Outlook CalDAV Synchronizer: The Swiss Army Knife for Classic Outlook
If you want to connect Outlook with Nextcloud, SOGo, Google, or any CalDAV server and ask an LLM like ChatGPT or Claude, you’ll almost always get the same recommendation: Outlook CalDAV Synchronizer. Deservedly. For years, the open-source add-in (AGPL license) has been the standard answer to “How do I get CalDAV into Outlook?” — something Microsoft still doesn’t offer natively.
And it does more than events: the add-in also syncs tasks and contacts, with Google as well as Nextcloud, SOGo, Horde, or any standards-compliant CalDAV/CardDAV server. Free, no paid tier, no strings attached on features.
Still, let’s be upfront: CalDAV Synchronizer is a tinkerer’s tool for a clearly defined case — technically inclined Windows users on classic Outlook with their own CalDAV server (say, a self-hosted Nextcloud). For most everyone else, a server-side sync is the better choice. Because the tool has a few structural properties you should understand before you invest an afternoon in setting it up.
What CalDAV Synchronizer Does Well
- Broad server support: Google, Nextcloud, SOGo, Horde, and any standards-compliant CalDAV/CardDAV server
- More than calendars: events, tasks, and contacts — unique in this class of tools
- Bidirectional sync with configurable intervals and conflict resolution
- Multiple profiles in parallel: several calendar pairs, each with its own settings
- Free and open source (AGPL), active community on GitHub
The project is still maintained: version 4.6.0 shipped in June 2025, and the repository shows activity in 2026 as well. The release cadence is slower than it used to be, but this is by no means a dead project.
Requirements: Classic Outlook desktop on Windows — Outlook 2007 through 2021 plus Microsoft 365 desktop are supported, along with .NET Framework 4.8.
The Structural Limits
1. It’s a COM add-in — and “New Outlook” doesn’t support COM add-ins. This is the most important point in 2026: Microsoft’s new Outlook for Windows is technically a web app and no longer runs classic add-ins. CalDAV Synchronizer simply doesn’t work there — and according to the developers, it won’t be able to, because the architecture doesn’t allow it. If you migrate from classic to New Outlook (or your IT department migrates you), your sync stops working overnight. Mac and web-only users are out from the start.
2. PC off = sync off. The add-in runs inside Outlook on your PC. If Outlook is closed, the machine asleep or off, nothing happens. The Nextcloud event you add from your phone on the go only reaches Outlook once your work machine is running again. For free/busy blockers against double bookings, that creates a time-window gap — exactly when other people are booking you. On top of that: one machine is a single sync node, and updates and configuration are yours to maintain.
3. Setup is not a quick side task. Let’s be honest here: for tinkerers, CalDAV Synchronizer is a dream — DAV URLs, sync profiles, conflict rules, mapping options, everything configurable. For everyone else, it’s an afternoon. You need your server’s correct CalDAV URL (for Google, additional API authorization), create a profile per calendar, and test that the direction is right. Doable and well documented — but a long way from “sign in and it works.”
4. Full event data, no visibility levels. The add-in transfers events completely. A graduated mode like “show only as busy in my work calendar” isn’t the concept — it’s a replication tool, not a privacy layer.
CalDAV Synchronizer vs. Kalender Sync at a Glance
| Outlook CalDAV Synchronizer | Kalender Sync | |
|---|---|---|
| Price | Free (open source, AGPL) | Free plan (1 sync), Basic from €4/month |
| Runs | As an add-in inside classic Outlook (Windows) | Server-side, 24/7, device-independent |
| Syncs while PC is off | No | Yes |
| ”New Outlook” | No (COM add-ins unsupported) | Yes — works via the official cloud APIs |
| Mac / web-only | No | Yes |
| Syncs | Events, tasks, contacts | Events (calendar specialist) |
| Supported providers | Outlook ↔ CalDAV/CardDAV servers (incl. Google) | 9 (Google, Outlook/M365, Apple iCloud, GMX, WEB.DE, mailbox.org, Infomaniak kSuite, CalDAV, iCal feeds) |
| Detail level | Full event data | Selectable per sync: busy only · title + time · full details |
| Setup | High (DAV URLs, profiles) — plan an afternoon | ~2 minutes, no installation |
| Privacy | Data stays local + on your servers | GDPR, hosted in Germany (Hetzner), DPA for business |
Provider breadth deserves a second look, too: CalDAV Synchronizer connects Outlook with CalDAV/CardDAV servers — and that’s it. Kalender Sync connects 9 provider types directly with each other and is the only tool in this comparison that also supports the German providers GMX and WEB.DE. An iCloud calendar, a GMX mailbox, or an iCal feed next to your Nextcloud? End of the road for the add-in — one more sync for Kalender Sync.
To be fair: if you run a self-hosted Nextcloud and care that no third party ever processes your events, CalDAV Synchronizer gives you a fully self-controlled chain. That’s a real argument — as long as the requirements (classic Outlook, Windows, a running PC) remain in place for good.
Our Verdict: Which Tool for Whom?
CalDAV Synchronizer is the right choice for a clearly defined niche: you’re a tinkerer, run your own Nextcloud/SOGo instance (or another CalDAV server), use classic Outlook desktop on Windows — and are staying on it —, want to sync tasks and contacts alongside events, and your PC is running whenever the sync needs to be current. If that’s your setup: within this niche, the add-in is unmatched.
For most everyone else, Kalender Sync is the better choice:
- The sync runs 24/7 server-side — even with your computer off: phone events, vacation, weekends, no double-booking gap
- Works with every device and every Outlook flavor — including “New Outlook”, Mac, web, and M365 without desktop apps (and survives the migration if your IT department mandates it)
- 9 provider types connected directly — including CalDAV (Nextcloud too), iCloud, GMX, WEB.DE, and iCal feeds
- Visibility levels per sync (“busy only”, “title + time”, full details) for a clean separation of work and private life
- GDPR-compliant with hosting in Germany and a DPA for business
- 2-minute setup instead of an afternoon — no DAV URLs, no profiles, nothing to maintain
Bottom line: CalDAV Synchronizer is the right tool for tinkerers with their own CalDAV server and classic Outlook. For everyone else — which is most people — a server-side service like Kalender Sync is the more robust and far simpler choice.
Try Kalender Sync for free — 1 sync free forever, setup in 2 minutes
Frequently Asked Questions
Does Outlook CalDAV Synchronizer work with “New Outlook”?
No. The new Outlook for Windows doesn’t support COM add-ins — CalDAV Synchronizer technically cannot run there. You need classic Outlook (2007–2021 or Microsoft 365 desktop). If you use New Outlook or are migrating to it, you need a server-side solution like Kalender Sync that works via the official cloud APIs.
Can I use it to bring my Nextcloud calendar into Outlook?
Yes — that’s one of the add-in’s main use cases, including tasks and contacts. The prerequisites are classic Outlook on Windows and a running PC. Alternatively, Kalender Sync connects Nextcloud via CalDAV server-side with Outlook/M365, Google, and other providers — no add-in, device-independent.
Is CalDAV Synchronizer still maintained?
Yes. Version 4.6.0 was released in June 2025, and the GitHub repository shows activity in 2026 as well. Development is calmer than it used to be, but the project is maintained.
Does the add-in sync while Outlook is closed?
No. It runs inside Outlook — without Outlook open on a running Windows PC, no sync happens. Kalender Sync, by contrast, syncs server-side around the clock, no matter which device you’re currently using.
