Convert MKV to MP4

How MKV to MP4 works

The file is read with a FileReader, rewritten in memory and offered as a download. This is why large PDFs are limited by your device's RAM rather than by an upload size limit. A 200 MB scan never crosses the network, so the slowest part is your own disk.

That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.

MKV is a container, not a codec

An .mkv file is a Matroska box that can hold almost anything: H.264 video, H.265 video, FLAC audio, ten subtitle tracks. The MP4 box accepts a narrower list, which is why some MKVs convert in a second and others need a full re-encode.

The remux option copies the streams without touching them. If the video is H.264 and the audio is AAC — the most common case — the result is byte-identical picture quality and a conversion that finishes before you switch tabs.

When the remux fails

Three codecs break it: FLAC or PCM audio, which MP4 has no slot for; older DivX/Xvid video; and bitmap subtitle tracks (PGS or VobSub). The engine simply refuses rather than producing a file that plays picture without sound.

The re-encode option fixes all three by decoding everything and writing H.264 + AAC. It is slower and slightly lossy, but it always plays.

Subtitles and chapters

Text subtitles (SRT or ASS) are dropped when moving to MP4, because the format prefers its own timed-text track. Burned-in subtitles — the ones that are part of the picture — survive untouched.

Chapters and most metadata do not cross the container either. If you need them, an MKV-aware tool is the right choice.

Remux or re-encode: the one decision that matters

An MKV is a container, not a codec. If the video inside is already H.264 and the audio is AAC, MP4 can hold exactly the same streams — so the tool copies them into a new box without touching a single frame. That is a remux: seconds, no quality loss, no CPU burn.

Re-encoding decodes every frame and compresses it again. It is the only option when the MKV carries something MP4 cannot store, which in practice means DTS or AC3 audio, FLAC, and image-based subtitles such as PGS. Pick remux first; if the result refuses to play, run it again with re-encode.

Why the file size ceiling is 220 MB

The engine is ffmpeg compiled to WebAssembly, running in your tab. It needs the input in memory plus room for the output, and browsers cap how much a single tab can allocate — on a phone that cap is far lower than on a laptop.

220 MB is the size that stays reliable on a mid-range machine. Above it the tab tends to die mid-conversion, which wastes more time than it saves, so the tool refuses upfront instead. For a 2 GB movie, a desktop encoder is the correct tool and this page will not pretend otherwise.

What happens to audio and subtitles

Remux keeps every track MP4 can carry: AAC and MP3 audio survive, as do text subtitle tracks (SRT/ASS converted by the player, not by ffmpeg). Anything MP4 cannot store is dropped, which is why a remuxed file sometimes has video but no sound.

Re-encode maps the first audio stream to AAC 128 kbps and leaves subtitle tracks alone. If the subtitles are the point — an anime episode, a foreign film — burn them into the picture with the add-subtitles tool after converting the container.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions