There's an important technical distinction between how this tool adds subtitles and what "burned-in captions" means, and it changes which platforms the result actually works on. This tool muxes your .srt file into the video as a soft, selectable subtitle track (using the mov_text codec inside the MP4 container) — the video and audio streams are copied untouched, and the subtitle track rides alongside them as separate, toggleable data. It does not currently burn the subtitle text into the video pixels themselves.
Why that distinction matters for where you're posting
A soft subtitle track works exactly like closed captions on YouTube or a Netflix show — the viewer's player has to support subtitle tracks and the viewer has to turn them on. That's the right fit for YouTube, a video hosted on your own site with an HTML5 <video> player, or any platform/player that reads MP4 subtitle tracks. It will not show up automatically on Instagram Reels, TikTok, or YouTube Shorts — those platforms either don't support selectable subtitle tracks at all, or don't surface them by default, which is exactly the "auto-muted feed" problem burned-in captions are meant to solve. If your goal is captions that are always visible on a short-form video platform, this tool's current soft-track output won't get you there; you'd need the text rendered directly into the video frame.
What you get either way
Because the video and audio streams are stream-copied rather than re-encoded, this step is fast and doesn't degrade video quality — the trade-off for that speed is that the subtitles are a separate track, not pixels.
Common use cases (with the platform note above in mind)
- Adding a toggleable subtitle track to a video hosted on your own site or YouTube
- Providing an accessible caption track for training videos, without altering the original video quality
- Overlaying a translated subtitle track onto existing footage for platforms that support subtitle tracks
FAQ
Will these subtitles show up on Instagram Reels, TikTok, or YouTube Shorts?
Not reliably — those platforms generally don't support selectable subtitle tracks. This tool currently produces a soft/selectable track, not captions burned into the video frame, so for short-form platforms it won't have the intended effect.
What subtitle format is supported?
Standard .srt (SubRip) files.
Does this re-encode my video, and will quality be affected?
No — video and audio are stream-copied unchanged; only the subtitle track is added. This is fast and lossless for the original footage.
Is the video uploaded to a server?
No. All processing runs in your browser via ffmpeg compiled to WebAssembly.
Privacy
Subtitle muxing runs 100% client-side. No video or subtitle data is transmitted to external servers.