MKV to MP4 Converter
Turn MKV videos into universally playable MP4 files. Stream-copy remux when possible (fast and lossless), automatic re-encode when not. Free, no sign-up, everything in your browser.
Powered by ffmpeg.wasm (WebAssembly). No upload, no account, no watermark.
How the MKV to MP4 converter works
MKV and MP4 are both containers: wrappers around the actual video and audio streams. Most MKV files already contain H.264 video and AAC or AC3 audio, formats that fit perfectly inside an MP4. In that case this converter simply repackages the streams, a process called remuxing. The video and audio data is copied bit-for-bit with no re-encoding, so the result is lossless and finishes in seconds even for long files.
Some MKV files use codecs that MP4 players and the MP4 container do not support well, such as VP9 or AV1 video, or DTS and TrueHD audio. When a plain remux is not possible, the converter automatically falls back to re-encoding: video is compressed to H.264 at CRF 23 and audio to AAC at 192k, which plays on virtually every device, browser and TV. The result panel always tells you which path was used: stream copy or re-encode.
Everything runs locally in your browser with ffmpeg.wasm, a WebAssembly build of FFmpeg. The engine (about 32MB) downloads once and is then cached. Your file is never uploaded to any server, which also means there are no queue times and no per-file fees.
Tips for best results
| Situation | What to do |
|---|---|
| MKV with H.264 video (most downloads and rips) | Nothing: the remux path handles it losslessly in seconds |
| MKV with VP9, AV1, DTS or TrueHD | Expect the slower re-encode path; the output plays everywhere |
| Multiple audio tracks or subtitles | The MP4 keeps the first video stream and all audio streams; subtitle tracks are dropped because MP4 cannot carry most of them |
| File much larger than 200MB | Remuxing large files is fine, but re-encoding them is slow on low-end devices; keep the tab in the foreground |
Frequently asked questions
Is MKV to MP4 conversion lossless?
Yes, when the stream-copy remux works. The video and audio data is copied into the MP4 container unchanged, so quality is identical and conversion takes seconds. If your file uses a codec MP4 cannot carry, the tool re-encodes it to H.264 + AAC instead, which involves a small, controlled quality trade-off at CRF 23. The result panel always shows which path was used.
Why is the conversion so fast sometimes and slow other times?
Remuxing only rewrites the container, so it runs as fast as your device can read and write the file. Re-encoding decodes and recompresses every frame on your device's processor, which takes roughly as long as the video itself. The tool always tries the fast path first.
Why did my file get re-encoded instead of remuxed?
Your MKV contains a codec that does not fit in MP4 or that MP4 players widely reject: typically VP9 or AV1 video, or DTS, TrueHD or similar audio. Re-encoding to H.264 + AAC makes the file play on phones, TVs, browsers and game consoles.
Is there a file size limit?
There is no hard limit, but the file is processed in browser memory, so files above about 2GB will not fit. Files under about 200MB convert most comfortably; remuxing larger files usually still works, while re-encoding them can be slow.
Does my video get uploaded anywhere?
No. The converter runs entirely in your browser with ffmpeg.wasm. After the one-time engine download you can go offline and it will still work. Your file never leaves your device.