Avidemux “Cannot Find a Demuxer” Error: 5 Fixes That Work

Q: Can ffmpeg fix my file?

In many cases, yes. Running ffmpeg with -c copy repackages the streams into a fresh, well-formed container without re-encoding, which lets Avidemux open files whose original container headers were broken or mislabeled.

“Cannot find a demuxer” is one of the most common Avidemux errors, and it has a precise meaning: a demuxer is the component inside Avidemux that reads a container format (MP4, MKV, AVI, TS, and so on) and separates its video, audio, and subtitle streams. When this error appears, Avidemux has looked at your file and could not match its container or codec to any demuxer it ships with. Below are five fixes, ordered from fastest to most involved.

Fix 1: Confirm the file actually plays

Before changing anything, open the file in VLC or your media player of choice. If another player also refuses it, the problem is the file itself — a failed download or a broken file — and none of the steps below will help. If the file plays fine elsewhere, move on to Fix 2.

Fix 2: Fix a wrong file extension

A file named video.mp4 is not necessarily an MP4 file. Files downloaded from the web are sometimes mislabeled, and Avidemux picks its demuxer based on the extension and the actual header. Check the real format with MediaInfo (free, cross-platform), then rename the file to the correct extension — for example .mkv if the container is Matroska.

Fix 3: Remux the file with ffmpeg

If the extension is right but the container headers are malformed, a remux usually repairs them. Install ffmpeg, then run:

ffmpeg -i input.mkv -c copy output.mp4

This repackages the streams into a clean container without re-encoding, so it takes seconds and loses no quality. Adjust the input and output names to match your file, then open the new file in Avidemux.

Fix 4: Update Avidemux

Older Avidemux builds lack support for newer containers and codecs, which is a common cause of this exact error. Download the latest stable release, 2.8.1, from our download page. If your file uses a very new codec, a 2.8.2 nightly build (the development line, not a stable release) may open files that 2.8.1 cannot.

Fix 5: Accept that the file may be corrupt

If VLC, ffmpeg, and the latest Avidemux all reject the file, it is almost certainly corrupt or incomplete — typically a truncated download. Redownload it from a reliable source, or obtain it from the sender again. If the file is damaged beyond the container level (e.g. broken video frames), no tool can restore what was never downloaded.

Tip: Do not “repair” a file by forcing random extensions on it over and over. Identify the real format first with MediaInfo, then pick the fix that matches the actual problem.

Frequently asked questions

What does “cannot find a demuxer” mean?

It means Avidemux looked at the file’s container format or codec and did not recognise either. The demuxer is the component that opens and reads a container (MP4, MKV, AVI, and so on); without one that matches, Avidemux cannot load the file at all.

Will updating Avidemux fix it?

Sometimes. Newer builds add support for newer containers and codecs, so moving from an old version to the latest stable release (2.8.1) can fix the error. If the file is simply corrupt, though, no update will help.

Can ffmpeg fix my file?

In many cases, yes. Running ffmpeg with -c copy repackages the streams into a fresh, well-formed container without re-encoding, which lets Avidemux open files whose original container headers were broken or mislabeled.

Does this error mean the file is broken?

Not necessarily. It can also mean the file extension is wrong, the container is unsupported, or the download is incomplete. If ffmpeg and other players also refuse it, the file itself is most likely damaged.