Turn MKV videos into MP4 files that play on iPhones, TVs, editors and social sites. When the video is H.264, HEVC or AV1 the streams are copied as they are, so even a multi-gigabyte movie converts in seconds with zero quality loss; anything else is re-encoded with ffmpeg right in your browser.
Last updated:
video.mkv
Size
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Length
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Container
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Tracks found
Conversion method
Starting…
Your MP4 is ready
MKV
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MP4
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How to use the MKV to MP4
Choose your MKV file: drop it on the box or click Choose MKV file. The tracks inside are listed straight away.
Pick the audio tracks you want to keep, for example only the English one.
Leave Copy streams selected for a fast, lossless conversion, or choose Re-encode to H.264 + AAC for maximum compatibility or a smaller file.
Press Convert to MP4 and watch the progress bar.
Preview and download the MP4 file.
MKV vs MP4: what changes when you convert
MKV (Matroska) and MP4 (MPEG-4 Part 14) are both containers: they hold one or more compressed video, audio and subtitle streams plus an index. Converting between them does not have to touch the picture at all. What matters is whether the codecs inside the MKV are allowed in MP4:
Stream in the MKV
What this tool does
Quality
H.264 / AVC video
Copied into MP4 (avc1)
Identical
HEVC / H.265 video
Copied into MP4 (hvc1, Apple-compatible tag)
Identical
AV1 video
Copied into MP4 (av01)
Identical
VP8, VP9, MPEG-2, MPEG-4 ASP, Theora
Re-encoded to H.264 with ffmpeg
Slight loss (CRF 20 to 27)
AAC, MP3, MP2 audio
Copied
Identical
AC-3 (Dolby Digital)
Copied (ac-3 with dac3 box)
Identical
Opus, Vorbis, FLAC, DTS, E-AC-3, TrueHD, PCM
Converted to AAC 192 kbps
Transparent for most listeners
SRT, ASS, PGS subtitles
Not included
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How the lossless copy works
The tool reads the Matroska header (EBML) to find the tracks, codecs, languages and codec setup data (the CodecPrivate element, which for H.264 is exactly the avcC record MP4 needs).
It walks through every cluster and block, noting where each video and audio frame sits in the file, its timestamp and whether it is a keyframe. Laced audio blocks (Xiph, EBML and fixed lacing) and header-stripped tracks are unpacked.
Matroska stores presentation times; MP4 needs decoding times plus composition offsets for B-frames. These are derived from the sorted timestamps, and an edit list keeps audio and video in sync (including the 21 ms AAC priming delay).
Finally it writes the MP4 index (moov, placed at the start for fast web playback) followed by the frames in their original interleaved order.
Because the frames are referenced rather than copied into memory, the conversion speed is limited only by how fast your disk can read the MKV.
Re-encoding settings explained
Re-encoding uses the x264 encoder inside ffmpeg.wasm with the veryfast preset. CRF (Constant Rate Factor) sets quality instead of a fixed bitrate: 18 to 20 looks visually lossless, 23 is the x264 default and a good balance, 27 and above gives small files with visible softening. Each step of 6 roughly halves or doubles the file size. Lowering the resolution to 720p cuts the size and encoding time by more than half compared with 1080p.
Frequently asked questions
How do I convert MKV to MP4 without losing quality?
Use Copy streams. MKV and MP4 are just containers: when the video inside is H.264, HEVC or AV1 and the audio is AAC, MP3 or AC-3, the tool moves the compressed streams into an MP4 container byte for byte. Nothing is re-encoded, so quality is identical and it takes seconds.
Why is Copy streams not available for my file?
MP4 cannot store every codec. VP8, VP9, MPEG-2, Theora and AVI-style MPEG-4 video must be re-encoded, so choose Re-encode to H.264 + AAC. Audio in Opus, Vorbis, FLAC, DTS, E-AC-3 or TrueHD is converted to AAC automatically while the video is still copied.
Can it convert large MKV files like full movies?
Yes. The quick copy reads the MKV in small pieces and builds the MP4 from references to the original file, so 4 GB and larger movies work. Re-encoding with ffmpeg in the browser is limited to about 2 GB because of browser memory limits.
Are subtitles kept?
No. Subtitles in MKV files (SRT, ASS or image-based PGS) are not copied, because most MP4 players ignore them. Keep the subtitles as a separate .srt file next to the MP4, or burn them into the picture with a subtitle tool.
What happens with multiple audio languages?
Every audio track is listed. Tick the ones you want; the first one becomes the default track in the MP4 and the others can be switched in players such as VLC, QuickTime and most TVs.
Is my video uploaded to a server?
No. The whole conversion runs in your browser. The quick copy uses plain JavaScript; re-encoding downloads the open-source ffmpeg converter (about 31 MB, cached afterwards) and runs it on your own computer.
Why does the MP4 not play in my browser preview?
Browsers only preview some codecs. HEVC plays in Safari and Edge, AC-3 audio is silent in Chrome and Firefox. The MP4 file itself is fine: open it in your video player, or choose Re-encode to H.264 + AAC for a file that plays everywhere.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.