Subtitle reading speed and line length, by language

Characters per second (CPS)

Reading speed is measured as the number of characters in a caption divided by the time it is on screen. For Latin, Cyrillic and Greek scripts, around 15–17 characters per second is comfortable for most adults; faster than about 20 and viewers start missing lines.

Denser scripts are read more slowly per character, because each character carries more. Chinese sits around 9 characters per second, Korean around 12, and Japanese lower still.

Characters per line

Two lines per caption is the usual maximum. For Latin-based languages, about 42 characters per line fits comfortably across a widescreen frame. Chinese and Korean fit in about 16 characters, and Japanese in about 13–16.

On vertical video the frame is narrower, so shorter lines — roughly 20–30 Latin characters — read better.

How long a caption stays on screen

Show each caption for at least about a second, so even a one-word line registers, and rarely for more than six or seven seconds, after which it starts to look stuck. Leave a short gap between consecutive captions so viewers notice the change.

Why translation changes the numbers

Translated text is rarely the same length as the original. Spanish, Portuguese and French translations of English typically run 15–30% longer and German up to 35%, while Chinese and Japanese use far fewer characters. The timing comes from the original speech, so a longer translation means a higher reading speed in the same time window.

The practical fix is to split the longest captions, or trim the wording, rather than to cut the time the caption is on screen.

• Latin, Cyrillic, Greek: about 15–17 CPS, about 42 characters per line.
• Arabic, Persian, Devanagari: about 15 CPS, about 42 characters per line.
• Chinese: about 9 CPS, about 16 characters per line.
• Korean: about 12 CPS, about 16 characters per line.
• Japanese: lower still, about 13–16 characters per line.

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