Container vs codec — why this works
An MKV file isn't a video format; it's a wrapper around video and audio streams.
So is MP4. When a TV, editor or phone "can't play MKV", it almost always could play the
H.264 video sitting inside it — it just doesn't speak the wrapper. Remuxing swaps the
wrapper and nothing else, which is why it's instant and lossless where real conversion is
slow and lossy. We also set the faststart flag, so the resulting MP4 streams
properly when uploaded to the web.
When you actually need a re-encode instead
If your MKV carries VP9/AV1 video or Opus/FLAC audio, MP4 can't hold those streams and a genuine re-encode is required. Browser tools are the wrong instrument for that job — a two-hour film would take hours — so we'd rather point you at HandBrake (free, desktop, excellent) than pretend. For everything else — which in practice is most MKV files — the remux above is the fastest possible path.
Only need the sound? Extract the audio track instead. Wondering about sizes? The MP4 will be within ~1% of the MKV — same streams, different box.
FAQ
How can MKV to MP4 conversion be this fast?
Because nothing is re-encoded. MKV and MP4 are containers — boxes holding the same video and audio streams. This tool performs a "remux": it moves the streams from the MKV box into an MP4 box untouched. A 4 GB file converts in roughly the time it takes to read and write it, with zero quality loss.
Why did my MKV fail to convert?
Remuxing only works when the streams inside are codecs MP4 supports — H.264/H.265 video with AAC/MP3/AC-3 audio covers the vast majority of MKV files. Files carrying VP9/AV1 video or Opus/FLAC/DTS audio need a real re-encode, which browser-based tools are too slow to do honestly — use HandBrake (free, desktop) for those.
Is there a file size limit?
The limit is your browser’s memory — files up to roughly 1.5–2 GB work reliably. Unlike upload-based converters there is no artificial cap, queue or paywall, because your file never goes to a server.
Will subtitles inside the MKV survive?
Text subtitles (SRT streams) are converted to MP4’s subtitle format when possible; ASS/SSA styled subtitles and image-based subtitles (PGS) are dropped, since MP4 cannot carry them. To keep styled subs, extract them first and use our subtitle converter.
Does converting MKV to MP4 lose quality?
Not with this tool. Remuxing copies the streams bit-for-bit — the video and audio are mathematically identical before and after. Quality loss only happens when tools re-encode, which this tool never does.