MKV subtitles — remux to MP4, then burn them in
Remux is a stream copy, not a re-encode — a 2 GB MKV converts in seconds · burning then runs 100% browser-local
Honest answer first: no browser can decode MKV, so no browser tool — BurnSub included — can burn subtitles into one directly. What actually works, and takes about ten seconds, is remuxing: copying the video and audio streams unchanged into an MP4 container. No re-encode, no quality loss, no upload. After that, BurnSub handles everything locally: generate captions with Whisper or import the SRT/VTT you already have, style them, and burn them in.
How to mkv subtitles
- 1
Remux the MKV to MP4
One line with the free ffmpeg tool: ffmpeg -i input.mkv -c copy output.mp4 — it copies the streams without re-encoding, so it finishes in seconds and loses no quality. (If the MKV has an embedded subtitle track you want, export it too: ffmpeg -i input.mkv -map 0:s:0 subs.srt)
- 2
Drop the MP4 into BurnSub
Open burnsub.com and drop the new MP4. It stays on your device.
- 3
Add your captions
Click "Import SRT/VTT" to load the exported track or any subtitle file — or let Whisper auto-generate captions locally.
- 4
Style the captions
Apply a preset (anime-katakana-vibe, cinema-letterbox, gaming-neon, etc.) or customize manually.
- 5
Burn and download
Captions are rendered into the frames and the MP4 downloads directly. Your original MKV is untouched, and nothing was uploaded at any step.
Frequently asked
Why can't browsers open MKV files?
Matroska (MKV) is a container format that browser <video> elements do not demux. The codecs inside (H.264, AAC…) are usually supported — which is exactly why a remux to MP4 works without re-encoding.
Does remuxing lose quality?
No. A remux (-c copy) copies the compressed video and audio streams bit-for-bit into a new container. Nothing is decoded or re-encoded.
Can BurnSub read subtitle tracks embedded in the MKV?
Not directly — there is no Matroska parser in the browser. Export the track with ffmpeg (-map 0:s:0 subs.srt) or a tool like SubtitleEdit, then use Import SRT/VTT. Image-based tracks (PGS, VobSub) need OCR first.
What if I drop an MKV into BurnSub anyway?
You get a clear message with the exact remux command instead of a cryptic codec error.
Is anything uploaded?
No. The remux happens on your machine with ffmpeg, and BurnSub processes the MP4 entirely in your browser.
Ready to burn mkv → mp4 (one-line remux), plus srt/vtt into mp4?
Free forever · No upload · No watermark · No signup
🔥 Start Burning