The numbers
| Rule | Value | Source |
|---|---|---|
| Characters per line | up to 42 | Public English timed text style guide |
| Lines per caption | up to 2 | Same guide |
| Adult reading speed | up to 20 CPS | Same guide |
| Children's programs | up to 17 CPS | Same guide |
| Break lines | after punctuation, before conjunctions and prepositions | Same guide |
The same publisher has guides for other languages with their own limits. If you work in another language, look up the guide for it.
How to calculate CPS
- Count the characters in the caption, spaces included, line breaks excluded.
- Measure the time between the start and end times.
- Divide characters by seconds.
Example:
5
00:00:10,000 --> 00:00:12,000
Never press the red button
before the countdown ends.
Characters: "Never press the red button" is 26 and "before the countdown ends." is 26, total 52. Time: 2 seconds. CPS: 26, which is above 20. Fix it by giving the caption at least 2.6 seconds, or by shortening the text.
Why speed matters
Viewers read and watch at the same time. When captions pass too fast, they look at the text and miss the picture, or miss the text. Dense captions with long words and numbers are harder than simple ones, so the same CPS can feel faster or slower.
How to fix captions that are too fast
| Option | Use when |
|---|---|
| Shorten the text | Meaning stays intact |
| Split into two captions | The sentence has a natural break |
| Extend the end time | The next caption starts later |
| Start the caption a little earlier | The speech starts right after a cut |
Keep a short gap between captions so they do not overlap.
Line breaks matter
Break lines after punctuation, before conjunctions and before prepositions. Do not separate an article from its noun, a first name from a last name, or a verb from its auxiliary. Prefer a bottom-heavy shape, with the longer line at the bottom, and avoid leaving only one or two words on the top line.
Check your file
Open the subtitle reading speed checker, load an SRT or VTT and see which captions exceed the limit. The tool runs in your browser, so the file is not uploaded.
To create captions with sensible splitting from the start, use the SRT generator. More about the file format is in how to create an SRT file.
Short vertical video
For Reels, TikTok and Shorts, many creators keep captions to a few words and change them faster than broadcast rules would. That is a style choice; test on a phone and read auto captions for Reels, TikTok and Shorts.
Make captions that read well
Start with the SRT generator. 10 free minutes at sign-up.
Sources
- English (USA) Timed Text Style Guide: https://partnerhelp.netflixstudios.com/hc/en-us/articles/217350977-English-Timed-Text-Style-Guide
Frequently asked questions
What is a good subtitle reading speed?
One public English style guide for timed text sets a maximum of 20 characters per second for adult programs and 17 for children's programs. Use these as limits for broadcast style work and go lower for fast or complex text.
How do I calculate characters per second?
Count the characters in the caption, excluding line breaks, and divide by the time it stays on screen in seconds. A 40 character caption shown for 2 seconds runs at 20 characters per second.
How many characters per line should a subtitle have?
The same style guide uses at most 42 characters per line and at most 2 lines per caption. TooLongToListen splits text to that limit.
Do short vertical videos follow the same rule?
Many short video creators use shorter captions and faster cuts. Treat the broadcast numbers as a safe ceiling for readability, and test on a real phone.
What can I do if my captions are too fast?
Shorten the text, split it into two captions, or extend the display time if the next caption allows it.
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