Convert MKV to MP4
Pick a Matroska .mkv and get an .mp4 — a lossless container remux when possible, re-encode when needed. Runs in your browser, nothing is uploaded.
Convert MKV to MP4 in your browser
A Matroska .mkv and an .mp4 are both just containers — boxes that wrap
already-compressed video and audio streams. When your MKV already holds the
codecs MP4 uses (H.264 or HEVC video with AAC audio, which is what a huge share
of MKV rips and recordings carry), converting it is just a remux: the video
and audio packets are copied straight into an MP4 wrapper with no re-encoding.
That's lossless, changes nothing about the picture or sound, and finishes in a
fraction of a second.
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.
Two modes
- Remux (default) — stream-copies the video + audio with
-c copyand writes a web-friendly+faststartMP4. Lossless, near-instant, no quality change. Use this whenever the MKV holds H.264/HEVC video and AAC audio. - Re-encode to H.264/AAC — MKV is a superset container, so it often carries codecs MP4 can't legally hold: VP8/VP9/AV1 video and FLAC/Vorbis/Opus audio. For those, re-encoding always produces a valid MP4; it's slower and lossy, so the quality slider (1–100, default 75) trades size against fidelity.
Both modes keep the video and audio and drop MKV-only extras — soft subtitles (SRT/ASS/PGS) and font attachments — that MP4 can't carry.
Worked example
Take a 6-second clip episode.mkv holding an H.264 video stream and an AAC
audio stream. Leave the mode on Remux and drop the file in. Out comes
episode.mp4 with the exact same H.264 and AAC streams — identical duration and
dimensions, no visible change — ready to play on a phone, TV or anything that
rejects .mkv.
If instead the MKV held VP9 video or FLAC audio, Remux would fail with a codec error; switch to Re-encode to H.264/AAC and it converts (re-encoding the picture to H.264 and the sound to AAC).
Limits
- Input and output are each capped at 10 MiB — trim or compress longer clips first.
- Remux mode requires MP4-legal codecs (H.264/HEVC + AAC); if it errors, use Re-encode mode.
- Soft subtitle and attachment tracks are dropped — MP4 can't carry most of them. The video and all audio are kept.
- Output is always an
.mp4(H.264/HEVC video, AAC audio). To target WebM instead, use the video-transcode tool.
FAQ
Does converting MKV to MP4 lose quality?
In Remux mode, no — it's a lossless container change. The exact H.264/HEVC video and AAC audio streams are copied into the MP4 wrapper untouched, so the result is bit-for-bit the same media, just in a different box. Only Re-encode mode is lossy, because it genuinely re-compresses the picture and sound.
Why is the conversion so fast?
Because in the default Remux mode nothing is re-encoded. Re-encoding video is
the expensive part; a remux only rewrites the container metadata and copies the
already-compressed packets across with -c copy, so a short clip converts in
well under a second.
My MKV won't remux — it errors. What now?
That means the MKV holds a codec MP4 can't carry — commonly VP8/VP9/AV1 video or FLAC/Vorbis/Opus audio. Switch the mode to Re-encode to H.264/AAC, which always produces a valid MP4. It's slower and lossy, so raise the quality slider if you want to preserve more detail.
What happens to subtitles and attachments in the MKV?
They're dropped. MKV can carry soft subtitles (SRT/ASS/PGS) and font attachments, but MP4 can't legally hold most of them — leaving them in makes an otherwise-convertible file error out. This tool keeps the video and every audio track and discards those MKV-only extras. If you need burned-in subtitles, use the video-caption-burner tool first.
What does the quality slider do, and when does it matter?
It only applies in Re-encode mode, where 1–100 maps onto a practical slice of ffmpeg's libx264 CRF scale (higher = better quality and a larger file): 100 is visually lossless (CRF 18), the default 75 is roughly CRF 24, and 1 is small and low quality (CRF 40). In Remux mode it's ignored entirely, because a stream-copy never re-encodes.
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 mkv-to-mp4 'url=https://example.com/input' 'mode=copy' 'quality=75'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/mkv-to-mp4/?url=https://example.com/input&mode=copy&quality=75Machine-readable descriptor: tool.json — title + parameters JSON Schema for agents.