Caption Readability Checker

Knowledge Base

What This Tool Checks

Six things, per cue: reading speed in characters per second (the single best readability predictor. A caption that flashes by unread might as well not exist), line length, lines per cue, minimum and maximum duration, the gap between consecutive cues (captions that butt against each other read as one flickering block), and overlapping timings, which are almost always an export error. Speaker labels and sound-effect tags count toward the character totals because viewers read them too.

The Defaults Are Conventions, Not Law

The preset numbers (17 CPS, 42 characters per line, two lines, 1 to 7 second durations) follow widely used professional subtitling conventions of the kind published in broadcaster and streaming-platform style guides. But every client and platform publishes its own numbers, they differ (children's content, for instance, typically uses much slower reading speeds), and they change. If you're delivering against a spec, check the current version of that spec and set the fields to match. The defaults are for creators who don't have one and want a sane baseline.

Fixing What It Flags

High CPS has two fixes: extend the cue's duration if there's room before the next one, or, usually better, trim the text. Captioning is allowed to lightly condense. Verbatim transcription that nobody can read fast enough serves no one. Line-length violations usually want a line break moved at a natural phrase boundary rather than mid-clause. To apply fixes, round-trip the file through the SRT Caption Editor, then re-check here. For the reasoning behind these guidelines, see the caption quality guide.

FAQ

Q: Where do the default thresholds come from?
A: They follow widely used professional subtitling conventions: roughly 17 characters per second reading speed, 42 characters per line, two lines per cue, and cue durations between about 1 and 7 seconds. They are conventions, not law: broadcasters and platforms publish their own style guides with different numbers, so if you're delivering to a specific client or platform, check their current spec and adjust the fields to match.

Q: Is my caption file uploaded anywhere?
A: No. The file is parsed and analyzed entirely in your browser with JavaScript: nothing is sent to a server, and the page works offline once loaded.