9:16 Vertical Video Subtitle Formatter
Format and split subtitle cues for TikTok, Instagram Reels, and YouTube Shorts. Enforces 18–24 CPL mobile limits and safe-zone margin positioning.
Drop your SRT, WebVTT, or ASS subtitle file here
Automatically splits long lines into mobile-optimized 9:16 safe-zone cuesFormatting Subtitles for 9:16 Vertical Video Feeds
The majority of modern social media consumption happens on vertical mobile feeds (TikTok, Instagram Reels, and YouTube Shorts). Subtitles designed for traditional 16:9 widescreen screens (which usually allow 37–42 characters per line) perform poorly on smartphones. They require users to shift their gaze across the entire screen width, and frequently collide with native platform overlays.
The 9:16 Mobile Subtitle Principles
1. 18 to 22 Characters Per Line
Keeping lines under 22 characters ensures the text can be displayed at 50px–64px font sizes, making dialogue effortlessly readable even on smaller smartphone screens.
2. The Single-Line Retention Advantage
Pacing subtitles as 1-line rapid bursts creates visual momentum, syncing subtitle appearances with spoken cadences to maximize viewer retention and average watch time.
3. UI Safe Zone Compliance
Placing subtitles between 35% and 65% of screen height avoids interference with the top header navigation, bottom sound marquee, account handle, and right-hand engagement icons.
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Frequently Asked Questions
Why do horizontal subtitles look bad on TikTok, Reels, and Shorts?
Traditional widescreen subtitles use 37 to 42 characters per line across two lines. On vertical 9:16 mobile screens, these lines wrap erratically, shrink to unreadable font sizes, or become blocked by platform UI elements such as usernames, caption text, and side engagement buttons.
What is the optimal character limit (CPL) for short-form video subtitles?
High-performing short-form creators aim for 18 to 22 characters per line, typically presented as punchy 1-line or 2-line bursts. This allows viewers to read dialogue instantly without shifting their eyes away from the visual action.
How does the tool keep subtitle timing accurate when splitting cues?
The reflow engine splits long cues at punctuation and natural word boundaries, then distributes the original timecode duration proportionally across each split segment based on character count.
What is the safe zone margin for vertical video?
The safe zone reserves the top 15% (search and navigation icons) and bottom 20% to 25% (sound track marquee, description, and like/share buttons). SubPreflight positions the dialogue safely above these overlays.
Can I export styled subtitles ready for burning or Aegisub?
Yes. In addition to split SubRip (.srt) and WebVTT (.vtt), you can export styled Advanced SubStation Alpha (.ass) files with custom font sizes, outline borders, and vertical margin offsets.