Free tool

MKV to MP4 Converter

MKV is a great recording format and an awkward delivery format: editors, phones and social platforms often refuse it. This tool remuxes the MKV's picture into an MP4 without re-encoding and converts the sound only if MP4 players can't handle it.

01 — Your file

Choose a file

02 — Result

Your converted file

Choose a file and press Convert to MP4. The conversion runs in this tab, so even large files never leave your computer or phone.

How to convert MKV to MP4

  1. Choose the MKV above and check the codecs on the file card.
  2. Press "Convert to MP4".
  3. Download the MP4 and read the result card to see what was copied and what was converted.

What is copied and what is converted

H.264 or HEVC picture
copied into the MP4 as it is: no quality loss, fast
AAC or MP3 sound
copied as it is
AC-3, E-AC-3, DTS, Opus, Vorbis or FLAC sound
converted to AAC at 160 kbps, which every MP4 player supports
Other picture codecs, such as VP9
re-encoded to H.264, which is slow in a browser

In our test, an MKV with H.264 video and AC-3 sound converted in under a second: the picture was copied and only the sound was re-encoded.

OBS recordings

OBS Studio can record to MKV because an MKV survives a crash that would leave an MP4 unreadable. OBS itself has a "Remux Recordings" option in its File menu that does the same job as this tool. This page is handy when you are on a different computer, or when the recording's sound uses a codec your editor rejects, because the converter fixes the sound in the same pass.

What doesn't come across

MKV can hold things MP4 handles poorly. The converter keeps the main picture and the first audio track. Additional audio tracks, such as separate microphone and game tracks or alternative languages, and subtitle tracks are not carried into the MP4. If you need a specific track or the subtitles, MKVToolNix or ffmpeg on the desktop can pick them. The MKV vs MP4 comparison goes into the differences.

Long recordings

Because the picture is copied rather than re-encoded, an hour-long H.264 recording converts in about the time it takes to read and write the file. The limit is the output size: the MP4 is assembled in your browser's memory, so recordings that would produce more than about 1.5 GB need a desktop tool. For transcription you don't need to convert at all; mydubly reads MKV on the MKV to text page.

How the free converter works

The converter is ffmpeg, the open-source media toolkit, compiled to WebAssembly so it runs inside your browser tab. Your file is read straight from your disk by that engine; it is never uploaded to mydubly or anywhere else, and no account is needed.

  • The engine is about 30 MB and downloads the first time you convert something. After that your browser normally keeps it.
  • Large inputs are fine because the file is read in place, but the result is built in memory, so outputs are capped at about 1.5 GB.
  • Speed depends on your device. Copying streams takes seconds; re-encoding video is the slow case.
  • Keep the tab open until the download button appears. Closing it stops the conversion.

More free tools

Turn the file into text

Frequently asked questions

Does MKV to MP4 lose quality?

Not for H.264 or HEVC video, which is copied. Sound in AC-3, DTS or Opus is converted to AAC, which is a small, usually inaudible change.

Why is my MKV not converting quickly?

If the picture isn't H.264 or HEVC (for example VP9), it must be re-encoded, which is much slower in a browser. The result card tells you which route was taken.

Are subtitles kept?

No. Subtitle tracks and extra audio tracks are not carried into the MP4. The main picture and first audio track are.

Is this the same as OBS's remux feature?

For H.264 recordings with AAC sound, yes. This tool also converts incompatible sound to AAC in the same pass.