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Citadel vs Nextcloud Calendar: catalog facts
Citadel	Nextcloud Calendar
Free Free and open source under the GNU GPL, with no paid edition.	Free Free app for Nextcloud servers.
Web, Self-hosted	Web, Self-hosted
Mail, calendar, contacts, forums, wiki and chat in one server Consistent rooms-based structure across features	Calendars stay on your own server CalDAV sync with standard clients
The rooms metaphor differs from conventional mail servers Requires running and maintaining your own server	Requires a running Nextcloud server Not a standalone app
Self-hosted	Self-hosted
Open source	Open source
License: GPL	License not stated
Not stated if an account is needed	Account needed
Not stated if it works offline	Not stated if it works offline
Official website Full listing and sources Checked October 2, 2026	Official website Full listing and sources Checked October 2, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.