Subtitle timing rules the pros follow (CPS, duration, gaps)
Professional subtitles obey a handful of numbers — reading speed, minimum and maximum duration, line length, and the gap between cues. Learn them once and every file you make reads cleanly.
Good subtitle timing is invisible. When it is right, you read the words without ever noticing them arrive or leave; when it is wrong, you feel it as a flash you could not catch or a line that lingers awkwardly. The difference is not taste — it is a small set of numbers that broadcast subtitlers have converged on over decades, and they hold up because they are built around how fast people actually read.
This is a reference you can keep. Each rule below has the number, the reason behind it, and how to catch violations. There is a summary table near the end you can bookmark, but the reasoning matters, because knowing why a rule exists tells you when it is safe to bend.
What reading speed should subtitles use?
Reading speed is measured in characters per second (CPS): the visible characters in a cue divided by its duration in seconds. A 51-character cue held for 3 seconds runs at 17 CPS. This is the most important number in subtitling because it directly controls whether a viewer finishes reading before the cue disappears.
The professional target is around 17 CPS for a general adult audience, with 20 CPS treated as a ceiling you cross only when the audio truly leaves no room. Slower audiences need less: children's programming often aims for 13 CPS or lower. When a cue runs over the limit you have exactly two levers — shorten the text so there are fewer characters, or lengthen the cue so there is more time. There is no third option, because CPS is just characters over seconds.
How long should a subtitle stay on screen?
The maximum comfortable duration for a single cue is about 6 to 7 seconds. Past that, a static line starts to feel abandoned: the viewer finished reading seconds ago and begins to wonder whether the subtitle is stuck or whether they missed a change. If a spoken line genuinely runs longer, the fix is to break it into two consecutive cues that follow the speech, not to hold one line for ten seconds.
The maximum is rarely the problem — most cues are shorter than the dialogue anyway. The frequent mistake is the opposite end of the scale, which the next rule covers.
How short can a subtitle be?
The minimum duration is about 0.8 to 1 second, even for a single word. Below roughly this threshold a cue flashes on and off before the eye can register that it appeared at all — the viewer sees a flicker, not text. A one-word reply like "Yes." still needs close to a second on screen, even though it is spoken in a fraction of that time.
This is why you cannot time subtitles purely to the audio. Speech has quick beats that are far shorter than a second, but a subtitle timed to match them would be unreadable. The cue has to hold past the end of the sound. When a rapid exchange produces cues that keep falling under the minimum, the answer is often to merge two short lines into one, which the subtitle editor does while keeping the combined timing sensible.
Why do subtitles need a gap between them?
When one cue ends and the next begins, they should not touch. Professional practice leaves a minimum gap of about two frames — roughly 40 milliseconds at 25 fps — between the out-point of one cue and the in-point of the next. The gap gives the eye a moment to register that the old subtitle has gone and a new one has arrived. Without it, two back-to-back cues can look like a single block that mysteriously changed its words.
The stricter version of this rule is the one that must never be broken: cues must not overlap. If cue two starts before cue one has ended, a player may show both stacked at once, or flicker between them, and the result is a mess. Overlaps usually creep in when you shift or stretch timing in bulk. Running the file through the subtitle timing checker flags every overlap and every sub-minimum gap so none slip through, and the subtitle sync fixer can realign a whole track that has drifted.
Scan a file for overlaps and CPS breaksHow many characters and lines per subtitle?
The line-length standard is 42 characters per line, with a maximum of two lines per cue. A subtitle is meant to be absorbed in a single glance, not scanned like a paragraph; a line much wider than 42 characters forces the eye to travel and breaks that glance. Two lines is the practical ceiling of what a viewer takes in before the cue changes.
When you must split a cue across two lines, break at a grammatical boundary — after a clause, keeping an article with its noun and a preposition with its phrase. A break in the wrong place reads worse than a slightly lopsided one. If a cue would need three lines to fit, it is carrying too much text for its duration, which is really a CPS problem in disguise; cut the words or split into two cues.
The rules in one table
Here is the whole set in one place. These are sensible defaults drawn from broadcast practice; individual broadcasters and streamers publish their own house guides that tighten or loosen a value here and there, but a file that satisfies this table will pass almost anywhere.
| Rule | Standard value | Why it exists |
|---|---|---|
| Reading speed | ≤ 17 CPS (20 max) | So viewers finish reading before the cue leaves |
| Minimum duration | ~0.8-1 second | Below this a cue only flickers, unreadable |
| Maximum duration | ~6-7 seconds | Longer feels stuck or abandoned |
| Line length | ≤ 42 characters | Read in one glance, no left-right scan |
| Lines per cue | 2 maximum | More than two overloads a single moment |
| Gap between cues | ≥ 2 frames (~40 ms) | Eye registers that a new cue arrived |
| Overlaps | Never | Players stack or flicker overlapping cues |
If a whole file breaks the reading-speed rule because it was retimed for a different frame rate or playback speed, the subtitle speed changer rescales the timing, and the subtitle shifter slides everything earlier or later when the track is simply offset. When the captions finally pass every rule and you want them fixed to the picture, you can add subtitles to a video directly in your browser.
Open a file and fix the timingFrequently asked questions
What CPS should I aim for?
Around 17 characters per second for a general adult audience, treating 20 CPS as a hard ceiling. Children's content should go slower, closer to 13 CPS. If a cue exceeds the limit, either trim the text or extend the cue.
What is the shortest a subtitle can be?
About 0.8 to 1 second, even for a single word. Anything shorter flashes by before the eye can register it, so a quick "Yes." still needs to hold on screen for roughly a second past when it is spoken.
Why should there be a gap between subtitles?
A small gap of about two frames lets the eye see that one cue has left and another has arrived. Without it, consecutive cues read as one block that changed its own words. Overlapping cues are worse still and must never happen.
Are these rules absolute?
They are strong defaults, not laws. Broadcasters and streaming services publish house style guides that adjust individual values, but the reasoning behind each rule stays the same, so a file that meets this table is safe almost everywhere.