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Timed by speech recognition

Captions that are already in time

Typing subtitles means listening to the same sentence four times and still guessing the timings. Speech recognition gives you a line per phrase with its start and end — you only have to fix the words it misheard, then choose where the text sits.

Short answerLast edited October 31, 2026

The Auto Captions Generator writes captions from the speech in a video, with a maximum number of characters per caption and a style you choose. Recognition runs on your device or on the optional server model, and the captions are styled when they are rendered on the video.

Two speakers, one timeline

Caption blocks do not sit on the video frame, they sit on time. Below, two voices share one timeline: each block is a cue, and the highlighted word is the one being spoken at that instant. The tool creates the same structure — cue by cue — and the editor lets you move a boundary without touching the text.

00:0000:0700:1400:2100:28
ASpeaker A
Welcome back to the studio Every line arrives already timed Then burn them in or export
BSpeaker B
Today we caption a long clip Fix the words, keep the timing
Who speaksWord being spoken

Three steps, no timeline work

1

Drop the file

The sound is extracted in the browser. With the AI server option it becomes a small mono MP3; with the on-device option nothing leaves the tab at all.

2

Correct the text

Each cue shows its start and end. Fix names and numbers, delete a bad line, merge what belongs together — the timings stay where they were.

3

Export or burn in

SRT for YouTube, VTT for the web player, or a new MP4 with the captions styled and burned into the picture.

Four caption styles

StyleLookWhere it works
Bold yellowLarge, capitals, centred above the middleShorts, Reels, TikTok: readable with the sound off
Clean whiteMedium outline, lower thirdInterviews, tutorials, anything that has to look calm
Small lower thirdWhite on a soft box, small typeDocumentary and corporate footage where text is a helper
Karaoke highlightEvery word fills with colour as it is spokenMusic, language lessons, high-energy edits
Captions burned into the picture can never be switched off. If the video also goes to a platform that generates its own subtitles, export the SRT/VTT file instead and keep the MP4 clean.
Read before you export
  • Play the first minute with the captions on
  • Fix names, brands and numbers
  • Split any line longer than two rows
  • Make sure no cue sits under 0.8 seconds
  • Check that the last line disappears with the speech

Caption questions

Is the video uploaded?
With the on-device option, no: the model runs in the tab. With the AI server option, only a small mono MP3 of the soundtrack is sent, once, for transcription — the video file itself never leaves your device.
Which languages are supported?
Around a hundred are recognised, and the language can be detected automatically. The on-device models are strongest in English and the larger European languages; the server option handles songs and heavy accents better.
Can I edit the timings as well as the text?
Yes. Every cue has editable start and end fields next to the text. The line count changes when you pick a different maximum length, because long cues are split at word boundaries.
What is the difference between SRT and VTT?
SRT is the older, universally accepted format. VTT is the web standard and supports styling and placement. Both are exported here; the VTT studio adds validation and a STYLE block.
How long can the video be?
The on-device models can handle long files, but slowly on a phone. The AI server option has a per-file upload limit shown next to the tool. For hour-long recordings, cut the video into parts first.
Do the captions stay in sync after burning?
Yes. The file is re-encoded at the original frame rate and the subtitle timings come from the same clock as the picture. Variable frame rate sources are converted to a constant rate, which is what keeps them aligned.

Why Captions Are No Longer Optional

Most video is watched with the sound off, at least at first. In a bus, in an office, in bed next to someone asleep. Captions stopped being an accessibility feature and became the default way a large part of the audience meets a video — and platforms know it, which is why an unlabelled clip is quietly ranked lower than a labelled one.

What speech recognition actually gives you

Modern recognition models do not return a paragraph. They return phrases with a start and an end time, which is exactly what a subtitle file needs. That is the whole trick: the hard part of captioning was never the typing, it was knowing when each line appears and disappears.

What the model does badly is also predictable. Proper nouns, brand names, numbers and technical vocabulary come out phonetically. Overlapping speech collapses into one line. Music and heavy background noise produce confident nonsense. This is why the editor exists: you are not starting from zero, you are correcting a draft.

The rules that make captions readable

Two lines, no more. Around 42 characters per line. A cue that stays on screen for at least a second, and usually between two and five. A reading speed under about 21 characters per second, so the eye can follow without racing.

Break lines at natural pauses, not in the middle of a phrase. Keep the text close to the spoken words — a caption that appears a second before the voice feels like a leak from the future.

Burned in or as a file

A subtitle file is flexible: players can resize it, translate it, turn it off. Burned-in captions are part of the picture: they survive any player, any download, any re-upload, and they can be styled to match the edit. Social video usually needs them burned in, because the platform's own captions will not match your design.

The safe choice is to export both. Keep the clean master, upload the MP4 with captions where reach matters, and hand the SRT to the places that need a file.

Captioning is also an edit

Once you read your own video as text, you notice the filler. “So, basically, kind of” repeated twelve times. A sentence that goes nowhere. Captions make those visible and easy to cut, which is why editors who caption their own work tend to produce tighter videos over time.

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Quick facts

  1. About 15 to 17 characters per second is the speed at which a viewer can still read a caption; a longer caption leaves the screen before it is read.
  2. Timing matters more than styling: a plain caption at the right moment beats a styled caption that appears half a second late.
  3. WebVTT is the format the HTML video element reads for captions, and it separates the millisecond with a dot.W3C
  4. The older SRT format has no formal standard; it survives because every player imports it.W3C
  5. A language model reads text in tokens, and one token is roughly four characters of English, so a 300-word answer is about 400 tokens.OpenAI Cookbook

Questions people ask

How do I add automatic captions to a video?
Load the video, choose the language and the maximum characters per caption, and run the recognition. Review the caption text and timings, then export a caption file or burn the captions into the video. Correct names and technical terms before you publish.
Are automatic captions accurate enough for YouTube or social media?
They are a good first draft for clear speech. Accuracy falls with background music, accents and fast overlapping talk. Platforms like YouTube let you upload a corrected caption file, which is worth doing for any video that will be watched widely.
How long should one caption stay on screen?
A caption should stay long enough to read at about 15 to 17 characters per second, and no more than about four seconds for a short line. Captions that leave the screen too early are the most common reason viewers stop reading.

What this page is about

  • WebVTT

    The W3C subtitle and caption format used by the HTML video element, with cue timings written as 00:00:01.000.

  • Large language model

    A model that predicts the next token of a text; the site sends the prompt to a hosted model and returns its answer.

  • H.264 / AVC

    The video codec standardised by ITU-T as H.264 and by ISO/IEC as MPEG-4 AVC, decodeable by every current browser.

Official sources cited on this page

  1. WebVTT: The Web Video Text Tracks FormatW3C
  2. How to count tokens with tiktokenOpenAI Cookbook
  3. Recommendation H.264: Advanced video coding for generic audiovisual servicesITU-T

Continue with the same file

Each of these opens what the tool above produces. Use “Use in another tool” under the result to carry the file over — it moves from tab to tab inside your browser, never to a server.

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