Validation Guidelines
What each validation check does, default thresholds, and links to industry style guides.
The subtitle tool validates entries against configurable rules. Choose a preset (Netflix, BBC, Amazon, etc.) or set thresholds manually.
Reading Speed
Max CPS (Characters Per Second)
Measures reading speed by dividing the character count of an entry by its display duration. Subtitles that exceed the CPS limit flash by too quickly for comfortable reading.
| Preset | Max CPS |
|---|---|
| Netflix | 20 |
| Netflix Kids | 13 |
| Amazon | 20 |
| BBC | 17 |
| Relaxed | 30 |
CPS Counting Mode
- All characters — counts every character including spaces and punctuation.
- Exclude spaces — counts only non-space characters (used by some European broadcasters).
Max CPL (Characters Per Line)
The maximum number of characters allowed on any single subtitle line. Long lines get cut off on smaller screens.
| Preset | Max CPL |
|---|---|
| Netflix | 42 |
| Netflix Kids | 42 |
| Amazon | 42 |
| BBC | 37 |
| Relaxed | 60 |
Max WPM (Words Per Minute)
An alternative reading speed metric. Netflix targets 300 WPM for adult content and 200 WPM for children's programming.
| Preset | Max WPM |
|---|---|
| Netflix | 300 |
| Netflix Kids | 200 |
| Amazon | 300 |
| BBC | 180 |
| Relaxed | 400 |
Max Lines
The maximum number of lines allowed per subtitle entry. Most standards limit this to 2.
Timing
Min Duration
The minimum display time for any subtitle entry. Very short subtitles are unreadable. Common minimum: 500–1000 ms (or ~12–24 frames at 23.976 fps).
Max Duration
The maximum display time. Subtitles left on-screen too long can feel stale or confuse viewers about whether new dialog is being spoken.
Min Gap
The minimum gap between consecutive entries. A gap of at least 2 frames (≈83 ms at 24 fps) lets viewers register the transition between subtitles.
Structural Checks
- Overlapping timecodes — two entries displayed at the same time.
- Chronological order — entries appear out of sequence.
- Empty subtitle text — entries with no visible text.
- Negative / zero duration — end time is at or before start time.
- Consecutive duplicates — adjacent entries with identical text.
- Leading/trailing whitespace — extra spaces or newlines around text.
- Sequence numbering — SRT index numbers are non-sequential.
- Unbalanced formatting tags — unclosed
<i>,<b>,<u>tags. - Double (repeated) words — repeated adjacent words like "the the".
- Missing terminal punctuation — entries that don't end with
.,!,?, etc. - Punctuation-only entries — entries containing only punctuation characters.
Industry Reference Links
- Netflix Timed Text Style Guide — comprehensive requirements for CPS, CPL, duration, positioning, and language-specific rules.
- BBC Subtitle Guidelines — guidelines for reading speed, shot changes, music/sound effects, and color usage.
- Amazon Direct Publishing Subtitle Requirements — format and timing requirements for Fire TV and Prime Video.
- EBU R 137: Subtitling — European Broadcasting Union recommendation for live and prepared subtitles.