Well-timed subtitles give viewers enough time to read each line comfortably: typical professional targets are a reading speed of up to about 17 to 20 characters per second, no more than two lines of about 42 characters, a duration between roughly 5/6 of a second and 7 seconds, and a short gap between consecutive subtitles.
These figures come from broadcaster and streaming style guides, and they vary by language, audience, and client. Treat them as sensible defaults rather than fixed laws, and follow your client's own guide when you have one.
Reading speed: characters per second
Reading speed is usually measured in characters per second (CPS): the number of characters in a subtitle divided by the time it stays on screen. Commonly cited targets come from Netflix's English style guide: around 17 CPS for children's programs and up to 20 CPS for adult content. Other guides use different numbers, and tools differ on whether spaces and punctuation count.
A quick example: a two-line subtitle with 70 characters needs about 3.5 seconds at 20 CPS, or about 4.1 seconds at 17 CPS. If the speaker only gives you 2.5 seconds, the subtitle is too dense. The usual fix is to condense the wording, not to hold the subtitle longer and fall behind the speech.
Line length and number of lines
- Maximum line length: 42 characters per line is the commonly used limit for English, including in Netflix's English guide. Some broadcasters use shorter limits, and languages such as Chinese and Japanese use far fewer characters per line.
- Maximum lines: two lines per subtitle.
- One line when it fits: if the text fits comfortably on a single line, many guides prefer to keep it there.
Minimum and maximum duration
A subtitle that flashes by gets missed, and one that lingers tends to be read twice. Typical limits look like this:
| Rule | Typical value | Why |
|---|---|---|
| Minimum duration | About 5/6 second (≈833 ms, 20 frames at 24 fps) | Long enough to notice and read a short line |
| Maximum duration | About 7 seconds | Longer subtitles tend to be reread |
| Gap between subtitles | At least 2 frames (≈83 ms at 24 fps) | Lets the eye register that a new subtitle appeared |
| Reading speed | Up to about 17 to 20 CPS | Viewers can read and still watch the picture |
| Line length | Up to 42 characters, 2 lines | Keeps eye movement comfortable |
Two frames is about 83 ms at 24 fps, 80 ms at 25 fps, and 67 ms at 29.97 fps. Without that gap, two subtitles of similar length that follow each other directly can look like one subtitle that never changed.
Shot changes, overlaps, and line breaks
Shot changes. A subtitle that starts or ends a few frames away from a cut creates a distracting flicker, because the picture changes just before or after the text. Common practice is to snap in-times and out-times to a nearby shot change, or to keep them clearly away from it. Exact rules vary from guide to guide.
Overlaps. Two subtitles on screen at once, where one cue ends after the next one starts, are almost always a timing error, and many players handle them badly. The exception is deliberate simultaneous speech. Even then, a common convention is to combine both speakers in one subtitle, with each line starting with a dash:
- Did you lock the door? - I thought you did.
Line breaks. Where you split a two-line subtitle affects how quickly it can be read:
- Break after punctuation, such as a comma or period, when possible.
- Break before conjunctions (and, but, because) and prepositions (in, on, with), not after them.
- Keep units together: an article and its noun ("the car"), an adjective and its noun, a first and last name, and a verb with its auxiliary ("has gone").
- When the sense allows it, aim for a pyramid shape, with a shorter top line and a longer bottom line.
SDH conventions
Subtitles for the deaf and hard of hearing (SDH) add information that hearing viewers get from the soundtrack:
- Sound effects in square brackets:
[door slams],[phone buzzing]. - Speaker labels when the speaker isn't obvious, such as an off-screen voice:
MARIA: Over here!or[Maria] Over here!, depending on the style guide. - Music marked with a music note, such as
♪ soft piano music ♪, with song lyrics framed by ♪ as well. - Tone of voice where it changes the meaning:
[whispering],[sarcastically].
If you need the opposite, a clean file for hearing viewers, you can remove SDH text such as bracketed sounds and speaker labels.
Applying the rules with the Fix tab
SubTweak's Fix tab, also available as the overlapping subtitle fixer, handles the mechanical parts of these rules in your browser:
- Fix overlaps with a minimum gap. Enter a gap such as 83 ms (two frames at 24 fps), or 80 ms for 25 fps material, so each cue ends at least that long before the next one begins.
- Set a minimum and maximum duration. For example, 833 ms and 7000 ms, so very short cues are lengthened and very long ones are shortened.
- Clean up. Remove empty and duplicate cues and trim stray spaces.
- Check again. Lengthening short cues can push them into the next cue, so run the overlap fix again after changing durations.
Reading speed and line breaks still need human judgment, because fixing them usually means rewording. Make those edits in the subtitle editor. If the whole file is early or late, sync it first; how to sync subtitles walks through the steps.
Quick answers
Are these rules mandatory? No. They are typical guidance, and specific clients, broadcasters, and languages set their own limits.
Is an overlap ever acceptable? Rarely. Deliberately simultaneous speakers are the main exception, and even then a single two-speaker subtitle is usually cleaner.