ALL VIDEO TOOLS23
MKV to MP4
Convert MKV to MP4 Online — Free & Lossless
Rewrap Matroska MKV files as MP4 without re-encoding the video.
About MKV to MP4
MKV is a superb container and a terrible one to hand somebody. Matroska will hold essentially any codec, which is why it is the default output of so many rippers and downloaders — and why iPhones, smart TVs, PowerPoint and most video editors refuse to open it. The video inside is very often plain H.264, so the fix is not a conversion at all: it is unwrapping the streams from Matroska and rewrapping them as MP4, with the compressed data copied across unchanged. That runs in milliseconds and the picture is identical to the source, because no frame is ever decoded. The tool inspects the streams before it starts, so it can tell you which case you are in rather than guessing. Where a stream cannot live in MP4 — MKV happily carries codecs MP4 has no tag for — you are told up front and offered an H.264 conversion as a separate choice, with its real cost named: that path re-encodes, so it is slow and lossy. FFmpeg runs as WebAssembly in the tab and nothing is uploaded.
Questions
Is the video re-encoded?
Not when the codec fits. Matroska very often holds plain H.264, which MP4 accepts directly, so the compressed stream is copied into the new container without being decoded — milliseconds of work and no quality cost. Where the codec does not fit MP4, you are offered an H.264 conversion as an explicit, clearly slower choice.
Do extra audio tracks and subtitles survive?
This is the part of Matroska that MP4 handles worst. Expect the main video and audio track to come across; do not count on additional language tracks, chapter data, or subtitle streams, which MP4 supports far more narrowly than MKV does. If those matter, keep the MKV as your master copy.
Why won't my TV or phone open MKV in the first place?
Matroska will hold almost any codec, which is exactly why hardware players avoid it — they cannot promise to decode what they might find inside. MP4 is a much narrower container, and that narrowness is what makes it safe to support everywhere.
How long does the conversion take?
Milliseconds to a few seconds for the rewrap itself, because no frame is decoded. In our measurements a stream copy into a new container ran at between several hundred and roughly 1,700 times realtime. The first run on a page also downloads the ~31 MB FFmpeg engine, which is the part you actually wait for.
Is there a size limit on the MKV?
1.5 GB. MKV rips are often larger than that; trimming the section you need first, or keeping the original, is the honest answer for a full-length film.
Is my file uploaded to a server?
No. Transmute processes everything locally in your browser using JavaScript and WebAssembly. Your files never leave your device — there is no server, no upload, no cloud processing.