Convert MP4 to MKV
Pick an .mp4 and get an .mkv — a lossless container remux that stream-copies every track with no re-encode. Runs in your browser, nothing is uploaded.
Convert MP4 to MKV in your browser
An MP4 and a Matroska .mkv are both just containers — boxes that hold
already-compressed video, audio and subtitle streams. Moving from one to the
other doesn't require touching the media inside. This tool does a lossless
remux: it runs ffmpeg -i in.mp4 -map 0 -c copy out.mkv, which stream-copies
every track from the MP4 straight into an MKV wrapper. Nothing is
re-encoded, so the picture and sound are bit-for-bit identical, and a short clip
converts in a fraction of a second.
-map 0 selects every stream in the input — the video, all audio tracks,
subtitles and data — rather than just the default video plus first audio. -c copy copies those packets across without invoking an encoder. Because MKV is a
superset container that accepts essentially every codec MP4 can carry (H.264,
HEVC, AV1, VP9, MPEG-4 video; AAC, AC-3, MP3 audio), the remux always succeeds —
there is no codec that "won't fit," so this tool has no re-encode fallback and
no settings to tune.
Everything runs locally with ffmpeg compiled to WebAssembly — your file is never uploaded to a server, and the page keeps working offline once it has loaded.
Why move to MKV?
MP4 handles multiple audio tracks and soft (selectable) subtitles poorly. MKV was built for exactly that. Remuxing to MKV first — losslessly, keeping your original streams untouched — gives you a container you can later add extra audio tracks or subtitle tracks to. If you instead want to re-encode (change codec, resolution or quality), that's a different job: use video-transcode or video-compress.
Worked example
Take a 6-second clip holiday.mp4 holding an H.264 video stream and an AAC
audio stream. Drop it in. Out comes holiday.mkv with the exact same H.264 and
AAC streams copied across — identical dimensions, identical duration, no visible
or audible change — now in a Matroska container ready to accept extra subtitle
or audio tracks. The conversion finishes almost instantly because nothing is
re-compressed.
Limits
- Input and output are each capped at 10 MiB — trim or compress longer clips first.
- This is a container remux only. It never changes codec, resolution or quality; to do that, use video-transcode or video-compress.
- Output is always an
.mkv(video/x-matroska). The remux keeps whatever codecs the MP4 already used — it doesn't convert them.
FAQ
Does converting MP4 to MKV lose quality?
No. This is a lossless container remux, not a re-encode. The exact video, audio
and subtitle streams are copied into the MKV wrapper untouched with -c copy,
so the result is bit-for-bit the same media, just in a different box. Only tools
that genuinely re-compress the picture (video-transcode, video-compress) change
quality.
Why is the conversion so fast?
Because nothing is re-encoded. Re-encoding video is the slow, expensive part; a
remux only rewrites the container metadata and copies the already-compressed
packets across with -map 0 -c copy, so a short clip converts in well under a
second.
Does it keep all my audio tracks and subtitles?
Yes. -map 0 selects every stream in the MP4 — the video, all audio tracks,
subtitles and data — and copies them all into the MKV. Nothing is dropped.
Because MKV is a superset container, whatever the MP4 held fits, which is exactly
why the remux always succeeds.
Is my video uploaded anywhere?
No. The conversion runs entirely in your browser using ffmpeg compiled to WebAssembly — the file never leaves your device, and the page works offline once it has loaded.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool mp4-to-mkv 'url=https://example.com/input'New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/mp4-to-mkv/?url=https://example.com/inputMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.