Free tier
Simple and School, and the paper tape, are free with no time limit. The advanced modes run on a per-session timer until you buy — no card and no account needed to try them.
Free to try
Get Castiel
The same engine and the same tape on every device. Simple and School are free to use, with no time limit. The advanced modes — Financial, Engineer, and Programming — run free for a session before stepping back; a small payment on most platforms removes that limit, and the Linux build is free in full. No ads, and no account to create.
i
Only the Microsoft Store edition is available today. The other builds are in progress — this page will list each one as it ships.
Desktop
Windows
The full instrument on Windows: every mode, the side panels, and the docked editable tape, unlocked with a one-time purchase. Available now on the Microsoft Store.
Get it from Microsoft Store — $2MSI installer
macOS
The full instrument on macOS, with every mode and the docked tape. The full version is a small yearly subscription through the Mac App Store, per Apple's rules. Coming soon.
Mac App Store
Linux
The complete app for Linux, free — the full unlimited equivalent of the Windows version. Available as an AppImage or a Flatpak. Coming soon.
AppImage
Mobile
Android
The same engine in your pocket, with a swipe-up tape drawer, unlocked with a one-time purchase. Coming soon.
Google Play
iOS
Castiel for iPhone and iPad, carrying the same modes and the same tape. The full version is a small yearly subscription through the App Store, per Apple's rules. Coming soon.
App Store
Watch
Samsung Watch
Simple mode on your wrist, for a quick sum away from the desk. Coming soon.
Galaxy Store
One small payment
A single small payment removes the session timer on most platforms — Linux already ships the full app for free. There are no ads on any platform; pricing and payment terms vary slightly by platform — see below for details.
Your data stays yours
Calculations, saved tapes, and settings live on your device. Connectors for currency rates or an AI provider are opt-in and use your own keys.
Prefer nothing to install?
A browser version of Castiel is under development. It is not available yet — see the web app page for what is planned.