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MP4MKV

Convert MP4 to MKV

This free MP4 to MKV converter changes your MPEG-4 Part 14 (.mp4) file into a Matroska Video (.mkv) file, right in your browser. There is nothing to install, no account to create, and no limit on how many files you convert.

100% private. Your file is processed locally in your browser with WebAssembly — it is never uploaded to a server, stored, or seen by anyone.

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The conversion runs on your own device using ffmpeg compiled to WebAssembly (ffmpeg.wasm), so your file is never uploaded to a server. MKV is best for archiving with multiple tracks and subtitles, which is the usual reason to convert from MP4.

Why convert MP4 to MKV?

  • Note: MKV has narrower support than MP4 - convert only when a specific app or workflow needs it.
  • The video is re-encoded to efficient H.264, so the MKV file is usually about the same size as your MP4.
  • Pick MKV when you need a file made for archiving with multiple tracks and subtitles.

MP4 vs MKV

PropertyMP4MKV
Full nameMPEG-4 Part 14Matroska Video
ContainerMPEG-4Matroska
Typical codecH.264 + AACH.264 / H.265
CompatibilityUniversalLimited
Relative sizeSmallMedium
Best forSharing and playing almost anywhereArchiving with multiple tracks and subtitles

How to convert MP4 to MKV

  1. Add your MP4 file

    Drag and drop your .mp4 file onto the upload area, or click to browse and pick it from your device.

  2. Start the conversion

    Click "Convert to MKV". Everything runs locally with ffmpeg compiled to WebAssembly (ffmpeg.wasm) - your file stays on your device.

  3. Wait a few seconds

    A progress bar shows the conversion. The first run is slightly slower while the engine loads, then it is cached.

  4. Download your MKV file

    When it finishes, download the .mkv file straight to your device.

Frequently asked questions

Is this MP4 to MKV converter free?
Yes. It is completely free with no signup, no watermark, and no limit on how many files you convert. It is supported by ads, which keeps the conversion itself free.
Are my files uploaded to a server?
No. The conversion runs locally in your browser using ffmpeg compiled to WebAssembly (ffmpeg.wasm), so your file is processed in memory on your own device and is never uploaded, stored, or seen by anyone.
Will I lose quality converting MP4 to MKV?
The video is re-encoded to MKV with a high-quality setting, so the result looks very close to the original MP4. Re-encoding is never bit-for-bit identical, but the loss is minimal.
Will the file get smaller or larger?
Usually similar. The video is re-encoded to H.264 at a high-quality setting, so the MKV file is typically about the same size as your MP4; the container itself adds little.
What is the difference between MP4 and MKV?
MP4 is MPEG-4 Part 14 (an MPEG-4 container, typically H.264 + AAC); MKV is Matroska Video (a Matroska container, typically H.264 / H.265). MKV is best for archiving with multiple tracks and subtitles, while MP4 is best for sharing and playing almost anywhere.
How long does the conversion take?
Most files finish in a few seconds to under a minute, depending on length and your device. The first conversion is a little slower while the engine downloads and caches, then later runs start instantly.

Related conversions

AVI → MKVMOV → MKVMP4 → MOVMP4 → AVIMP4 → WebM

Need more formats or options? Use the full Change Video Format.