MKV to MP4 Converter

MKV and MP4 are boxes, not video

An MKV and an MP4 can hold exactly the same picture, frame for frame. Matroska (.mkv) is an open container built to carry almost anything: H.264, H.265, VP9, AV1, MPEG-2, DTS, FLAC, Opus, uncompressed PCM, any number of subtitle tracks, chapters, cover art, even the font files a subtitle track needs. MP4 is the ISO base media format, and the list of things it is allowed to carry is much shorter. That single difference is the whole of this conversion. In most cases you are not changing the video at all - you are lifting the same encoded frames out of one box and setting them down in a box that more players, editors and televisions will open.

It also explains where MKVs come from. Screen recorders write MKV because the format survives a crash mid-recording. Disc rips use it because it can hold the original video untouched alongside five audio languages and a dozen subtitle tracks. Downloads use it because nobody has to ask what codec is inside. And then the file reaches the last mile - a TV over USB, a Windows machine with no codec pack, PowerPoint, an editor, a phone - and the last mile only speaks MP4.

Whether yours is a copy or a real re-encode

Most MKVs contain H.264 or H.265 video with AAC, AC-3 or MP3 audio. All of those are legal inside an MP4, so this converter copies the streams across instead of decoding and re-encoding them. That is a remux: the output holds byte-identical video, it takes seconds rather than minutes no matter how long the film is, and there is no quality argument to have because nothing was re-drawn. The tool inspects your file first and tells you in plain words which of the two it is about to do, before you press Convert.

A re-encode is only forced when the codec inside genuinely cannot live in an MP4, or when you ask for something that changes the pixels.

If the result copies an HEVC stream and then refuses to play, the container was never the problem: the device cannot decode H.265. Run the file through the HEVC to H.264 converter instead, which re-encodes deliberately and produces something a decade-old machine will open. If your MKV turns out to hold VP9 - some downloads do - the same logic that governs WebM to MP4 applies here, and a real encode is unavoidable.

What to choose when a re-encode is unavoidable

When the picture has to be re-drawn, quality is set by the Low, Medium and High presets, which are constant-quality settings rather than fixed bitrates. High keeps the file visually very close to the source and costs the most time and space; Medium is the sensible default for a film you want to watch rather than archive; Low exists for getting a large file under a sharing limit. Because they are constant-quality, a static talking-head clip stays small while an action scene is given the bits it needs, instead of every minute being padded to the same size.

Leave resolution and frame rate on "original" unless you have a reason. Dropping 1080p to 720p halves the pixel count and roughly halves the encode time, which matters in a browser; converting a variable frame rate to a constant one matters if an editor is choking on the file, and that has its own guide. Whatever you pick, a re-encode is never lossless, no matter how high the setting - only a copy is.

Subtitles, extra audio tracks and chapters

This is the honest cost of moving to MP4, and it is worth reading before you delete the original. The converter keeps the first video stream and the first audio stream. Subtitle tracks are dropped. Second and third audio tracks - the commentary, the dubbed language - are dropped. Chapters and any attached fonts do not come across. The tool counts those tracks while it inspects your file and warns you exactly how many it is about to leave behind, so nothing disappears silently.

If the first audio track turns out to be the wrong language, or the MP4 plays with a picture and no sound because a DTS track was involved, the reasoning is set out in the guide on video that plays without audio.

The honest limits of doing this in a browser tab

The converter is FFmpeg compiled to WebAssembly, running inside this page. Your file is read from disk into the tab and never leaves it, which is why there is no upload wait, no queue and no account. It also imposes two real limits that a server-side converter does not have, and MKVs are exactly the sort of file that meets them.

• Files over 2 GB are refused outright. A 32-bit WebAssembly heap cannot address them. This is not a paywall; nothing on the free tier is being withheld.
• Above roughly 300 MB the page warns you, because the conversion wants free memory of a few times the file size. A remux is the cheap case and a 1 GB file is often fine on a desktop with headroom; a full re-encode of the same file on a laptop with a dozen tabs open may run out of memory instead.
• The build is single-threaded, so a genuine re-encode is slow - think roughly real time at 720p and slower than that at 1080p. A remux is near-instant however long the video is, which is the case most MKVs land in.

Nothing here can open a DRM-protected file, and no converter can. If you have a 6 GB disc rip, the browser is the wrong tool and the Android app or its iPhone counterpart is the right one: it uses the phone's hardware encoder and has no size ceiling.

Why people arrive on this page

Almost always one of five reasons. A smart TV shows the MKV in its USB file list and then reports an unsupported file. Premiere Pro or DaVinci Resolve will not import it, and the editor's format list quietly excludes Matroska. A phone will not open it at all. A cloud converter refused the file because the free tier stops well short of a movie-sized upload - which is an architectural consequence of every byte having to cross the network twice, not a policy anyone chose to be unkind. Or the file is simply going somewhere corporate, and MP4 is the format nobody will ask questions about.

All five have the same fix, and in most cases it is a copy rather than a conversion. If you are not sure what is inside your file before you start, you can check the codec in your browser - this tool reads the container, codec, profile, resolution, frame rate and track counts and shows them to you before anything happens.