Subtitle line length and reading speed: the numbers worth knowing
Professional subtitling runs on a small set of numbers that look arbitrary until you understand what each one is protecting. They are worth knowing even if you never subtitle anything professionally, because they are the difference between captions a viewer stops noticing and captions a viewer has to work at.
42 characters a line, two lines at most
For Latin-script subtitles, 42 characters is the usual maximum line length, and no caption should run to more than two lines. Netflix's timed-text style guide uses exactly those figures; other broadcast guidelines sit close to them, some a little shorter. The limit is about eye travel, not about fitting on screen. A long line makes the eye cross most of the picture and come back, and at that point the viewer is reading the subtitle instead of watching the video. The two-line limit protects the picture: a third line starts covering things worth seeing. On a phone the same 42 characters occupy a much larger share of the screen, which is an argument for shorter rather than longer.
Break the line where the sentence breaks
When a caption needs two lines, where you split it changes how fast it reads. Break at a grammatical boundary — after a clause, before a preposition, between subject and verb if there is nothing better — and keep phrases that belong together on one line. The other half of it is balance. Two lines of similar length read faster than one full line and a single trailing word, because the eye's return journey is predictable. That is why a good re-wrap looks for the split closest to the middle rather than simply filling the first line and pushing the remainder down. Never split a word across lines, and try not to leave an article or a preposition stranded at the end of the first line.
About 17 characters a second
Reading speed is measured in characters per second: the number of characters in the caption, divided by how long it is on screen. Around 17 is the common target for adult viewers. Children's programming runs slower, often nearer 13. Some fast-paced material is cut faster than 17 and gets away with it, though the viewer pays for it in attention. The practical use is as an alarm rather than a rule. If a handful of captions in a file exceed it, they are probably dense lines of dialogue and they are fine. If most of the file exceeds it, either the captions need longer on screen or the text needs cutting — and cutting is what professional subtitlers actually do. A subtitle is not a transcript; dropping a filler word to let the line breathe is normal practice, not a compromise.
Minimum and maximum time on screen
A caption needs roughly a second on screen no matter how short it is. Less than that and it registers as a flicker — the viewer sees that something appeared and has no idea what it said. At the other end, a caption held much past six seconds starts to look frozen. If the text genuinely needs that long, it is too much text for one caption and wants splitting. Leave a small gap between consecutive captions too: a couple of frames of clear screen. Without it, two captions can read as one block that changed halfway through, and the viewer re-reads to work out what happened.
When to break the rules
These numbers are guidance shaped by broadcast, and broadcast has constraints your video may not share. A tutorial the viewer can pause tolerates denser captions than a drama. A social clip watched without sound is read, not glanced at, and the captions are the primary channel rather than a support for it. What does not change is the reason behind each number. Break 42 characters and you have made the eye travel further. Break 17 characters a second and you have asked the viewer to read faster. Do it knowing which cost you accepted, rather than because a tool never mentioned it.
Frequently asked questions
Why 42 characters and not 40 or 45?
It is a convention rather than a measurement — the figure in Netflix's timed-text guidance for Latin-script languages, and close to what several broadcasters use. Anything in the high thirties to low forties behaves much the same; what matters is having a limit and re-wrapping to it.
Do these numbers apply to every language?
No. Character-based scripts such as Japanese and Chinese carry far more meaning per character and use much lower limits. The 42-character and 17-character-a-second figures are for Latin-script languages.
What counts as a character — does a space count?
Yes. Line length is measured in characters including spaces and punctuation, which is how a subtitle tool and a player both count it.
Is it acceptable to cut words out of the dialogue?
Yes, and it is standard practice. Subtitling condenses: filler, repetition and false starts go first, then redundancy the viewer can hear anyway. The aim is that the caption can actually be read in the time available.
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