German consumer mailboxes.
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", real time. 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, and their AVV is included with Business and available on request otherwise.
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.
Where Keeper.sh fits 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.
Any CalDAV server. 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.
What their comparison gets wrong about Keeper.sh Free
Their page describes Keeper.sh Free as a 30-minute sync, which is right about half of it. We read your calendars every minute on every plan, including Free. The 30 minutes is how often the free plan writes the copies out, and on Pro both halves run every minute.
Everything else on their page matches what we ship, including their description of self-hosting.
What Kalender Sync has that Keeper.sh does not
Paperwork. They offer a signed AVV under Art. 28 DSGVO from Munich's PPJ Venture Labs UG (HRB 308771); Keeper.sh publishes no AVV or DPA for the hosted service.
German hosting. Databases at Hetzner in Falkenstein, with tokens encrypted at rest.
Teams. A quote-based Team plan from five users adds central management, single sign-on, audit logs and invoiced billing.
Speed and Sharings. Covered above.
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 how some organisations prefer to settle the paperwork question.
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?
Keeper.sh, for $5 flat however many calendars you connect, event titles stripped by default, and Fastmail working today where theirs is still on a roadmap. Kalender Sync has the signed German data agreement, sharing into other people's calendars, a team plan and a German-language product.
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.
Other comparisons
Still weighing tools? These cover the rest of the field.
- Keeper.sh vs CalendarBridge
- Keeper.sh vs Morgen
- Keeper.sh vs OneCal
- Keeper.sh vs Reclaim.ai
- Keeper.sh vs SyncDate
- Keeper.sh vs SyncThemCalendars
- Nine calendar sync tools compared on price, speed and privacy
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.