Convert video to MKV
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Worked example
MKV is a container rather than a codec, so converting into it is often just repackaging.
| Extension | .mkv |
|---|---|
| MIME type | video/x-matroska |
| Playback | Limited support |
| From MP4 without re-encoding | Yes |
| Video codec | libx264 |
| Audio codec | aac |
The equivalent command
ffmpeg -i holiday-clip.mp4 -c:v libx264 -crf 23 -pix_fmt yuv420p -preset veryfast -c:a aac -b:a 160k -threads 1 -y out-holiday-clip.mkvThis is exactly what runs in the browser. Copy it to run the same job locally on a file too large for a tab.
What to expect
Size is only predictable when you set a bitrate. These figures are for the bitrate example above:
| At 1500 kbps for 60s | 12.1 MB |
|---|---|
| Rough encode time | 5:38 |
At a constant quality the encoder lets the bitrate follow the content, so the output size genuinely cannot be predicted in advance — a slide deck and handheld footage of leaves differ by an order of magnitude at the same setting.
Why it matters
Because MKV can hold almost any codec, an MP4's existing H.264 and AAC streams can usually be copied into it untouched — instant, and lossless. That makes MKV the right choice when you want to change the wrapper without touching the content. The cost is reach: browsers do not play MKV, so it is a storage and playback-software format rather than a sharing one.