You keep a work calendar and a personal one, and neither knows about the other. Both Keeper.sh and SyncThemCalendars fix that by copying your busy time between them.
They take different shapes. SyncThemCalendars covers three providers. Keeper.sh reaches more kinds of calendar, charges $5 flat however many connections you run, and copies your busy time while your event titles stay behind.
Does either one sync both ways?
Both do, and they get there differently.
Their two-way sync page puts it plainly. One-way copies events from one calendar to another. Two-way "mirrors both calendars", so a change on either side shows up on the other, and it is a per-sync setting you toggle.
On Keeper.sh each connection runs in one direction, so mirroring work and personal takes two connections. Same result on your calendars, one more step at setup. The free plan's three connections cover one mirrored pair plus one more calendar going one way.
Which calendars does each one connect?
SyncThemCalendars supports Google Calendar, Microsoft Outlook and Office 365, and Apple iCloud. Their own FAQ lists those three, and we could not find any mention of generic CalDAV on their site.
Keeper.sh supports Google, Outlook, iCloud, Fastmail and any CalDAV server. That last part is the difference. If your calendar lives at mailbox.org, Posteo, Nextcloud, Infomaniak or a server you run, Keeper.sh can read and write it.
We also read iCal and ICS links. Those links are read-only by design, so nothing can write back to them.
If your calendars are all Google, Outlook or iCloud, none of this affects you.
How fast does a change reach your other calendar?
Their two-way sync page says "Google and Outlook notify us the moment something changes, so copies update in moments." For iCloud, the same page says they check "every few minutes."
Keeper.sh reads every calendar every minute, on every plan including Free. We write the copies out every minute on Pro, and 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.
On iCloud, neither of us is instant. Apple publishes no way for a calendar tool to be told an event changed, so every product in this category checks iCloud on a timer.
What does each one cost?
SyncThemCalendars charges by how many syncs you run, and does not charge for connecting more calendars or accounts. Their three tiers each cover one user, with a 14-day free trial.
| Keeper.sh | SyncThemCalendars | |
|---|---|---|
| Free plan | 2 linked accounts, 3 connections, no card | 14-day trial, no card |
| Entry paid plan | $5/month, unlimited connections | $3.99/month, 5 syncs |
| Middle plan | None | $7.99/month, 16 syncs |
| Top plan | None | $17.99/month, 36 syncs |
| Yearly option | $42/year | $47.88, $95.88, $215.88 |
Paying monthly, run five syncs or fewer and they are cheaper by a dollar. Run more than five and we are cheaper, because $5 on Keeper.sh Pro has no connection limit at all.
Paying yearly there is no crossover. Keeper.sh Pro is $42 a year against their cheapest $47.88, because their yearly price is just twelve monthly ones.
The other real difference is the free plan. Ours never expires, with 2 accounts and 3 connections. Theirs is 14 days, and then they ask for payment.
Where Keeper.sh fits better
More calendar types. CalDAV and Fastmail, as above.
It is open source. Keeper.sh is AGPL-3.0. The sync engine is on GitHub and you can read it before trusting it with your calendar.
You can run it yourself. Every paid feature is free on your own instance. See below.
Unlimited connections for $5. No counting syncs.
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. Pro is what shows the real detail.
An API and an agent connector. Keeper.sh has a REST API, plus an MCP server with 14 tools, so assistants like Claude can list, create, edit, delete and RSVP to events. Both run on the free plan, capped at 25 requests a day, and uncapped on Pro.
One feed for every calendar. Keeper.sh gives you a single private iCal link combining the calendars you choose to include, readable by anything that takes a calendar feed.
What SyncThemCalendars has that Keeper.sh does not
Two-way mirroring in one toggle, where Keeper.sh takes two connections, and Google and Outlook copies that update "in moments" by their own pages.
Seven languages — English, French, German, Japanese, Spanish, Italian and Portuguese — where Keeper.sh is English only.
A lower monthly price at small setups, $3.99 for five syncs against our $5.
Privacy set per sync and per field, where Keeper.sh sets it once on the calendar events come from.
What about data handling?
SyncThemCalendars does not publish a DPA or a security page that we could find. Their privacy policy, dated March 2019, says data is "stored and processed in United States, or where we or our partners, affiliates and third-party providers maintain facilities."
Keeper.sh publishes no data processing agreement for the hosted service. If your employer requires a signed one, run Keeper.sh on your own server instead. Your database, your server, and nobody else in the path to sign an agreement with.
Running Keeper.sh yourself
Running Keeper.sh yourself gets you the full product. Every Pro feature switches on automatically, including per-minute writes and the uncapped API.
Take this route when the data cannot leave your control. You hold the database, the tokens and the logs, and nobody else sits in the path.
It is genuine work. 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 fits if your calendars go beyond Google, Outlook and iCloud, or you run more than five connections. It also fits if you want event titles kept off the copy, or a copy you run yourself. SyncThemCalendars has one-switch two-way sync, a two-minute setup, seven languages, and a lower monthly price at five syncs or fewer.
Does Keeper.sh only sync every 30 minutes?
No. We read every calendar every minute on all plans. The 30-minute figure is how often the free plan writes copies out, and Pro writes every minute.
Can I try both?
Yes. Keeper.sh has a free plan that does not expire. SyncThemCalendars has a 14-day trial without a card.
Other comparisons
Still weighing tools? These cover the rest of the field.
- Keeper.sh vs CalendarBridge
- Keeper.sh vs Kalender Sync
- Keeper.sh vs Morgen
- Keeper.sh vs OneCal
- Keeper.sh vs Reclaim.ai
- Keeper.sh vs SyncDate
- Nine calendar sync tools compared on price, speed and privacy
Sources
Everything above about SyncThemCalendars comes from their own public pages, checked on the date this post was published:
- syncthemcalendars.com — homepage, pricing and FAQ
- Two-way calendar sync — sync direction, update speed and privacy fields
- Apple iCloud calendar sync — iCloud checking interval
- Google calendar sync — instant updates from Google
- Compare — pricing model
- Privacy policy — data storage location
Prices and features change. If something here is out of date, their page wins, and we would appreciate you telling us.