Why Is ffmpeg-web Not Working in My Browser?

ffmpeg-web runs entirely in your browser using WebAssembly (specifically ffmpeg.wasm), so when it fails, the cause is almost always one of four things: your browser lacks WebAssembly support, the file is too large or in an unsupported format, the browser is blocking a required capability (memory, threads, or storage), or a network problem prevented the WebAssembly binary from loading. If none of those apply and it still won't run, the native desktop version of ffmpeg-web is the fallback, since it uses a real local FFmpeg build instead of a browser sandbox.

Confirm your browser supports WebAssembly and ffmpeg.wasm

ffmpeg-web's browser mode depends on WebAssembly, so an old or restricted browser will fail before any conversion starts.

• Use a current version of Chrome, Edge, Firefox, or Safari. WebAssembly has been supported in all major browsers for years, so a failure here usually means an outdated build or a locked-down environment.
• If you're on a managed work computer or a kiosk browser, WebAssembly may be disabled by policy. Test the same page in a personal browser profile to rule this out.
• Some browsers restrict WebAssembly compilation for very large modules. If the page loads but conversion never begins, this is a likely cause.

What you'll see: the interface loads, but clicking convert does nothing, or the console reports a WebAssembly compile/instantiate error.

Check the file type and size

The browser version is bounded by what ffmpeg.wasm can handle in a single tab, which is far less than a desktop FFmpeg install.

As a rule of thumb, short clips and small audio files convert reliably in the browser; long videos or high-bitrate footage are where the browser version tends to hit its ceiling. There is no fixed size limit published for ffmpeg-web itself — the practical limit is your browser's available memory.

Look for memory, permission, and storage limits

• Memory: ffmpeg.wasm loads the input file and the FFmpeg runtime into the same tab. If the tab is already using a lot of memory, or the file is large, the browser can kill the operation. Close other tabs and retry.
• Storage / private mode: private or incognito windows often restrict the storage the page needs to stage files. Try a normal window.
• Downloads permission: if the conversion finishes but no file appears, check whether the browser blocked the automatic download. Look for a download-blocked icon in the address bar.
• Network: the WebAssembly binary is fetched when the page loads. A blocked CDN, an aggressive ad blocker, or an offline connection will stop ffmpeg-web from initializing even though the page itself renders.

Try the native desktop version

ffmpeg-web also ships a native desktop version that runs FFmpeg directly on your computer rather than through ffmpeg.wasm. This removes the WebAssembly, memory, and browser-permission constraints entirely, so it's the right move when:

• The browser version consistently fails on files you need to convert.
• You're working with large videos or batch jobs.
• You're on a browser or machine where WebAssembly is restricted.

The trade-off is that you install and run a local application instead of using the page directly, but you get the full capability of a native FFmpeg build.

Quick diagnostic order

1. Try a small, common file (e.g., a short MP4 or MP3) in a current browser in a normal window.
2. If that works, the problem is file size or format — move to the desktop version.
3. If even a small file fails, check the browser console for WebAssembly errors and disable extensions/ad blockers.
4. If it still fails, switch to the native desktop version, which bypasses the browser entirely.