Subtitle formatting rules that actually affect comprehension
· 5 min read
Subtitle guidelines look like bureaucracy until you watch someone try to read a badly-cut caption track. The constraints exist because reading while watching is a genuinely hard task, and small formatting choices decide whether a viewer keeps up.
Reading speed
Broadcast guidelines generally cap subtitles at 15 to 20 characters per second. Above that, viewers stop reading and start skimming — and once they skim, they miss the line entirely. If your captions feel rushed, the fix is almost never a smaller font; it is fewer words per cue.
Line length
Roughly 42 characters per line, two lines maximum. Longer lines force horizontal eye movement across the frame, which pulls attention off the picture. In practice a 5-to-8 word line is comfortable for most speech.
Where to break
This is the one most tools get wrong. A subtitle should break at a grammatical boundary — after a clause, before a preposition — not at whatever word happens to be number seven.
Splitting "we went to the / shop on Tuesday" reads badly. "We went to the shop / on Tuesday" reads naturally, because the break lands where you would pause anyway.
CaptionsThis has two segmentation modes for exactly this. Fixed word count is predictable and fast; natural sentences breaks on punctuation and caps at a maximum length. For dialogue, natural sentences almost always reads better.
Timing
- Minimum around one second on screen, even for a single word — anything shorter reads as a flicker.
- Maximum around six seconds, or the viewer re-reads the line and loses the picture.
- Leave a couple of frames between consecutive cues so the change is visible.
None of this requires special software. It requires being able to edit line breaks and timings without breaking sync — which is the whole reason word-level timing matters.