How Long Should a Subtitle Line Be? Readability Rules That Matter

The working rules: keep a subtitle line around 37–42 characters, use at most two lines per subtitle, hold it on screen long enough for roughly 15–17 characters per second of reading, and break lines at phrase boundaries rather than wherever the width runs out. Those four numbers, applied consistently, separate subtitles people read effortlessly from subtitles people rewind.

Where these numbers come from

They're not arbitrary — they descend from decades of broadcast practice. Public style guides like the BBC's subtitle guidelines and the Netflix timed-text style guides converge on the same territory: line lengths in the high thirties to low forties, two lines maximum, and reading speeds tuned so a viewer can read the line and still watch the picture. The exact numbers vary by broadcaster, language and audience; the shape of the rules doesn't.

The four rules, and why each exists

1. Line length: ~37–42 characters

Eyes read subtitles in one or two fixations — short lines get absorbed at a glance, long ones force actual reading, which steals attention from the video. Wider lines also collide with player margins and vertical phone video. If your tool exports lines much beyond 42 characters, tighten it.

2. Two lines, never three

Three lines cover the picture and signal that the cue is trying to hold too much speech. Split the cue in two instead. When using two lines, aim for balance — a long top line over a short bottom line (or the reverse) reads worse than two even ones, and many guides prefer the bottom line slightly longer to keep the text block low.

3. Reading speed: ~15–17 characters per second

The invisible rule that decides comfort. A 40-character line needs about 2.5 seconds on screen; give it one second and even fast readers miss the end. Adult broadcast targets sit around 15–17 cps in Latin scripts (children's content much lower). This is also why translated subtitles overflow: the translation grows, the timing doesn't, and cps quietly climbs past readable.

4. Break at phrase boundaries

The line break is punctuation for the eye. Break after clauses and before conjunctions; never separate an article from its noun, a preposition from its phrase, or a name from its title. "The head of the research team / presented the findings" reads itself; "The head of the / research team presented the / findings" fights you word by word.

The quick reference

What changes per language and format

• CJK scripts carry more meaning per character — line limits drop (Netflix's guides use per-language limits, e.g. considerably shorter lines for Japanese and Korean) and cps rules are counted differently.

• Vertical short-form video narrows usable width: shorter lines, bolder type, and burned-in styling — the open-caption world we covered in open vs closed captions.

• Accessibility captions add non-speech information ([applause]), which competes for the same character budget — one more reason to keep dialogue lines lean.

How much of this do you do by hand?

Less than you'd think. A good subtitle generator applies length limits, phrase-aware breaks and timing when it creates the file — machine line-breaking has quietly become good. Your review pass then hunts the residue: a crowded line after a fast exchange, a break that splits a name, a cue flashing too briefly after tight editing. Fix those in the editor (a free subtitle editor works on any existing SRT/VTT), and the file meets the rules without you counting characters once.

Frequently asked questions

Is 42 characters a hard limit?

It's a strong convention, not physics. Broadcasters pick 37–42 by house style; consistency matters more than the exact digit. Past the mid-forties, real readability costs appear.

How do I measure characters per second?

Characters in the cue (spaces included) divided by seconds on screen. Editors that display cps make the check ambient; otherwise spot-check your densest cues.

Should subtitles match the speech exactly?

Verbatim when speed allows; lightly condensed when it doesn't. Trimming "I mean, I think that basically…" to keep cps readable is standard professional practice, not censorship.

Do these rules apply to auto-generated captions?

They're exactly what auto-captions break — word-chasing fragments with no phrase logic, as we showed with YouTube's auto-captions. Rules like these are why generated-then-reviewed files read so differently.

The bottom line

Four numbers do the work: 42 characters, two lines, 15–17 cps, breaks at phrases. Let the generator enforce them, spend your minutes on the exceptions, and your subtitles disappear into the viewing — which is the entire goal.