Convert MKV to MP4 online

Matroska in one paragraph

MKV, short for Matroska, is an open container format from 2002. It can hold almost any video and audio codec, any number of audio tracks, several subtitle tracks, chapters and attached fonts in one file. That flexibility made it the standard for ripped films, anime fan releases, OBS recordings and archived TV broadcasts.

MP4 is stricter. It supports fewer codecs and only basic text subtitles, but in exchange it plays almost everywhere: iPhones and iPads, the Photos app on Windows, smart TVs from a USB stick, browsers, messengers and every editing app. When an MKV refuses to play or import, converting to MP4 is usually the fastest fix.

Remux or re-encode: what decides the speed

A container is just packaging. If the video inside the MKV can be stored in MP4 as is, there is no need to decode and compress it again; the stream can be copied into a new package. This is called remuxing, and it takes seconds even for a two-hour film because the work is mostly reading and writing the file.

The browser tool first reads which codecs the MKV contains, then:

1. H.264 video is copied unchanged. Any other video codec is re-encoded to H.264 at CRF 23 with the ultrafast preset and 8-bit 4:2:0 colour, which runs at roughly real-time speed on a laptop and slower on a phone.
2. AAC audio is copied as well. Anything else is converted to AAC at 128 kbit/s, because MKV audio is often FLAC, AC-3, DTS or Opus, which many MP4 players cannot decode.
3. The file is written with its index at the start, so it starts playing before it is fully loaded.

If FFmpeg still refuses the copy, for example because of broken timestamps, the tool falls back to a full re-encode.

The result card shows the size before and after. A size close to the original usually means the video was copied; a very different size means it was re-encoded.

Know what is inside before you convert

Which path you get depends on the video codec, and that also decides how compatible the MP4 will be.

• H.264 (AVC). The most common case for OBS recordings and most downloads. Copied bit for bit, no quality change, plays everywhere.
• HEVC (H.265). Common in 4K and 10-bit releases. Re-encoded to H.264, because many browsers, older phones and TVs cannot play HEVC. Expect this to take a while for long 4K files, and the result to be larger.
• VP9 and AV1. Re-encoded to H.264 as well; iPhones, older TVs and most editing software do not take them in an MP4.
• Anything else, such as Theora or MPEG-2, is re-encoded the same way.

For long files that need re-encoding, the server option on the page is faster. Server processing always re-encodes to H.264 with the slower medium preset at CRF 23 and 8-bit 4:2:0 colour, which gives better compression than the browser's fast preset. Free accounts get one server job a day up to 500 MB; Premium is unlimited and accepts files up to 4 GB.

Tracks, subtitles and other extras

An MKV often holds more than one stream. FFmpeg's automatic selection keeps one video track and one audio track, choosing the audio with the most channels. Practical consequences:

• A film with English and a dubbed language keeps only one of them, which is not necessarily the one marked as default.
• Subtitles are not included in the MP4. Image-based subtitles such as PGS from Blu-ray cannot be stored in MP4 at all, and styled ASS subtitles lose their styling even when converted. Keep the separate subtitle file, or turn it into a format your player understands with subtitle converter.
• Chapters, cover art and attached fonts are not carried over.

5.1 surround audio is encoded to AAC with its channel layout intact, so a home theatre receiver still gets six channels.

When the conversion fails or the file is too big

• Browser runs out of memory. The browser keeps the whole input and output in memory. A 1.8 GB film may fail on a phone even though the limit is 2 GB. Use a laptop, or the server option for larger files.
• Re-encoded file is much larger. The ultrafast preset favours speed over efficiency. Running the result through compress video with the Light preset shrinks it with little visible change.
• Only a part is needed. Cut the clip first with trim video; converting a five-minute excerpt is far quicker than a full episode.

Nothing is uploaded in browser mode, and the original MKV is never modified; you download a new MP4 next to it.