SRT to VTT converter

What actually changes

The two formats carry the same information and differ in three small ways, all of which this converter handles.

• A WEBVTT signature is added as the first line. A VTT file without it is invalid and browsers refuse to load it.
• Timestamps switch from a comma before the milliseconds to a period: 00:00:01,000 becomes 00:00:01.000.
• Sequence numbers are dropped, since VTT treats a line before a timestamp as an optional cue identifier rather than an index.

Why browsers need VTT

HTML5 video only understands WebVTT. If you point a <track> element at an .srt file, the browser will fetch it and then silently show no subtitles, which is the single most common reason captions fail to appear on a self-hosted video.

Most upload-based platforms are more forgiving. YouTube and Vimeo both accept SRT directly, so you only need this conversion when you are serving the video yourself.

Serving the converted file

Two things trip people up after a correct conversion. The file has to be served with a text/vtt content type, because some servers send .vtt as a download and the browser will not parse it. And if the video and the subtitle file sit on different domains, the subtitle request needs CORS headers, since a track element is subject to the same rules as a script.