How to convert files offline, in your browser

A website normally needs the internet for every page. A site that works offline adds a service worker: a small script the browser keeps, which can answer requests from a saved copy when the network isn’t there. FixMyFiles uses one. It saves the homepage on your first visit, saves each page as you open it, and saves each engine the first time a tool uses it, so whatever you have used once is ready next time, connection or not.

This works because converting a file never needed the internet in the first place. The audio, video and image engines run inside the tab, on your device. The connection was only ever for fetching the page and its code, and once those are saved, there is nothing left to fetch.

It is also why offline and private go together. A converter that keeps working in airplane mode cannot be uploading your files, because there is nowhere for them to go.

Run each tool you’ll need once. Opening a page saves the page; running the tool saves its engine. The Audio Converter handles MP3, WAV, M4A, OGG, Opus and FLAC with your browser’s own decoders, and fetches the FFmpeg engine (a one-time 31 MB download) the first time you convert a specialist format such as WMA or AMR. Image tools load their encoders as they are needed.

Tools that use a model download it once. Remove Background fetches its cutout model the first time (about 88 MB), and Image to Text fetches the model for each language you read. Both are kept by the browser afterwards.

Install it as an app, if you like. In Chrome or Edge on a computer, use the install icon in the address bar, or the Install app button under a tool when it appears. In Safari on a Mac, choose File → Add to Dock. On an iPhone, tap Share → Add to Home Screen; on Android, open the Chrome menu and choose Install app or Add to Home screen. The app opens in its own window and uses the same saved copy.

Turn on airplane mode (or switch off Wi‑Fi), open the tool from a bookmark or the app icon, and convert a short file. If it works now, it will work on the plane. The line under each tool changes to say you are offline when the browser reports no connection.

Pages you have never opened won’t be there: offline, the site can only show what it has saved, and a page it hasn’t seen shows a short offline notice instead. The search box needs its index, which is saved the first time you search.

Most offline audio jobs are short and common: WAV to MP3 to make a recording smaller, M4A to MP3 for a voice memo, trimming the silence off the start of a take, or evening out the volume of an interview. All of them run inside the tab, so once a tool has been used it needs nothing from the network.

Speed depends on your device rather than on a server queue. A laptop converts a typical song in a second or two; a long recording on an older phone takes longer, and a phone may pause a tab that is in the background to save battery. Keep the tab in front until the download appears.

Video works the same way. Compressing a video without uploading it covers the extra things to know when the files are large.

Clearing site data or browsing history for the site removes it. The next visit while online saves it again.

Private and incognito windows forget everything when they close, so nothing is kept for next time.

Low storage. When a disk is nearly full, browsers may remove saved data from sites you haven’t used in a while. Installed apps tend to be kept.

Safari on iPhone and Mac can clear saved data for websites you haven’t opened for a few weeks. A Home Screen or Dock app is exempt from that, which is the best reason to install it if you rely on it offline.

A new version of the site replaces the saved copy the next time you are online. Until then, the version you have keeps working.