MP4 to SRT

Some MP4 files already carry a subtitle track (mov_text). This tool doesn't read those: it listens to the audio and writes new subtitles from the speech, which works for any MP4, including phone recordings and screen captures that never had subtitles.

The result is a standard SRT with up to 42 characters per line and two lines per subtitle. Rename it to match the MP4 (video.mp4 and video.srt) and most players load it automatically.

Transcription is done by Whisper, the open-source speech recognition model from OpenAI, running inside your browser. The model downloads once (40 to 250 MB depending on the accuracy you pick) and is cached, so later files start straight away. Your video is never uploaded.

With a recent Chrome or Edge on a computer with a graphics card, it runs on the GPU and takes a fraction of the video's length. Without one it runs on the CPU, which can take about as long as the video itself. Files up to 30 minutes work.

Whisper understands about 100 languages and adds punctuation. Accuracy is good on clear speech, and lower with music, heavy accents or several people talking at once: check names and numbers before you publish.