Glossary
What is an SRT file?
By the KeyMo team · Updated
An SRT file is a plain-text subtitle file, named after SubRip, that stores each caption as a numbered block with a start time, an end time and the text. A player reads it and shows the words over the video at those times. The file contains no video or audio.
What an SRT file is
Open an .srt file in a text editor and you will see the whole format. Each block has an index number, a line with a start time and an end time, and one or two lines of text, followed by a blank line. YouTube recommends SubRip or SubViewer to anyone new to caption files for that reason: they need only basic timing and can be edited in an ordinary text editor.
Here is a three-line sample, with the timing line as YouTube shows it.
| Number | Times | Text |
|---|---|---|
| 1 | 00:00:01,000 --> 00:00:03,500 | We stopped hiring for skill. |
| 2 | 00:00:03,500 --> 00:00:06,000 | We started hiring for taste. |
| 3 | 00:00:06,200 --> 00:00:09,000 | That one change fixed the team. |
The times are hours, minutes, seconds, then a comma, then milliseconds. The arrow sits between the start and the end.
The rules that trip people up
- The millisecond separator is a comma, not a period. YouTube's own example uses a comma.
- The file must be plain UTF-8 text for YouTube to accept it.
- YouTube reads only the basic version of SRT. Style markup is not recognised, so colours and fonts you add will not carry over.
- YouTube's example numbers each block, and by convention a blank line separates one block from the next.
- There is no official specification for SRT in the sources we read. The format is defined by habit and by the tools that read it.
SRT vs WebVTT
| SRT | WebVTT | |
|---|---|---|
| Header | None | The file must begin with WEBVTT |
| Millisecond separator | Comma | Period |
| Style and position | None in the basic version YouTube reads | CSS-style blocks, cue settings, speaker voice tags |
| Spec | No formal specification found | W3C Candidate Recommendation Draft, 20 May 2026 |
| Name in the W3C accessibility pages | Named as a caption format | Called the most common caption format on the web |
YouTube accepts both. If you need styling or positioning, WebVTT is richer, but YouTube lists WebVTT as an advanced format in initial implementation, with limited styling. See VTT, SRT vs VTT and the SRT to VTT converter.
Worked example: why a burned-in clip does not need one
An SRT file is a separate sidecar: the video and the text travel as two files, and the viewer's player draws the words. A clip with captions drawn into the picture needs no sidecar to show them. They are part of every frame. KeyMo's clips are 1080x1920 MP4 files with the captions in one of 12 styles, so the words are visible without uploading anything alongside. You would reach for an SRT when you want a viewer to switch captions on and off, or to upload a translation to a platform that supports caption tracks. See open captions and closed captions.
Common mistakes
- Saving a file as .txt after editing, so no tool recognises it.
- Sending a file in the wrong text encoding. YouTube wants plain UTF-8.
- Converting to VTT and forgetting that the comma becomes a period.
- Expecting the file to change how a burned-in caption looks. It can't.
Sources: YouTube Help: Supported subtitle and closed caption files (checked 11 October 2026); W3C: WebVTT (checked 11 October 2026); W3C WAI: Captions/Subtitles (checked 11 October 2026).