Burn subtitles into a video

How Add Subtitles to Video works

The file is read with a FileReader, rewritten in memory and offered as a download. This is why large PDFs are limited by your device's RAM rather than by an upload size limit. A 200 MB scan never crosses the network, so the slowest part is your own disk.

That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.

Soft versus burned captions

A soft subtitle is a separate track inside the container: the player draws it, and the viewer can turn it off. It is elegant, and it breaks the moment the video is uploaded somewhere that ignores extra tracks — which is most social platforms.

Burning draws the text into the frames themselves. Nothing can switch it off, nothing can lose it, and the caption survives re-compression by Instagram or WhatsApp. The cost is that the original clean video is gone unless you kept a copy.

What an SRT file actually is

Plain text, three parts per block: a number, a time range, and the caption. Hours:minutes:seconds,milliseconds separated by an arrow, then the text until a blank line. That is the whole format — you can fix a typo in Notepad.

WebVTT is the same idea with a WEBVTT header and dots instead of commas. Both are accepted here, and both are converted by libass into rendered frames.

Why the timing is not editable here

A subtitle editor is a different tool with a timeline, waveform and keyboard shortcuts. Building a bad one would help nobody.

The workflow that works: get the .srt from a transcription service or write it, fix the timings in a text editor or the existing subtitle tool set, then burn the final version here.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions