SRT and VTT are close cousins, not twins
SubRip (.srt) is the oldest, most widely produced caption format. It is what almost every transcription tool, editing app, and download button hands you. WebVTT (.vtt) is the format the web platform actually understands: it is the only caption format the HTML5 <track> element is required to support. The two look nearly identical at a glance, which is exactly why dropping a raw .srt into a <track> tag fails silently: the browser parses the file, hits a line it does not expect, and shows no captions at all.
The three changes this tool makes
The conversion is small and reversible, but every part of it matters. First, a WEBVTT header line is prepended, because VTT files that lack it are rejected outright. Second, the decimal separator in every timecode flips from a comma to a dot: SRT writes 00:00:01,000 while VTT requires 00:00:01.000. Third, the bare numeric sequence counter that SRT puts before each cue is stripped, because VTT does not use cue numbers (and a stray number on its own line can confuse a strict parser). Everything else is preserved byte-for-byte: the text, the line breaks, and the timing.
Why this runs entirely in your browser
A subtitle file is plain text, often holding a full transcript of a private call, a customer demo, or unreleased footage. There is no reason for it to touch a server. This tool reads the file directly in your tab with the browser's own FileReader, rewrites the timecodes in memory, and hands you a download. No network request leaves your machine. You can confirm it yourself: open your network tab, run a conversion, and watch it stay empty. The same privacy-first principle drives the rest of Clipy, the free screen recorder these tools were built around.
Skip subtitle wrangling entirely
Converting captions by hand is a chore that exists only because your recording tool did not give you the right format to begin with. Clipy auto-generates captions on every screen recording: transcribed, time-aligned, and ready to play on the share page without you touching a single .srt or .vtt file. If you find yourself round-tripping subtitle formats often, record in Clipy instead and let the captions come with the video for free.