Proton Calendar

What Is Proton Calendar?

The conventional wisdom is that calendar apps do not need heavy encryption. What is there to protect? Meeting times? Lunch plans? The assumption is that calendar data is low-sensitivity — unlike email or documents, it does not contain secrets worth encrypting.

That assumption collapses under scrutiny. Your calendar reveals where you are, who you meet, what doctors you visit, which investors you are pitching, when you travel, and how you spend every working hour. A calendar is a metadata-rich record of your life. Google Calendar can read every event on your schedule. Apple Calendar encrypts at rest but holds the keys. Most calendar services can access your data whenever they choose — or whenever a government request compels them to.

Proton Calendar is built on the principle that this access should not exist. Created by Proton AG — the Swiss company behind ProtonMail and ProtonVPN, founded in 2014 by scientists at CERN — Proton Calendar encrypts all event details on your device before they reach Proton's servers. Titles, descriptions, locations, attendees, and notes are all encrypted with keys that only you hold. Proton cannot read your schedule. A subpoena served to Proton would yield encrypted data that Proton itself cannot decrypt.

Launched in 2020, Proton Calendar is the newest component of the Proton ecosystem (Mail, Drive, VPN, Pass, Calendar). It is available on web, Android, and iOS, with a free tier included in every Proton account. The entire codebase is open-source and has been independently audited.

The limitations are real: Proton Calendar has fewer features than Google Calendar, no CalDAV support for third-party calendar apps, limited integrations with non-Proton services, and sharing calendars with non-Proton users requires workarounds. It is a calendar that prioritises one thing — privacy — and makes meaningful compromises on convenience to achieve it.

Key Features

End-to-End Encryption

Every event in Proton Calendar is encrypted on your device before being transmitted to Proton's servers. The encryption covers event titles, descriptions, locations, attendees, and any attached notes. The encryption keys are derived from your Proton account credentials and never leave your device. This is zero-access encryption: Proton's servers store your calendar data but cannot read it. Even Proton employees with server access see only encrypted blobs. This is the same encryption architecture that powers ProtonMail, applied to scheduling.

Proton Ecosystem Integration

Proton Calendar integrates tightly with the broader Proton suite. Event invitations arrive through Proton Mail. File attachments can be stored in Proton Drive. The overall privacy guarantee extends across all Proton services — your email, your calendar, your files, and your VPN traffic are all encrypted under the same architecture. For users who have committed to the Proton ecosystem, the calendar is a natural extension. For users who use Proton Calendar in isolation from Gmail or Outlook, the integration value is limited.

Multiple Calendars and Views

You can create multiple calendars with colour coding for different contexts (work, personal, family). Views include day, week, and month layouts. Recurring events, event reminders, and time zone support cover the core scheduling functionality. The interface is clean and straightforward — Proton's design language is consistent across its products, with a professional, minimal aesthetic.

ICS Import

Proton Calendar supports importing events from ICS files, which is the standard export format for Google Calendar, Apple Calendar, and Outlook. This makes initial migration feasible: export your existing calendar as an ICS file, import it into Proton Calendar, and your events are re-encrypted under Proton's zero-access architecture. The import works well for one-time migration, though there is no continuous sync from external calendars.

Open Source and Audited

The web, Android, and iOS clients are all open-source, published on GitHub. The codebase has been independently audited by security researchers, providing third-party verification that the encryption implementation works as claimed. For users evaluating Proton's privacy claims, the ability to inspect the source code and reference independent audits provides a level of transparency that proprietary calendar services cannot offer.

Pricing

Proton Calendar is included free with every Proton account. The Proton Free tier provides one calendar with end-to-end encryption, 500 MB of Proton Mail storage, and basic event management. For individuals who need a private calendar with no cost, this is sufficient.

Mail Plus at EUR 3.99 per month adds up to 25 calendars, 15 GB of storage, calendar sharing with other Proton users, and a custom email domain. The calendar-specific upgrade — going from 1 to 25 calendars and gaining sharing — requires this tier.

Proton Unlimited at EUR 9.99 per month includes all Proton products (Mail, Calendar, Drive, VPN, Pass) with 500 GB of storage and priority support. For users who want the full Proton privacy suite, this is the most cost-effective option.

Proton Business at EUR 12.99 per user per month adds user management, priority business support, and custom domains for team use.

The pricing is competitive with other Proton services but represents a premium compared to Google Calendar (free, unlimited) or even Apple Calendar (free with iCloud). The premium is the price of zero-access encryption and Swiss privacy — a cost that is either essential or irrelevant depending on your threat model.

EU Compliance & Privacy

Proton AG is a Swiss company headquartered in Geneva, operating under Swiss privacy law — which, while not EU law, is among the strongest privacy frameworks in the world. Switzerland has received an adequacy decision from the EU, meaning data transfers between the EU and Switzerland are permitted without additional safeguards.

All Proton data is stored in Swiss data centres, some of which are located in former military bunkers. The end-to-end encryption means that even with physical server access, the data is unreadable without the user's keys.

Proton cannot comply with data access requests because it cannot decrypt user data. This is a structural privacy guarantee rather than a policy-based one — it is not that Proton promises not to read your data, but that Proton is architecturally incapable of reading it.

For European users, Switzerland's privacy-friendly jurisdiction, combined with end-to-end encryption and open-source transparency, creates a privacy profile that matches or exceeds what EU-based services offer. The 9.5 EU compliance score in our assessment reflects this exceptional positioning.

Who It's Best For

Privacy-conscious individuals who want their schedule encrypted with zero-access architecture, preventing any third party — including the service provider — from reading their events.

Proton ecosystem users who already use ProtonMail and want their calendar integrated within the same encrypted environment.

Professionals handling sensitive scheduling data — lawyers, journalists, healthcare providers, executives — whose calendar contents could be exploited if accessed by unauthorised parties.

Users migrating away from Google who want to remove Google Calendar alongside Gmail as part of a broader de-Googling strategy.

The Verdict

Proton Calendar does one thing and does it uncompromisingly: it encrypts your schedule so that nobody — not Proton, not a hacker, not a government — can read it without your keys. The feature set is basic compared to Google Calendar. The lack of CalDAV support limits flexibility. Integrations with non-Proton services are minimal. But for users whose threat model includes calendar data — and more people should be in that category than currently realise it — Proton Calendar provides a level of privacy that no mainstream calendar comes close to matching. It earns 6.8 overall, reflecting the tension between exceptional privacy (9.5 EU compliance) and limited functionality (5.5 feature depth, 4.5 integration ecosystem). It is the right tool for a specific, important need.

Frequently Asked Questions

Can I share calendars with non-Proton users?

Proton Calendar supports sharing via public links that expose event data in an unencrypted ICS format. This means the shared calendar loses its end-to-end encryption. Sharing between Proton users preserves encryption. For users who need encrypted shared calendars, all participants must have Proton accounts.

Why does Proton Calendar not support CalDAV?

CalDAV requires the server to read and process calendar data to respond to client queries, which is incompatible with Proton's zero-access encryption architecture. Since Proton cannot read your calendar data, it cannot serve it via standard CalDAV protocols. This limits Proton Calendar to Proton's own apps.

Can I use Proton Calendar for team scheduling?

Proton Business plans support shared calendars among team members. However, features like scheduling assistants (finding mutually available times) and room booking are not available. For teams with complex scheduling needs, Proton Calendar's team features are basic compared to Google Calendar or Microsoft Outlook.

Is Proton Calendar available on desktop?

Proton Calendar is available via the web interface and through the Proton Desktop app (which bundles Mail, Calendar, and Drive). There is no standalone desktop calendar application, and the lack of CalDAV support means you cannot use Proton Calendar with third-party desktop calendar apps like Thunderbird or Apple Calendar.

How does Proton Calendar compare to Tutanota's calendar?

Both offer end-to-end encrypted calendars from European providers. Proton is Swiss-based; Tutanota (now Tuta) is German-based. Both encrypt all event data. Proton offers a more polished interface, mobile apps, and the broader Proton ecosystem. Tuta integrates its calendar more tightly with its email client. Both are strong choices for encrypted scheduling; the decision often comes down to which email/calendar ecosystem you prefer.