Subtitle file size guide
Subtitle files are tiny compared to video files, but their size varies depending on duration, dialogue density and format. This guide gives you realistic expectations.
Typical sizes
| Content | Runtime | SRT size | VTT size |
|---|---|---|---|
| TV episode | 22 min | 15–25 KB | 15–25 KB |
| TV drama episode | 45 min | 30–45 KB | 30–50 KB |
| Feature film | 90 min | 60–90 KB | 60–100 KB |
| Feature film | 150 min | 100–150 KB | 100–170 KB |
| Short video | 5 min | 3–8 KB | 3–8 KB |
What determines size
- Runtime — longer video, more cues, bigger file.
- Dialogue density — dense dialogue (fast-paced comedy) produces more cues than sparse dialogue (slow drama).
- Language — CJK subtitles often use fewer characters but each character is 3 bytes in UTF-8, so file sizes may be similar to English.
- Cue count — each cue has a fixed overhead (cue number, timestamps, blank line). More cues means more overhead.
- Average cue length — long cues (many characters) produce larger files.
- Format overhead — VTT has slightly more overhead than SRT due to the header and optional cue identifiers.
Why subtitle files stay small
A 90-minute film with 800 cues produces an SRT file of roughly 80 KB. That is smaller than a single JPEG image. The reason is that subtitle text is very compressible — plain UTF-8 text with no styling has excellent compression ratios, and even uncompressed it is only a few hundred bytes per cue.
This is why subtitles are often embedded in video containers without meaningfully increasing file size. The subtitle file is negligible next to the video bitrate.
Formats and their overhead
Different formats have different overheads:
- SRT — minimal. Cue number, timestamps, dialogue, blank line. Roughly 60–80 bytes per cue plus dialogue.
- WebVTT — slightly more. Header, optional cue identifiers, timestamps. Roughly 70–90 bytes per cue plus dialogue.
- JSON — much more. Each cue is a JSON object with quoted field names. Roughly 150–250 bytes per cue plus dialogue.
- ASS — high. Style definitions, formatting tags, dialogue. Roughly 100–200 bytes per cue plus dialogue.
JSON and ASS files can be 2–3× larger than SRT for the same content.
Reducing file size
Subtitles are already tiny — reducing size rarely matters. But if you need to:
- Compress with gzip — subtitle text compresses extremely well, typically 70–85% reduction.
- Use SRT instead of JSON — if you only need to distribute text, SRT is the most compact.
- Remove formatting tags — tags add bytes without adding dialogue content.
- Remove unused cues — for old files with many empty cues, cleaning them reduces size.
Large file handling
Subtitle Convert handles files up to around 15 MB without performance issues on most devices. This is roughly 5,000–10,000 cues, which is larger than any practical subtitle file (feature films have 500–2,000 cues). If you encounter a subtitle file over 15 MB, something is likely wrong (duplicated content, wrong format, encoding issue) and you should investigate before processing.