Subtitle formats explained
Subtitle formats all hold text and times, and they differ in what else they can carry: screen positions, fonts, colours, speaker names. Most of the time you only need to know which format your player or upload form takes and how to get your file into it, and the answer is usually SRT. Here's each format, where you'll run into it, and the tool that converts it.
SRT
The plainest format and the most widely accepted: a number, a start and end time, then the words. Players, editors, YouTube and social upload forms all take it. It can carry italics, bold and colour tags but no screen positions. What is an SRT file? covers it in full, and Convert to SRT turns any other text format here into one.
VTT (WebVTT)
The web's subtitle format, and the only one the HTML5 video player reads. It looks like SRT with a WEBVTT line on top and dots instead of commas in the times, and it adds screen positions and speaker tags. Zoom transcripts and YouTube Studio downloads come as VTT. Tools: SRT to VTT, VTT to SRT, VTT to text and VTT to Word.
ASS and SSA
SubStation Alpha (SSA) and the newer Advanced SubStation Alpha (ASS) carry full styling: fonts, colours, outlines, positions anywhere on screen, karaoke timing and animation. Anime fansubs use them and Aegisub is the usual editor, but many TVs and Plex apps can't draw them. Tools: ASS to SRT and SRT to ASS.
SMI (SAMI)
Microsoft's caption format, written like a web page with HTML-style tags. It's still common in Korea and in old Windows Media files. Tools: SMI to SRT, and Convert to SMI, which can save EUC-KR for older Korean TVs.
SUB (MicroDVD) and MPL2
Two older text formats. MicroDVD times each line by frame number, so it only lines up at the frame rate it was made for. MPL2, common with Polish subtitles, counts in tenths of a second. DVD rips also use .sub for a completely different format made of pictures, VobSub, covered below. Tools: SUB to SRT, and MPL2 files convert on Convert to SRT.
SBV
YouTube's own simple caption format and one of the download options in YouTube Studio. Almost nothing outside YouTube reads it. Tool: SBV to SRT.
LRC
Lyrics files for music players, with a time at the start of each line so the words scroll with the song. Enhanced LRC adds a time to each word. Tool: LRC to SRT.
TTML and DFXP
An XML caption format used by broadcasters and streaming services, also saved as .dfxp or .xml. Desktop players mostly don't read it. Tool: TTML to SRT.
PGS (SUP) and VobSub (IDX/SUB)
Picture subtitles from discs. PGS is the Blu-ray format, usually a .sup file, and VobSub is the DVD one, an .idx and .sub pair. Each line is an image of text, so nothing can edit or restyle it until it has been turned into text. Tools: SUP to SRT and IDX/SUB to SRT. Extract subtitles pulls either kind out of an MKV first.
Plain text, Word and PDF
Not subtitle formats, but often where subtitles need to end up: a transcript to quote, a script to review, a page to print. Tools: SRT to text, TXT to SRT for the way back, SRT to Word and SRT to PDF.
Which format to use
SRT, unless something tells you otherwise. Use VTT for video on your own web page, ASS when you want styling and your player can show it, and SMI only for the older Korean players that need it. When a file won't open, converting it to SRT usually fixes it.
Tools for fixing the timing
Shift subtitle timing moves every line earlier or later. The frame rate converter fixes subtitles that drift further out as the video goes on. Two-point sync corrects both from two lines you know the right times for, and auto sync matches the file to the video for you. Fix subtitle timing repairs overlapping lines and lines that flash past, and the reading speed checker finds lines too fast to read.
Tools for cleaning up the text
Clean up a subtitle file does ten jobs in one pass, for files that arrived messy. Remove SDH takes out speaker names and sound labels like [door slams]. Remove formatting strips tags, remove line breaks joins split lines, the case converter fixes ALL CAPS and fix encoding repairs garbled accents. Add pinyin is for learners reading Chinese subtitles.
Tools for editing, merging and styling
The subtitle editor opens any subtitle file in your browser so you can fix wording and times line by line. Merge subtitles joins two files, such as CD1 and CD2 or two languages to show at once, and split subtitles cuts one into parts. Change subtitle color and move subtitles on screen handle the look.
Tools for subtitles and video
Extract subtitles pulls subtitle tracks out of an MKV or MP4. MP4 to SRT and the auto subtitle generator write subtitles from a video's speech, and MP3 to SRT does it from a recording. Burn subtitles and add subtitles to video put them into the picture so they show everywhere.
Questions
What is the most common subtitle format?
SRT. Nearly every player, editor and video site reads it, which makes it the safest format to save in.
What's the difference between SRT and VTT?
VTT adds a WEBVTT header, writes times with a dot instead of a comma, and can place text on screen and tag speakers. SRT can't do either, and more software reads it. VTT to SRT converts between them. The full comparison, with when to use which, is in SRT vs VTT.
Which subtitle formats does YouTube accept?
SRT is the safest choice. YouTube also takes VTT, SBV, SAMI, TTML and several others.
Can I convert any subtitle format to any other?
Text formats, yes: the subtitle converter handles them. Picture formats from discs have to be turned into text first, on SUP to SRT or IDX/SUB to SRT.