MKV to MP4 Converter

What is inside an MKV file

MKV is the Matroska container, an open format from 2002 designed to hold almost anything: any video codec, several audio tracks in different languages, subtitle tracks, chapters and attachments such as fonts. That flexibility is why MKV is popular for archived films and recordings — and why some devices, editors and TVs refuse it.

Most MKVs people want to convert hold H.264 or HEVC video, which MP4 accepts as is. Their audio is more varied: AAC copies straight across, while Dolby Digital (AC-3), DTS, FLAC or Opus are re-encoded to AAC, which takes seconds rather than minutes because audio is cheap to encode.

Where MKV files come from

The MKV files people bring here are mostly a recording from OBS Studio, which has long recommended MKV so that a crash cannot corrupt the whole file, a film or TV episode from a personal archive or a disc backup, or footage exported from software or cameras that write Matroska. In practice most of these convert by copying the streams, so expect seconds rather than minutes.

The converter does not guess. When you drop the file it reads the container's own description of its streams — codec, resolution, frame rate, audio tracks — and shows you, before you press anything, whether the MP4 will be a copy or a re-encode.

What happens to your MKV

The converter lists every track the MKV contains. If there are several audio tracks, you choose one — MP4 can carry several, but most players only ever play the first, so one well-chosen track is the reliable option. The video is then copied when it is H.264 or HEVC, and re-encoded to H.264 when it is anything else (VP9, AV1, MPEG-4 Part 2, MPEG-2).

Subtitle tracks are not carried over: MKV commonly uses SRT or ASS subtitles, which MP4 cannot hold in the same form. If you need them, extract them separately and use the Subtitle Converter to tidy them up.

How long MKV to MP4 takes

With H.264 video, the whole job is a remux plus, at most, an audio re-encode — under a minute for a full-length film on a decent laptop, most of it spent reading the file.

With VP9 or AV1 video the video has to be re-encoded, and a film-length file is then a very long job in a browser tab — roughly eight minutes per minute of 1080p at the default setting — so a desktop encoder such as HandBrake is the better tool for a whole film.

Problems you may run into

The MP4 has no sound on the TV — if the MKV's track was DTS or AC-3 and your TV handles AAC, the converted file plays. If the TV still shows no audio, check that you picked the right track in the Audio track box.

A very large file fails part-way — the browser tab has a memory ceiling of a few gigabytes, and the output is built in memory. Files above roughly 2 GB are risky; split them, or convert with a desktop tool.

OBS recording ends abruptly — an MKV from a crashed OBS session is still readable up to the crash, which is exactly why OBS uses MKV. The MP4 will contain everything up to that point.

When not to convert MKV to MP4

If your player already handles MKV — VLC, Plex, Kodi, most smart TVs and Android — there is nothing to gain by converting, and you would lose subtitle and extra audio tracks.

For OBS users: OBS's own File → Remux Recordings does this same container change on your machine without the browser's memory ceiling, which suits multi-gigabyte recordings better.

Your MKV is never uploaded

Converting an MKV on a website normally means uploading it, waiting in a queue, and downloading the MP4 back — with a copy left behind on the service's storage.

This converter skips all of that. ffmpeg, compiled to WebAssembly, runs inside this tab and reads your MKV straight from your disk; the MP4 is assembled in memory on your machine and handed to you as a download.

Only the engine travels: about 32 MB, fetched the first time you use any of the video tools and kept for the session. Disconnect afterwards and the conversion still runs.