A web page that installs to your home screen and then behaves like an app. Everything is computed on the device: nothing you type is sent anywhere, and there is no account or login.
iPhone and iPad: open this page in Safari, tap the Share button, then Add to Home Screen. It has to be Safari — the other browsers on iOS cannot install it.
Android and desktop: the browser menu offers Install or Add to Home screen, and Chrome often shows an install icon in the address bar.
The first visit stores the whole app on the device, so afterwards it opens with no signal at all. That is the point of installing it rather than bookmarking it. Values you type are kept locally too, so a calculation is still there when you come back to it.
Just open it. Each launch checks for a newer version and takes it if there is one, falling back to the stored copy when there is no connection. The build number at the foot of the page tells you which version you are looking at.
If the build number looks stale, close the app properly — swipe up to the app switcher and swipe it away — then reopen. That forces a fresh check rather than resuming the old session.
Radiocommunication engineering, offline