MKV to MP4 Converter

To convert MKV to MP4 without losing quality, drop the file above, keep the audio tracks you want and press Convert to MP4. The page copies the video and sound into an MP4 container without re-encoding them, so every frame is identical to the MKV and the conversion takes about as long as copying the file. Subtitles are not carried over; the page tells you which ones and gives the ffmpeg command that keeps text subtitles.

How to convert MKV to MP4 online

1. Add the file. Drop one or more MKV files on the box above, or press Choose MKV files. WebM files work too, since WebM is a form of Matroska.
2. Read the check. For each file the page lists the video track and every audio track with its codec, language, resolution or channels and duration, names every subtitle track, and says what MP4 can keep.
3. Pick the audio tracks. All the audio tracks that MP4 can hold are ticked. Untick a dubbed language or a commentary you do not need; the first video track is always kept.
4. Press Convert to MP4. In Chrome and Edge on a computer you choose where to save and the MP4 is written to disk as it is built. In other browsers it is assembled in memory, then downloaded.

With several files, convert them one by one, or, in Chrome and Edge on a computer, press Convert all into a folder: you pick a folder once and every file that can be converted is written there in turn, with a new name if a file with the same name already exists.

MKV vs MP4: what is the difference?

MKV and MP4 are containers, not video formats. A container is the box that holds the compressed video, the sound tracks and the timing that keeps them in sync. The picture itself is compressed by a codec such as H.264 or H.265, and the same H.264 video can sit in either box.

• MKV (Matroska) is an open container from 2002. It accepts almost any codec, any number of audio and subtitle tracks, chapters and attached fonts. That flexibility is why films, series and screen recordings often come as MKV.
• MP4 (MPEG-4 Part 14) is the container that every phone, TV, browser, video editor and social network reads. It accepts fewer codecs and its subtitle support is limited, but it plays almost everywhere.

So an MKV that will not play on an iPhone, in QuickTime, on a TV or in Premiere is usually not broken: the box is the problem, not the video inside. Moving the same video into an MP4 box fixes it.

Why a remux is lossless and fast

Most converters decode every frame and compress it again. That takes minutes per film, uses the processor at full load and always loses a little quality, because lossy compression is applied a second time. This page does something else: it reads the compressed video and audio packets from the MKV and writes the same packets into an MP4, with new timing tables. This is called remuxing.

Nothing is decoded, so the result is bit for bit the same picture and sound. The time it takes is the time needed to read the MKV and write the MP4 once, which is why a full film usually takes seconds to a minute rather than an hour. The MP4 ends up about the size of the MKV, minus any audio track you dropped.

The MP4 keeps its index (the moov box) at the end of the file, which every player reads. If you upload the file to a web server for streaming, you can move the index to the front without re-encoding with ffmpeg -i video.mp4 -c copy -movflags +faststart video-web.mp4.

Which codecs can go from MKV to MP4

The ffmpeg command is shown for every file, even when the page can do the job, so you can run the same conversion on a whole folder from a terminal.

Subtitles, chapters and attachments

The page copies the video and audio tracks only. Subtitle tracks, chapters and attached files (often fonts for styled subtitles) are left out, and the file check names each subtitle track that will be missing so you are not surprised later.

MP4 has its own text subtitle format, mov_text. SRT, ASS and WebVTT subtitles can be converted to it by ffmpeg while the video and sound are copied. The page prints the command for your file, for example:

ffmpeg -i film.mkv -map 0:v:0 -map 0:a:0 -map 0:s:0 -c:v copy -c:a copy -c:s mov_text film.mp4

ASS styling (fonts, colours, positions) is lost in mov_text, which only keeps the text. Picture-based subtitles from Blu-ray (PGS) and DVD (VobSub) cannot become MP4 text at all: keep the MKV, or burn them into the picture, which means re-encoding the video.

Sound that plays everywhere

MP4 can hold Opus, FLAC, AC-3, E-AC-3, DTS and PCM, but players differ. QuickTime, older Apple devices and many TVs may not play Opus or FLAC from an MP4; browsers and the Windows apps often skip AC-3 and DTS; most players refuse Vorbis in MP4. When the MKV carries one of these, the page converts anyway, says which players may stay silent, and prints the safest variant: the video copied as it is and only the sound turned into AAC.

ffmpeg -i film.mkv -map 0:v:0 -map 0:a:0 -c:v copy -c:a aac -b:a 192k film.mp4

Re-encoding the sound is quick (sound is a small part of the file) and AAC at 192 kb/s sounds the same as the original to most listeners on stereo sound. For 5.1 sound, raise it to -b:a 384k.

Other ways to convert MKV to MP4

ffmpeg

The reference tool, free on Windows, macOS and Linux. The basic remux is:

ffmpeg -i input.mkv -map 0:v:0 -map 0:a -c copy output.mp4

-c copy copies without re-encoding, -map 0:v:0 takes the first video track and -map 0:a every audio track. Without -map, ffmpeg keeps one video and one audio track only. Adding -map 0 to take everything fails as soon as the MKV has SRT subtitles, because MP4 needs -c:s mov_text for them.

VLC

In VLC, open Media, Convert / Save, add the MKV, press Convert / Save, and choose the profile Video - H.264 + MP3 (MP4). As it is, that profile re-encodes everything. Click the wrench icon next to it and tick Keep original video track on the Video codec tab and Keep original audio track on the Audio codec tab to copy instead. VLC handles one file at a time and shows no codec warning.

HandBrake

HandBrake always re-encodes the video. That is the right tool when the MKV holds MPEG-2 or Xvid, or when you want a smaller file, but it is slow and loses some quality for a file that only needed a new container.

Online converters

Sites such as CloudConvert and FreeConvert upload your file, convert it on their servers, and let you download the result. That means waiting for the upload of a multi-gigabyte film, a size limit on free plans, and a copy of your video on someone else's server. Most of them re-encode by default.

Limits

• Memory. Chrome and Edge on a computer write the MP4 to disk as it is built, so the size is limited by your disk. Other browsers (Firefox, Safari, phones) keep the MP4 in memory: the page warns above 500 MB and refuses above 1 GB (300 MB on phones), and the ffmpeg command takes over.
• One video track. The first video track is copied. MKV files with several video angles are rare; ffmpeg's -map 0:v keeps them all.
• Track names. Audio languages and track titles such as Commentary are copied into the MP4. Not every player shows the titles.
• Cut or damaged files. An MKV whose recording was cut short is converted up to the cut. If the start of the file is damaged, the page cannot read it and says so; ffmpeg is more lenient.
• Encrypted or compressed tracks. Tracks with a content encoding other than header stripping are skipped by the reader and do not appear in the list.