The number
A counter, starting at 1 and going up. It marks where one caption ends and the
next begins.
Video tower · floor
What an SRT file is, and the four lines it repeats
An SRT file is a plain text file with a very short structure, and the quickest way to understand it is to look at one. This example is published by YouTube in its own documentation.
YouTube publishes this example of a SubRip file. The whole format is here. That is the entire specification as a reader needs it: a number, a time range,
the words, a blank line, repeated. Nothing else is required, which is why YouTube says these
files “can be edited using any plain text editing software”.What is actually inside one
1
00:00:00,599 --> 00:00:04,160
>> ALICE: Hi, my name is Alice Miller and this is John Brown
2
00:00:04,160 --> 00:00:06,770
>> JOHN: and we’re the owners of Miller Bakery.
3
00:00:06,770 --> 00:00:10,880
>> ALICE: Today we’ll be teaching you how to make
our famous chocolate chip cookies!
4
00:00:10,880 --> 00:00:16,700
[intro music]
The separator between blocks. Easy to lose when editing by hand, and the file stops working
without it.The time range
hours:minutes:seconds,milliseconds, twice, joined by -->. Note the
comma before the milliseconds rather than a full stop.The blank line
Once the shape is clear, so is what the format does not involve. There is no special program required to make one, no format to convert into, and nothing hidden. If you can type, you can write a subtitle file — the work is in getting the timings right, not in the file itself.
Two constraints are not about structure at all, and YouTube states both in the same sentence.
Two constraints from YouTube, and between them they explain most rejected files. UTF-8 is
a text encoding — a file saved in something else may upload and then show the accented and
non-Latin characters as nonsense, because the words were correct and the encoding was not. And
markup is not read: bold tags, colours and positioning put into an SRT are either ignored
or make the file unreadable, depending on where they land.“Only basic versions of these files are supported. No style info (markup) is
recognized. The file must be in plain UTF-8.”
The encoding one is worth expanding, because it is a property of how the file was saved rather than of what is written in it. UTF-8 is a way of representing characters; a file saved another way can contain exactly the right words and still not be the file YouTube says it requires. YouTube states the requirement and not the consequence of missing it, so this page will not describe what a wrongly-encoded file does. What is worth knowing is where the setting lives: text editors generally offer the encoding as a choice at the moment of saving, and UTF-8 is the one the requirement names.
The markup one is simpler: SRT has no way to say “put this in the corner” or “make this red”. Anything of that kind belongs in a different format.
YouTube divides them into basic and advanced, and says what each division buys you.What a caption file is for, and when SRT is the wrong one
That division is the useful thing to take away when choosing. SRT is the format that does one job — words on a clock — and does it in a file anybody can read and edit. The advanced formats exist because that job is sometimes not enough: a caption that has to sit away from a burned-in graphic, or a colour that distinguishes two speakers, cannot be expressed in the four lines above.
Opening one, and why nothing seems to happen. An SRT file is plain text, so any text editor opens it — Notepad, TextEdit, anything. What it will not do is play: it holds no video and no audio, only words and timings. If you double-click one and get a window of numbers, that is the file working correctly. To see it as subtitles, the file has to be given to something that has the video too — a player, or the platform you are uploading to. Which of the two you are doing decides where the file needs to go, and that is a question for the player’s or the platform’s own instructions rather than for the file.
One more thing follows from the format being plain text, and it is the reason these files are worth keeping rather than regenerating. An SRT file is editable forever, by anything. A transcript produced automatically will have errors in it — names, technical words, anything spoken quickly — and correcting them is a matter of opening the file and typing. There is no proprietary editor to install and no version to be locked out of.
What this page will not do is tell you which tool to make one with. There are many, they change constantly, and recommending any of them would mean running it against real recordings for long enough to have a view worth publishing. What can be said without testing anything is that the output you want from any of them is this file — plain, UTF-8, four lines repeated — because that is the one that will still open in a decade.
The floors below take it three ways: the file itself, the video it belongs to, and getting the words out of a recording in the first place.
Where to start
Three ways in. If you have one and it will not open, start with the first.
- “I double-clicked it and got numbers.”
- That is correct behaviour — the file itself
- “The accents came out as gibberish.”
- That is the encoding — the two constraints
- “I need the words out of a video.”
- Go to getting the words
The file itself
Four lines repeated, in plain text, with no program required to read or write it. The neighbouring floors are the other formats that carry a video around.
MOV to MP4Apple documents that QuickTime will not do this. Windows ships something that will, at a fixed frame rate.Open this floor →
MP4 and convertingAn mp4 converter changes a box, or its contents. Knowing which explains four seconds against four hours.Open this floor →
MP4 to MP3Getting the audio out of a recording without re-encoding it, and how to tell whether yours did.Open this floor →
The two constraints
Plain UTF-8, and no markup. Between them they explain most files that are rejected, and most files that upload and then look wrong.
Compressing videoUsing a video compressor to get under a limit while keeping it watchable, and what to sacrifice.Open this floor →
Compressing on iPhoneThere is no compress button, and for most reasons you do not need one. What Apple documents, in order.Open this floor →
Screen recording on WindowsTwo recorders are built in, they do different things, and neither is advertised.Open this floor →
Getting the words
A caption file starts as a transcript. These floors are about producing one, and about the recording it comes from.
Transcribing audioWhat the free routes do well, and where a paid service earns its money.Open this floor →
Screen recording on iPhoneThe control is not there until you add it, and the microphone hides behind a long press.Open this floor →
Recording a callWhat each platform tells the other people, and the part that is not about software.Open this floor →
What this tower will not do
It will not recommend a subtitle tool. Choosing one means running it against real recordings, and this site does not publish verdicts it has not earned.
It will not send you to a converter for a file you can already open. An SRT is plain text; anything that offers to convert it into readable form is offering to do what your text editor does.
And it will not tell you SRT can do styling. YouTube states that no markup is recognised, and a page that suggested otherwise would be setting up a file that fails. What holds instead is simple: the SubRip example with its numbering and time codes, the statement that a caption file carries the text plus time codes and that some also carry position and style information, the recommendation of SubRip and SubViewer for anyone new because they need only basic timing information and can be edited in any plain text editor, the note that the main difference between the two is the format of the start and stop times, the requirement that files be plain UTF-8 with no markup recognised, and the description of the advanced formats as the ones to use for control over styling or positioning are quoted from YouTube’s own help documentation, listed below.
Where this page got its facts
- YouTube Help, Supported subtitle and closed caption files (that a subtitle or closed caption file contains the text of what is said in the video together with time codes for when each line should be displayed, and that some files also include position and style information which is especially useful for deaf or hard of hearing viewers; that anyone new to creating caption files is recommended to use SubRip (.srt) or SubViewer (.sbv) because they only require basic timing information and can be edited using any plain text editing software; that only basic versions of these files are supported, that no style information or markup is recognised, and that the file must be in plain UTF-8; that the main difference between SubRip and SubViewer files is the format of the caption start and stop times; the published SubRip example showing a sequence number, a start and end time in hours, minutes, seconds and milliseconds separated by an arrow, and the caption text; and that the advanced file formats including SAMI, TTML, DFXP and iTT are the ones to use for more control over styling or positioning) — support.google.com, read 22 August 2026.
Written by Alberto Gulotta
Founder and editor of AI Tools Primer, writing from Palermo, Italy. Thirty-five years of taking computers apart, starting with a Commodore 64 — the long version is on the about page.
Something wrong on this page? Write to aitoolsprimer@gmail.com and it gets fixed.
Written on 22 August 2026.
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