Estimate subtitle duration for lines.
Subtitle timing is governed by characters per second rather than words, because character count predicts reading effort better across languages and handles numbers and names correctly. Broadcast standards commonly target 15 to 17 characters per second for adult audiences and lower for children's programming. Two hard constraints then bound the result: a cue must stay on screen at least about one second to be noticed at all, and no more than about seven seconds before it feels stuck. Line length is the third rule — a maximum of 42 characters per line across at most two lines, because a third line covers too much of the frame.
Cue duration
Seconds = characters / target characters per second, bounded by the minimum and maximum
Line count
Lines = ceiling(characters / maximum characters per line), capped at 2 by convention
Because character count is a better predictor of reading effort and it works consistently across languages with different average word lengths. It also handles numerals, names and abbreviations correctly, where word counting treats a long technical term the same as a short one.
Because a third line obscures too much of the picture and forces the eye to travel too far from the action. Two lines of up to 42 characters is the widely adopted standard, and it also encourages splitting long sentences at natural grammatical breaks.