Converting Video Formats with Avidemux: MP4, MKV, AVI and More

"Convert" can mean two different things. Sometimes you only need to change the container, for example from MKV to MP4 so a TV or phone plays it. Sometimes you need to change the codec inside, such as turning an old DivX AVI into H.264. Avidemux handles both, and knowing which one you need saves a lot of time.

Container change (remux)

If the streams inside the file are already supported by the target container, keep Video and Audio Output on Copy, change only Output Format and save. H.264 video with AAC audio, for instance, moves between MKV, MP4 and MOV without re-encoding. The save takes seconds and quality stays identical.

Full re-encode

When the codec must change, choose a video encoder in Video Output:

Open Configure under the encoder, choose Constant Rate Factor and leave the value between 18 and 23 for x264 (20 to 28 for x265). Lower numbers mean a bigger, better file. Then set Audio Output to AAC (FDK or lav) at 128 to 192 kbps, and pick the muxer.

Common targets

• MP4 for phones and TVs: x264 + AAC + MP4 Muxer.
• MKV for archives with subtitles: copy or x265 + AAC/FLAC + Mkv Muxer.
• AVI for old players: Mpeg4 ASP (xvid) + MP3 (lame) + AVI Muxer.
• WebM for websites: VP9 + Vorbis/Opus + WebM Muxer.

Things to check before saving

Make sure the frame rate stays the same unless you add a Change FPS filter on purpose. Look at the Video > Properties panel to see the source resolution and codec, and compare it with what the target container supports. Finally, test a short section with File > Save on a 30-second selection before committing a long file.