An offline file converter
Convert files with the network off. The converter is downloaded once and then runs entirely on your own machine, on a plane or behind a firewall.
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Why an offline converter is possible at all
A converter that needs the internet needs it for one reason: the conversion is happening somewhere else. Remove that and the network stops being necessary. Everything this site does after the first load is arithmetic on your own processor, and arithmetic does not require a connection.
What gets cached, and when
The interface caches itself the first time you open the site — a few hundred kilobytes of markup, styles and scripts. Image conversion works immediately after that, because the codecs are already inside your browser.
Video and audio need FFmpeg, which is about 31 MB. It downloads the first time you run a video job and is then kept permanently, so the second job starts instantly and the hundredth job on a train works exactly as well as the first.
Getting it ready before you lose signal
If you know you are about to be offline, run one small video conversion while you still have a connection. That pulls the engine down and caches it. After that the site is fully self-contained and behaves the same with the network off as with it on.
Installing it as an app
Because everything runs locally, the site can be installed like a native application: its own window, its own icon, a launcher entry, and files that open straight into it from your file manager. Nothing about the conversion changes — it simply stops looking like a browser tab.
Questions
Can I convert files with no internet connection?
Yes, once the page has loaded. The site is a progressive web app: the interface and the conversion engine are cached on first use, and after that everything runs with the radio off. Video work needs the 31 MB engine downloaded once before you disconnect.
Do I need to install anything?
No, though you can. The site works as an ordinary web page with no installation. If you install it as an app, it gets its own window and appears in your launcher, but the conversion behaves identically either way.
Will it still work on a plane?
Yes. That is the honest test of whether a converter is really local, and it is the reason the offline support exists rather than being a checkbox feature.