Subtitle formats explained: SRT vs VTT vs ASS

At a glance

SRT: the one almost everything accepts

An SRT file is a list of numbered blocks. Each block has a sequence number, a start and end time joined by -->, one or more lines of text, and a blank line before the next block.⁶

1
00:00:00,800 --> 00:00:03,200
Welcome to the tutorial.

2
00:00:03,500 --> 00:00:06,400
Today we will make <i>subtitles</i>.

• Times are hours:minutes:seconds,milliseconds, with a comma, not a period.¹
• SRT has no formal specification, and the Library of Congress notes there is no character-encoding standard for it.¹ That is why the same file can show broken accents in one player and not another. Save it as UTF-8, which is what YouTube requires.⁸
• Formatting tags borrowed from HTML (bold, italic, underline, colour) work in some players, but YouTube ignores styling in SRT files.^1,8

WebVTT: SRT’s stricter web cousin

WebVTT began in 2010 as “WebSRT” and was built on SRT.² It is now a W3C Candidate Recommendation Draft, the format the HTML <track> element uses for subtitles and captions on web pages.^3,5

WEBVTT

NOTE Converted from tutorial.srt

1
00:00:00.800 --> 00:00:03.200
Welcome to the tutorial.

2
00:00:03.500 --> 00:00:06.400 line:10% align:center
<v Presenter>Today we will make <i>subtitles</i>.

• The file must start with WEBVTT, be UTF-8, and is served as text/vtt.^3,4
• Times use a period before milliseconds, and the hours part can be left out when it is zero.³ Cue numbers are optional identifiers.³
• Settings after the timestamp place a cue on screen: vertical, line, position, size and align.⁴ NOTE blocks are comments, and STYLE blocks hold CSS.³

ASS: for subtitles that need to look a certain way

SubStation Alpha (SSA) and its successor Advanced SSA (ASS) come from the SubStation Alpha editor and support positioning, karaoke and style management.⁶ A file is laid out like an INI file in sections.⁶

[Script Info]
ScriptType: v4.00+
PlayResX: 1920
PlayResY: 1080

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,Arial,56,&H00FFFFFF,&H000000FF,&H00000000,&H80000000,0,0,0,0,100,100,0,0,1,3,1,2,60,60,50,1

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.80,0:00:03.20,Default,,0,0,0,,Welcome to the tutorial.
Dialogue: 0,0:00:03.50,0:00:06.40,Default,,0,0,0,,{\an8}Today we will make {\i1}subtitles{\i0}.

• [Script Info] holds the script resolution; PlayResY matters because font sizes and positions are scaled by it.⁶
• [V4+ Styles] defines named styles: font, size, colours, outline, alignment and margins.⁶
• [Events] lists each Dialogue line with its style, and times are in hundredths of a second.⁶
• Override tags in braces change one line: {\an8} moves it to the top centre (positions follow a phone keypad, 1 to 9), and {\i1} … {\i0} turns italics on and off. Colours are written in blue-green-red order.⁷

The catch: none of the upload help pages we checked (YouTube, Vimeo, Adobe Premiere) lists ASS.^8,9,10 To keep the styling, burn the subtitles into the video, or ship them inside an MKV file, which can store SSA/ASS subtitles with their styles.⁶

Which platforms accept which format

From each platform’s own help or documentation page. “Not listed” means the page we read doesn’t name the format, not that it was tested and failed.

The VLC row combines the features page and the VideoLAN Subtitles wiki, which was last edited in April 2019.^11,12

Which one should you use?

• Uploading to YouTube, Vimeo or an editor: SRT. It is on every list in the table above that accepts uploads.
• A video player on your own website: WebVTT, because that is what the <track> element reads.⁵ Vimeo also recommends it.⁹
• Styled typesetting, signs or karaoke: ASS, then burn it in or keep it in an MKV, since upload platforms don’t list it.
• Not sure: start with SRT. It carries so little beyond text and timing that converting it to the other two loses almost nothing.

Converting between formats

SRT to WebVTT by hand: add a line reading WEBVTT and a blank line at the top, change the comma before the milliseconds in every timestamp to a period, and save the file as UTF-8 with a .vtt extension. The SRT sequence numbers can stay; WebVTT treats them as cue identifiers.³

With FFmpeg: it reads and writes all three formats,¹³ and picks the output format from the file extension:

ffmpeg -i tutorial.srt tutorial.vtt
ffmpeg -i tutorial.srt tutorial.ass
ffmpeg -i tutorial.ass tutorial.srt

Converting ASS to SRT or WebVTT keeps the text and timing but drops what the simpler format can’t express: fonts, positions, colours and karaoke effects.

Make an SRT from a recording with ClapClip

ClapClip’s transcription app runs on Windows 10 and 11 and writes a standard .srt file and plain text from the same pass, with timestamps taken from the recognised speech rather than fixed intervals. The audio is processed on your PC, not uploaded.

1. Add the recording. Video: MP4, MOV, MKV or WEBM. Audio: MP3, WAV, FLAC, AAC, M4A or OGG. You don’t need to extract the audio first.
2. Pick the spoken language and choose SRT as the output.
3. Review the draft: names, numbers and jargon first, then line breaks (one or two lines per cue).
4. Play the video with the subtitles on to check timing, then save the file as UTF-8 with the same name as the video, for example tutorial.srt next to tutorial.mp4.
5. Need WebVTT or ASS? Convert the finished SRT with one of the methods above.

The full walkthrough, with review and timing tips, is in How to Create SRT Subtitles from Video on Windows. For the tool itself, see the SRT generator.

See ClapClip transcription

Sources

Each source was read on October 29, 2026.

1. SubRip Subtitle format (SRT), fdd000569 · Library of Congress, Sustainability of Digital Formats
2. Web Video Text Tracks Format (WebVTT), fdd000567 · Library of Congress, Sustainability of Digital Formats
3. WebVTT: The Web Video Text Tracks Format (Candidate Recommendation Draft, 20 May 2026) · W3C Timed Text Working Group
4. Web Video Text Tracks Format (WebVTT) · MDN Web Docs
5. <track>: the embed text track element · MDN Web Docs
6. Subtitles: SRT and SSA/ASS subtitles · Matroska specification
7. ASS Override Tags · Aegisub documentation
8. Supported subtitle and closed caption files · YouTube Help
9. How to add captions or subtitles to my video · Vimeo Help Center
10. Supported file formats for captions (updated Jan 7, 2026) · Adobe Premiere Help
11. VLC features · VideoLAN
12. Subtitles (last edited 1 April 2019) · VideoLAN Wiki
13. General Documentation: Subtitle Formats · FFmpeg