Subtitle Line-Breaking: Line Length, Line Count, and Reading Pace
Viewers read subtitles on the video’s clock, not the text’s — overlong lines go unread, over-chopped cues flicker anxiously. Line-length standards are, at heart, pacing.
Line length: how many characters per line
Chinese convention caps a line at 15-18 characters (punctuation included): what an average reader finishes within a "2-4 second cue". Break overlong sentences at natural pauses — commas, periods, before conjunctions like "but" or "so" — never separating a modifier from the word it modifies.
English convention caps at 37-42 characters per line (two lines max): a broadcast-subtitle standard validated over decades. Chinese and English need not break at matching points — each language breaks at its own natural pauses; viewers read meaning, not formatting.
Line count and cue splitting: the two-line boundary
Two lines maximum per cue — three starts occluding the subject, forcing viewers to choose between reading and watching. Beyond two lines the fix is not smaller font but splitting into two cues: divide the sentence, give each roughly half the duration, and the reading rhythm actually steadies.
Splitting follows speech pace: fast talkers need 1.5-2 second cues; slow interviews can hold 3-4 seconds. The criterion is "readable within the on-screen time" — far more important than mechanically even slicing.
Line breaks at the format level
In SRT and VTT a line break is simply a newline in the text: press enter while editing and the player renders two lines. ASS supports explicit tags for finer control. Conversions preserve line breaks — which is why "spot-check a long cue after format conversion" belongs on the pre-publish checklist.
For automated transcripts: cues break at speech pauses, naturally close to standard, but proper nouns can split awkwardly — skim long cues before burning subtitles. Fixing breaks before a burn costs an order of magnitude less than redoing it.