How to remux MKV to MP4 – and when you have to re-encode
By Muhammad Sohaib and Luqman Ahmad · NexixLabs · Updated
Converting a video can mean two very different things. Re-encoding decodes every frame and compresses it again, which takes time and always loses a little quality. Remuxing just moves the existing video and audio into a different file format – it takes seconds and loses nothing.
Whether you can remux depends on what is inside the file. This guide explains how to tell, using MKV to MP4 as the main example.
Containers and codecs
A video file is a container – MKV, MP4, MOV, WebM, AVI – that holds one or more streams. Each stream is compressed with a codec: H.264, HEVC, VP9 or AV1 for video; AAC, Opus, MP3, AC3 or FLAC for audio. The extension only tells you the container.
Remuxing works when the target container accepts the codecs already inside the file. MKV accepts almost anything; MP4 is pickier, but it accepts the most common video codecs.
When MKV to MP4 is lossless
Most MKV files today contain H.264 or HEVC video, and those can go into an MP4 unchanged. In that case the video is copied bit for bit: the conversion is limited only by how fast your disk can read the file, and the picture is identical to the original.
Audio is where MKV files differ most. AAC can be copied straight across. Other audio formats – AC3, DTS, FLAC, Opus or Vorbis – are re-encoded to AAC by Asset Compressor so the MP4 plays on every device. Re-encoding audio is quick and, at a good bitrate, sounds the same.
What doesn’t come along
MKV is often used for films with several audio languages and subtitle tracks. Asset Compressor keeps the main video track and the main audio track; extra audio tracks and subtitles are not carried over to the MP4. If you need a particular language, make sure it is the file’s default track before converting.
Older formats need re-encoding
Legacy formats usually hold codecs that MP4 players don’t handle, so they are re-encoded to H.264 with AAC audio. This runs in software and is slower than a remux, but it makes the video playable on any phone, TV or browser:
- AVI: usually DivX or Xvid video with MP3 audio. AVI files that already contain H.264 can be remuxed.
- WMV: Microsoft’s WMV and VC-1 codecs, which are not allowed in MP4 and always need re-encoding.
- FLV: old Flash video. Newer FLVs with H.264 can be remuxed; Sorenson Spark and VP6 need re-encoding.
- 3GP: old phone videos with H.263 video and AMR audio, which modern players often don’t support.
- MPG: MPEG-1 or MPEG-2 video, which re-encodes to much smaller H.264 files.
How to remux in your browser
Open the MKV to MP4 converter and add your file. With the default “Original” quality, Asset Compressor checks the codecs and copies every stream that MP4 accepts; anything else is re-encoded automatically. You don’t need to know the codecs in advance.
If you also want a smaller file, choose a quality level such as Balanced. That switches to re-encoding, which takes longer but usually halves the size.
Remux or re-encode? A summary
Use this as a quick reference:
- Same quality, fast, same size: remux (MKV or MOV with H.264/HEVC to MP4).
- Smaller file: re-encode at a lower quality or to a target size.
- Old or unusual codec: re-encode to H.264 for compatibility.
- Web delivery: re-encode to WebM (VP9) or keep MP4 (H.264) as a fallback.