Remux MKV to MP4 — Convert Without Losing Quality

Repackage an MKV into an MP4 without re-encoding a single frame. A remux just moves the existing video and audio streams into a new container — it's instant, and since nothing is recompressed, the result is bit-for-bit the same quality as the source.

Remux to MP4 →
  • No upload wait
  • Nothing leaves your device
  • Free, no account
  • No artificial size limits

What does "remux" mean?

Every video file has two layers: the codec (how the picture and sound are actually compressed — H.264, HEVC, AAC, and so on) and the container (the box that holds those streams — MKV, MP4, MOV). Converting normally means decoding the video and re-encoding it into a new codec, which takes time and always costs a little quality.

Remuxing skips all of that. If the MKV's video and audio are already in a codec MP4 can hold — which is true for the huge majority of MKVs, since H.264 and HEVC are the most common codecs inside them — the converter just lifts the existing streams out of the MKV box and drops them straight into an MP4 box. No decoding, no re-encoding, no quality lost. It's closer to a fast file-format swap than a conversion.

Why remux instead of convert?

  • Zero quality loss — the video and audio bits themselves never change.
  • Finishes in seconds, even for a large file — there's no encoding work to do.
  • Uses almost no CPU or battery compared to a full re-encode.
  • The perfect fix for "this MKV won't play on my iPhone/TV/editor" when the codec inside it is already fine — MP4 is just the wrong box.

When remux works — and when it doesn't

Remux only works if the codec already inside the MKV is one MP4 is allowed to hold — in practice, H.264 or H.265 (HEVC) video with AAC or MP3 audio. That covers most MKVs, since H.264/H.265 are by far the most common video codecs used in the format.

It won't work if the MKV holds something MP4 can't carry — VP9 video, or audio like DTS, TrueHD, FLAC, or Vorbis. In that case the remux will fail outright rather than produce a broken file, and you'll need to actually re-encode instead. Our regular MKV to MP4 converter handles that case — it decodes and re-encodes whatever's inside the MKV into H.264 or H.265, so it always works, at the cost of a short re-encode and (at high quality settings) a visually lossless amount of recompression.

Subtitles and extra audio tracks

Like our standard MKV to MP4 conversion, remux keeps the first video and first audio track and doesn't carry over embedded subtitle tracks — MP4 handles subtitles differently from MKV. If you need those, keep the original MKV and play it in a player like VLC that renders MKV subtitles natively.

How it works

Remuxing runs entirely on your device using a WebAssembly build of ffmpeg — there is no upload step, and nothing about your video is ever sent anywhere. Because there's no encoding work involved, it's typically the fastest thing this converter does, regardless of file size. We only supply the code; your own device performs the whole operation, and the file stays 100% private throughout.

Frequently asked questions

What does "remux" mean?
Remuxing repackages a video's existing streams into a new container without decoding or re-encoding them. It's the difference between re-cooking a meal (converting) and just moving it to a different plate (remuxing) — the contents don't change, only the container around them.
Will remuxing lose any quality?
No. The video and audio data are copied exactly as they are — nothing is decoded, recompressed, or altered. The output is bit-for-bit identical in quality to the source, just inside an MP4 container instead of MKV.
Why did my remux fail?
Remux only works if MP4 can legally hold the codec already inside your MKV — in practice, H.264 or H.265 video with AAC or MP3 audio. If your MKV holds VP9 video or audio like DTS, TrueHD, FLAC, or Vorbis, MP4 can't carry it as-is, so the remux fails outright. Turn remux off (or use the regular MKV to MP4 converter) to re-encode instead — that always works.
How is this different from just renaming the file extension?
Renaming the extension doesn't touch the file's actual structure, so most players and devices still won't recognize it. Remuxing genuinely rebuilds the container — reading the MKV's real internal structure and writing a valid MP4 file around the same streams — so the result is a real MP4 that any MP4-compatible player accepts.
Are subtitles and extra audio tracks kept?
No — remux keeps the first video and first audio track only, the same behavior as our standard MKV to MP4 conversion. Subtitle tracks aren't carried over; MP4 handles subtitles differently from MKV, so keep the original MKV if you need them.
Is it really instant?
Close to it. Since there's no decoding or encoding involved, remux is limited only by how fast your device can read and rewrite the file — typically a few seconds even for a multi-gigabyte MKV, versus the minutes a full re-encode can take.

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