YouTube caption file formats

YouTube's help splits supported files into three groups:

• Basic: SubRip (.srt), SubViewer (.sbv or .sub), MPsub (.mpsub), LRC (.lrc) and Videotron Lambda (.cap). Text and timing only; styling is ignored.
• Advanced: SAMI (.smi), RealText (.rt), WebVTT (.vtt), TTML (.ttml) and DFXP (.dfxp). WebVTT supports positioning and bold, italic and underline; TTML supports styling and positioning.
• Broadcast: Scenarist (.scc), EBU-STL (.stl) and other TV formats.

SubRip is the most widely supported caption file anywhere: numbered cues, a start and end time with a comma before the milliseconds, then the text. A blank line ends each cue.

1
00:00:00,500 --> 00:00:03,200
Welcome back to the channel!

2
00:00:03,400 --> 00:00:06,900
Today we're testing the Sony A7 IV.

WebVTT starts with a WEBVTT line, uses a dot before the milliseconds, doesn't need cue numbers, and allows settings after the times (such as line:10% to move a cue up) and simple tags like <b> and <i>.

WEBVTT

00:00:00.500 --> 00:00:03.200
Welcome back to the channel!

00:00:03.400 --> 00:00:06.900 line:10%
Today we're testing the <b>Sony A7 IV</b>.

SBV (SubViewer) is the plain format YouTube has long used for caption downloads: start and end separated by a comma, single-digit hours, and no cue numbers.

0:00:00.500,0:00:03.200
Welcome back to the channel!

0:00:03.400,0:00:06.900
Today we're testing the Sony A7 IV.

YouTube's player has its own timed-text format, srv3, usually saved as .ytt. It carries what the other formats can't show on YouTube: colours, background boxes, outlines, YouTube's caption fonts and placement anywhere on the frame. Tools like CaptionMe and YTSubConverter write it.

In January 2026 YouTube switched off srv3 and YTT caption files, saying they caused playback failures for some viewers and that the change was temporary. Before relying on styled captions, upload a test file to an unlisted video and check it in the player. If styling must always show, it has to be burned into the video, which means viewers can't turn it off or pick a language.

The free caption converter reads SRT, VTT, SBV, ASS and YouTube timed text and writes any of them, in your browser. For styled output it previews the caption as you set fonts, colours and position.