MKV to MP4 with the video copied, not re-encoded
An MKV that plays perfectly on your computer can be useless on an iPhone, in a video editor or on a TV’s USB port. This MKV to MP4 converter rewraps Matroska files as MP4 in your browser. H.264 and HEVC video and AAC, Opus or MP3 audio are copied without re-encoding, so the picture is identical and a long film converts in about the time it takes to read it.
The tricky part of many MKV files is the soundtrack. Films and TV recordings often carry Dolby Digital (AC-3), Dolby Digital Plus (E-AC-3) or DTS audio, which browsers can’t decode. For each such track you decide: keep it unchanged (MP4 can carry AC-3 and E-AC-3, and TVs, Apple devices and VLC play it), leave it out, or convert it to AAC with ffmpeg.wasm, which is downloaded only if you agree. Even then, the video itself is copied untouched.
How to use the MKV to MP4 Converter
- Drop your MKV files on the upload area.
- Read the plan under each file. Video and AAC, Opus or MP3 audio show Copy; subtitle tracks are listed as dropped.
- If a track shows Choose (Dolby or DTS audio), pick “Convert to AAC with ffmpeg.wasm”, “Keep … as is” or “Leave this track out”.
- Press Convert to MP4. If you chose ffmpeg.wasm, confirm the one-time 32 MB download first.
- Check the MP4 in the preview, then download it or every file as a ZIP.
MKV to MP4 Converter features
Lossless video
H.264 and HEVC streams are copied packet for packet — no re-encoding, no quality loss and no long wait.
Every audio language kept
Dual-audio and multi-language MKVs keep all their tracks and language labels in the MP4.
Dolby and DTS your way
Keep AC-3 or E-AC-3 as they are, drop them, or convert AC-3, E-AC-3 and DTS to AAC — decided track by track.
Clear subtitle report
SRT, ASS, PGS and other subtitle tracks are listed with their language, so you know exactly what isn’t carried over.
Batch and ZIP
Convert a whole season at once and save it as one archive.
Why MKV doesn’t play everywhere
Matroska (MKV) is an open container that can hold almost any codec and any number of audio and subtitle tracks, which makes it ideal for films, TV recordings and archives. MP4 is stricter, and that strictness is exactly why phones, tablets, smart TVs, editors and upload sites support it so reliably.
When the codecs inside an MKV are ones MP4 also supports — H.264 or HEVC video with AAC audio is by far the most common pairing — converting is just a matter of moving the streams into a new container. That is what happens here, and why the plan usually reads Copy on every line.
What happens to each kind of track
| In the MKV | In the MP4 |
|---|---|
| H.264, HEVC, AV1 or VP9 video | Copied unchanged |
| VP8 video | Re-encoded to H.264 |
| AAC, Opus or MP3 audio | Copied unchanged |
| FLAC, Vorbis or PCM audio | Re-encoded to AAC |
| AC-3 / E-AC-3 (Dolby) | Your choice: keep, leave out, or convert to AAC with ffmpeg.wasm |
| DTS | Your choice: leave out, or convert to AAC with ffmpeg.wasm |
| SRT, ASS, PGS or VobSub subtitles | Not copied (listed in the plan) |
| Chapters and attached fonts | Not copied |
About ffmpeg.wasm and Dolby audio
Browsers don’t ship decoders for AC-3, E-AC-3 or DTS. To convert them, the tool can use ffmpeg.wasm: the FFmpeg project compiled to WebAssembly (@ffmpeg/core 0.12.10, licensed GPL-2.0-or-later), run unmodified in its own background worker. It is fetched from jsDelivr only after you confirm, then cached by your browser. Only the audio passes through it; Mediabunny (MPL-2.0) then puts the new AAC track next to the copied video.
Keeping AC-3 or E-AC-3 as is costs nothing and preserves the original surround mix — a good choice for TVs, Apple devices and VLC. Convert to AAC when the MP4 must play in Chrome or Firefox, or on devices without Dolby support. Subtitles can be saved beforehand with Extract Subtitles or drawn into the picture with Burn Subtitles.
MKV to MP4 Converter FAQ
How do I convert MKV to MP4 without losing quality?
Drop the MKV here and keep “Keep original” for both codecs. H.264 or HEVC video and AAC audio are copied into the MP4 unchanged, so there is no quality loss at all.
Why is there no sound after converting?
The track was probably AC-3, E-AC-3 or DTS and was left out — or kept as Dolby audio, which Chrome and Firefox can’t play. Convert it to AAC with ffmpeg.wasm instead and the MP4 will have sound everywhere.
What happens to the subtitles?
Subtitle tracks can’t be copied by the browser engine, so they are listed and left out. Save them first with Extract Subtitles, or burn them into the picture with Burn Subtitles.
Will the MP4 play on an iPhone or smart TV?
H.264 with AAC plays everywhere. HEVC plays on iPhones, Macs and most recent TVs. If the MKV uses something rarer, such as VP8, choose H.264 and the video is re-encoded.
Why is the MP4 smaller than the MKV?
Usually because a Dolby, DTS or subtitle track was left out, or a large lossless audio track (FLAC or PCM) became AAC. Copied tracks don’t change size.
Is it safe to convert my files here?
Yes. Everything runs in your browser and the MKV is never uploaded. ffmpeg.wasm, when you allow it, also runs on your device.