Charts · 36

How long text needs on screen

In one test render my own video app gave a 12-word line a second and a half. Nobody can read that, and a fixed 2-second card is not much better: it asks for 360 words a minute from people whose average on a still page is 238. The app now has floors: a line in a silent text note holds for at least a quarter of a second a word plus 0.3 seconds, 3.3 seconds for twelve words, and its caption cards hold longer. Read your own line at each speed below, then see every rule side by side.

What it feels like

Press play and read the line before it goes.

Every rule, to scale

Your line: 12 words, 62 characters. Edit it above, or try
0 s2 s4 s6 s8 s14812162024words on the lineBBC, 160 wpmmy card rulemy note rule238 wpm2 s cardyour line, 12 words
Your line, 12 words, by each rule
RuleSeconds2 s gives
A fixed 2 s card2.0n/a
Silent reading, 238 wpm3.0 2.4 to 4.183%
Netflix, 20 characters a second3.1 3.1 to 3.665%
My note rule3.361%
BBC subtitles, 160 to 180 wpm4.5 4.0 to 4.550%
My card rule4.842%

Hover a line or a row for where the rule comes from. The bands are the published ranges: 175 to 300 words a minute for most adult silent readers, 160 to 180 for BBC subtitles. The last column is how much of each rule a fixed 2 s card gives, measured against its most lenient end, so it is the kindest reading.

Notes

My two rules are in the app’s text-beat assembler (TextBeatAssembler in the PostReady codebase). The note rule, noteReadableSeconds, is words ÷ 4 + 0.3 s, which is 240 words a minute with a beat to find the line; it is the floor for silent text notes, short videos that loop, so a viewer gets a second pass. The card rule, readableSeconds, is slower: the larger of 1.8 s and words ÷ 3 + 0.5 s, plus 0.3 s for each word with a number in it, 0.2 s for an acronym, 0.15 s for a word of nine letters or more, plus a 0.15 s fade in and out that does not count as reading time. The curve on the chart is the card rule for plain short words; the table prices the exact line. A fixed 2 s card holds about 6 words by the note rule, 7 at the average silent reading speed and 6 at the BBC’s faster subtitle rate.

Reading speed: Brysbaert’s meta-analysis of 190 studies puts adult silent reading of English non-fiction at 238 words a minute on average, fiction at 260, with most adults between about 175 and 300; reading aloud averages 183. That is reading a still page. On screen the text competes with the picture: subtitle standards run slower, 160 to 180 words a minute at the BBC (0.33 to 0.375 s a word, and a target minimum of around 0.3 s a word), and up to 20 characters a second for adults and 17 for children’s programmes at Netflix, never under 20 frames. Text that moves is slower again: in Uetsuki and colleagues’ study of dynamically presented Japanese text, the optimum rate was about 6 letters a second, against an average of 11.3 for silent reading, roughly half. On the other side, eye-tracking by Szarkowska and Gerber-Morón found most viewers coped with subtitles at up to 20 characters a second and still followed the picture, and that slow subtitles caused more rereading: the standards are a floor for comfort, not a ceiling on ability.

Sources: Marc Brysbaert, How many words do we read per minute? A review and meta-analysis of reading rate, Journal of Memory and Language 109 (2019); BBC Subtitle Guidelines, section 4, Timing; Netflix, Timed Text Style Guide: General Requirements and Subtitle Timing Guidelines; Masaki Uetsuki, Junji Watanabe, Hideyuki Ando and Kazushi Maruya, Reading traits for dynamically presented texts, Frontiers in Psychology 8 (2017); Agnieszka Szarkowska and Olivia Gerber-Morón, Viewers can keep up with fast subtitles: evidence from eye movements, PLOS ONE 13(6) (2018).