MKV & WebM in Avidemux: Open, Edit, Convert

MKV (Matroska) and WebM are container formats — wrappers that hold video, audio, subtitle, and chapter streams inside a single file. Avidemux handles both of them: you can open them, cut them without re-encoding, and convert them into other containers. This guide covers exactly how.

Opening MKV in Avidemux

MKV is one of the best-supported containers in Avidemux. Opening a file normally triggers an indexing pass — for large files this can take a while, which is normal. Let it finish: cancelling the indexing mid-way can leave navigation inaccurate. If the file plays, you are done; if not, see the troubleshooting notes below.

WebM and VP9 support

WebM files (usually VP8 or VP9 video with Opus or Vorbis audio) open in most Avidemux builds. One caveat: hardware VP9 decoding depends on your build and your GPU — some builds accelerate it, others fall back to software decoding. There is no universal guarantee, so performance on VP9 files varies from system to system.

Lossless cutting of MKV and WebM

Both formats support copy-mode (lossless) cutting: set Video Output and Audio Output to Copy, mark your segment boundaries on keyframes (use the up/down arrow keys to jump between keyframes), and save. Cutting exactly on keyframes keeps the result clean; cutting between keyframes in copy mode can cause a brief glitch at the start of the cut.

Converting MKV to MP4 without re-encoding

To change only the container — MKV to MP4 — without touching quality or waiting for a re-encode:

  1. Open the MKV file in Avidemux.
  2. Set Video Output to Copy.
  3. Set Audio Output to Copy.
  4. Set Output Format to the MP4 muxer.
  5. Save the file with a .mp4 name.

The process takes seconds and produces an MP4 with identical quality, because the streams are moved, not re-encoded.

Note: Copy-mode conversion only changes the container — the actual video and audio data are untouched, so quality is exactly preserved. If you need a smaller file or a different codec, that requires a re-encode instead.

Tip: If an MKV refuses to open at all, try remuxing it first with ffmpeg -i input.mkv -c copy fixed.mkv — broken container headers are the most common cause. See also our guide to the “cannot find a demuxer” error.

Frequently asked questions

Can Avidemux open MKV?

Yes. MKV (Matroska) is well supported in Avidemux — opening, lossless cutting, and converting to other containers all work. Very large files may take a moment to index; let the indexing pass finish rather than cancelling it.

Can Avidemux open WebM?

Yes, in most builds. WebM files with VP8 or VP9 video and Opus or Vorbis audio open normally. VP9 hardware decoding depends on your build and your GPU — it varies, so do not assume hardware acceleration is available on every system.

How do I convert MKV to MP4 without losing quality?

Open the MKV, set Video Output and Audio Output both to Copy, set the Output Format to the MP4 muxer, and save with an .mp4 name. Copy mode moves the streams into a new container without re-encoding, so quality is untouched.

Why is my VP9 file slow?

VP9 is computationally demanding to decode. Whether your build uses your GPU for VP9 depends on the build, and on software decoding a high-resolution VP9 file can easily stutter. Converting to MP4 with copy mode keeps the VP9 stream, so to reduce playback load, re-encode to H.264 or lower the resolution instead.