MKV to MP4 Converter

Convert a Matroska .mkv into an MP4 that plays on any device and app. Runs on your device — nothing is uploaded.

By LK Forge · Published 6 August 2026 ·

Drop your file here, or click to choose Processed privately in your browser — nothing is uploaded

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100% private. Your file is processed in your browser with ffmpeg (WebAssembly) and is never uploaded. Big or long files take longer and use more memory — for those, trim first.

What is the difference between MKV and MP4?

MKV is the Matroska container, an open format published in 2002 that can hold an unlimited number of video, audio, subtitle and chapter tracks in one file. MP4 is far more restricted but far more widely supported: Safari, iOS, Android, smart TVs and nearly every social platform play MP4 and reject .mkv outright, even when the H.264 video inside the MKV is identical. This converter re-encodes to libx264 at CRF 23 on the veryfast preset with AAC audio at 160 kbps, and keeps only the first video and first audio track — embedded subtitle streams and secondary audio tracks are dropped, which is what most people want from a clean, playable MP4. The output is written with -movflags +faststart so it starts playing while it loads. Because ffmpeg.wasm runs single-threaded inside the browser tab, a long 1080p film can take a while — but the file never leaves your device, and the engine downloads once per session.

How to Use the MKV to MP4 Converter

Three steps, all in your browser.

  1. Load your MKV

    Drag a .mkv file onto the box or click to choose one. MKV is common for downloaded or ripped video.

  2. Convert to MP4

    Click Convert. The tool re-encodes the video to H.264 and the audio to AAC inside an MP4 container.

  3. Download the MP4

    Preview and download the .mp4 — ready to play on phones, TVs and the web, and to upload to sites that reject MKV.

What happens to my subtitle, audio and chapter tracks?

The container mismatch, not the video codec, is what you actually lose.

Matroska is deliberately permissive: it holds an unlimited number of audio tracks, soft subtitles as SRT, styled ASS/SSA or bitmap PGS, chapter markers, cover art, and even the font files a subtitle track needs. MP4 does none of that.

Track / featureIn the MKVIn the MP4 you getWhy
Main videoAny codecH.264, CRF 23Re-encoded for universal playback
Main audioAny codec, incl. FLACAAC, 160 kbpsMP4 has no FLAC or Vorbis path
Extra audio tracksUnlimitedDroppedOne track is kept for compatibility
SRT subtitlesSoft, selectableDroppedUse a sidecar .srt, or burn them in
ASS / SSA subtitlesStyled and positionedNot representableMP4 carries only 3GPP timed text
PGS / VOBSUBBitmap imagesNot representableMP4 has no bitmap subtitle track
ChaptersFull chapter listMay not surviveVerify before deleting the source
Attached fonts, cover artSupportedDroppedMP4 has no attachment box

Two consequences worth planning for. ASS/SSA and PGS subtitles have no MP4 equivalent at all — if the subtitles matter, extract the .srt from the MKV before converting and keep it as a sidecar, or burn it into the picture. And a multi-language rip keeps a single audio track, so a foreign-language file comes out in whichever track the source marks as default. Check that before you convert, not after.

There is a cost this tool does not hide: it always re-encodes to H.264. A large share of MKV files already contain H.264, and for those a stream copy would have been instant and mathematically lossless. Re-encoding them at CRF 23 spends a generation of quality, and a lot of your device’s time, on a change that is really only to the wrapper. For other legacy containers see AVI to MP4, whose problem is the opposite — an old wrapper holding an old codec.

Frequently Asked Questions

How do I convert MKV to MP4?
Load your .mkv file and click Convert. The tool re-encodes it to an MP4 with H.264 video and AAC audio, entirely in your browser. When finished you can preview and download the MP4 — no software to install and nothing uploaded.
Why won’t my MKV play, and does MP4 fix it?
Many devices, browsers and social platforms don’t support the Matroska (MKV) container even when the video inside is standard. Converting to MP4 with H.264/AAC — the most widely supported combination — makes the clip play in Safari, on iPhones, on smart TVs and on sites that reject .mkv uploads. The re-encode uses H.264 at CRF 23 with AAC audio at 160 kbps.
Is my video uploaded anywhere?
No — your MKV never leaves your device. Everything runs locally with ffmpeg compiled to WebAssembly, so the file is never uploaded, logged or stored. This is ideal for large personal videos. The video engine (about 10 MB) downloads the first time you convert in a session; after that, no further network requests are made.
How long does it take?
How long an MKV takes depends on its length and resolution and on your device. Because the work happens in your browser, a long 1080p or 4K movie can take a while. Shorter clips convert quickly.
Will subtitles or extra audio tracks be kept?
The tool keeps the main video and audio track for maximum compatibility. Embedded subtitle tracks and secondary audio streams in the MKV are not carried across, which is what most people want for a clean, playable MP4. If you need those tracks, keep the original MKV alongside the converted file.

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