# Convert MP4 to MKV
Free MP4 to MKV converter in your browser. Lossless stream-copy remux — keeps video, audio and subtitle tracks, nothing re-encoded or uploaded.
## Run it
- **CLI:** `gizza tool mp4-to-mkv 'url=https://example.com/input'`
- **Web:** https://gizza.ai/tools/mp4-to-mkv/
- **Agents:** machine-readable descriptor (parameters JSON Schema) at https://gizza.ai/tools/mp4-to-mkv/tool.json
## Inputs
- `file` — MP4 video _(file; accept: video/mp4,video/x-m4v,video/*)_
## Output
- MKV video (video)
## Query parameters
Open the tool pre-filled and auto-run via URL:
- `url` — fetch the input file from a public URL (CORS-permitting)
Example: `https://gizza.ai/tools/mp4-to-mkv/?url=https://example.com/input`
---
## 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.
## Related tools
- [Convert MKV to MP4](https://gizza.ai/tools/mkv-to-mp4/): Free MKV to MP4 converter that runs in your browser. Lossless stream-copy remux for H.264/HEVC clips, or re-encode VP9/AV1. Nothing is uploaded.
- [Convert MOV to MP4](https://gizza.ai/tools/mov-to-mp4/): Free MOV to MP4 converter that runs in your browser. Lossless container remux (stream-copy) for H.264/HEVC clips, or re-encode ProRes. Nothing is uploaded.
- [Extract M4A Audio from MP4](https://gizza.ai/tools/mp4-to-m4a/): Extract an MP4's audio track to M4A in your browser. Lossless stream-copy remux — no re-encode, no quality loss, nothing uploaded.
- [Convert Video to H.264 MP4](https://gizza.ai/tools/video-to-h264/): Force-transcode any video to universally-playable H.264 High-profile MP4 (yuv420p, faststart, AAC) in your browser. Make clips that won't play work everywhere.
- [Transcode a Video](https://gizza.ai/tools/video-transcode/): Convert any video to MP4 or WebM right in your browser — pick a target format and quality. Re-encodes locally with ffmpeg, nothing is uploaded, free.