MKV to MP4
Get a Matroska file into a container that TVs, phones and editors will actually open.
Your file
Drop a file here, or click to choose
It stays on this device. Nothing is sent anywhere.
Settings
What gets lost
MKV can hold several audio tracks, several subtitle tracks and chapters. MP4 handles one audio track cleanly here, so extra tracks are dropped in conversion and the first of each is kept. If the MKV carries subtitles you want to keep, extract them and use the subtitle burner.
The video itself is usually h.264 or h.265 already, so the picture survives the trip well.
Why MKV exists and why nothing plays it
Matroska was designed to hold anything: any number of video, audio and subtitle tracks, chapters, attachments, fonts for the subtitles, and metadata about all of it. That generality is exactly why it is the format of choice for archives and rips, and exactly why consumer hardware avoids it. A TV, a games console or a phone implements a fixed set of things it can decode, and a container that might hold anything is a support problem rather than a feature. MP4 is narrower, older, and consequently understood by everything.
Decide about the extra tracks before you convert
This is the one conversion where information genuinely goes missing, so it is worth a look at media info first to see what is inside. A film rip commonly carries two or three audio tracks, an original language and one or two dubs, plus several subtitle tracks. Only the first audio track survives here, and the subtitles do not survive at all.
If the first track is not the one you want, that is a problem this tool cannot solve for you, and the answer is a desktop tool that lets you pick. If the subtitles matter, the practical route is to extract the SRT elsewhere and then burn it into the picture with burn subtitles, which makes it permanent and unswitchable but also makes it work on every device.
Size, and what will not change it
MKV files have a reputation for being enormous, and the container has nothing to do with it. A 12 GB MKV is 12 GB because it holds a high-bitrate encode, often h.265 at near-source quality, and rewrapping it as MP4 changes the wrapper rather than the contents. If the goal is a smaller file, that is a re-encode and compress video is the tool. If the source is h.265 and the destination struggles with it, this conversion to h.264 will typically make the file larger rather than smaller, which surprises people; h.265 is simply more efficient.
Common questions
Why is my MKV file so large to start with?
MKV is often used for high-bitrate rips and archives. The container is not the reason, the bitrate inside it is.
Can I keep the subtitles?
Not as a separate switchable track. Burn them into the picture instead.