What this checker looks for
Good subtitles are easy to read without the viewer noticing them. Four things decide that, and this tool checks each one for every subtitle in your file:
- Reading speed (CPS). Characters per second: the number of visible characters divided by how long the subtitle is
on screen. Above the limit, viewers miss words or stop watching the picture to read.
- Line length (CPL). Characters per line. Long lines force the eye across the whole screen and can run off the
edge on phones.
- Duration. Subtitles that flash by in under a second are hard to notice, even when they are short.
- Overlaps and gaps. Two subtitles on screen at the same time confuse players and viewers. A small gap between
them (two or three video frames, about 80 milliseconds) lets the eye see that the text changed.
Formatting tags such as <i> and position codes such as {\an8} are not counted as characters.
How to use it
- Drop an
.srtor.vttfile into the box, or paste the subtitles. - Pick a rule set, or set your own limits.
- Press Check. You see how many subtitles break each rule and a list of every problem with its time and text.
- Press Fix what can be fixed, check the list again, and press Download fixed file.
Which rules to choose
| Rule set | Reading speed | Line length | Good for |
|---|---|---|---|
| Standard | 17 CPS | 42 | Most YouTube videos, films and courses |
| Relaxed | 20 CPS | 42 | Fast talkers, viewers used to subtitles |
| Children | 13 CPS | 37 | Younger audiences and language learners |
| Vertical video | 17 CPS | 32 | Shorts, Reels and TikTok |
These limits follow common practice in professional subtitling. Broadcasters and streaming services publish their own style guides with slightly different numbers, so if you are delivering to one of them, use Custom and enter theirs.
What the automatic fix does
- Re-wraps long lines into balanced lines within your limit, up to the number of lines you allow. Lines with
formatting tags are left as you wrote them, so styling is never broken.
- Extends subtitles that are too fast or too short into the free time before the next subtitle, never past it.
- Trims overlaps so each subtitle ends just before the next one starts, keeping the gap you chose.
Some problems need a person: when there is simply too much text for the time available, the only real fixes are shorter wording or merging two subtitles. The tool tells you how many of those are left and lists them so you can edit them in your subtitle editor.
Why it matters on YouTube
Captions on YouTube are shown at the times in your file. Fast or overlapping captions make viewers turn them off, and people watching with the sound off, which is common on phones, may simply scroll away. Clean subtitles also make your video easier to translate.
FAQ
Does it change my wording? No. It only moves line breaks and end times.
Can I check auto-generated captions? Yes. Download them from YouTube Studio as .srt and check them here; auto-captions often have very short subtitles that the fix extends.
Is my file uploaded? No. Everything runs in your browser.