The thresholds the tool checks against
**Words per minute (≤160 WPM, soft warning at 140)**. The most common readability issue. Faster cues are physically possible but make captions feel rushed and reduce comprehension, especially for non-native readers and viewers with reading-related accessibility needs.
**Characters per line (≤32 chars)**. Most caption display environments break lines around this length anyway. Lines longer than this on the source file get reflowed by the player in unpredictable ways. The BBC's subtitle guidelines use 37 characters as their hard maximum; US-broadcast captioning typically uses 32.
**Lines per cue (≤2 lines)**. Three-line cues consume too much screen real estate, especially on phones. They also tend to indicate a single thought broken across too many cue boundaries — usually a sign that the source needs editing for cue density rather than just cue formatting.
**Cue duration (≥1 second, ≤6 seconds)**. Cues shorter than 1 second flash on and off too fast to read. Cues longer than 6 seconds usually indicate that what should be multiple cues was merged into one.