Fix subtitle problems before you publish.
Validate, auto-repair and convert SRT and WebVTT files. Everything runs in your browser. Your subtitles never leave your device.
- Timing collisions & gaps:Flags negative durations, inverted timecodes, and overlapping cues.
- Reading speed (CPS) & CPL:Enforces YouTube (42 CPL / 21 CPS) and mobile safe zones.
- Safe structural auto-repair:Normalizes BOM, CRLF, and comma separators with zero dialogue drift.
Drop your SRT or WebVTT file here
or click to browse from device
From broken subtitle to clean export
Four straightforward steps. Run locally with instant feedback.
- 01
Drop & inspect
SRT or WebVTT parsed locally in milliseconds.
- 02
Plain-English flags
Every issue names the cue, the rule and recommended fix.
- 03
Safe auto-repair
Structure is repaired safely. Your dialogue stays untouched.
- 04
Verify & export
Review changes and export your clean, compliant subtitle file.
Subtitle Toolkit & Utilities
Fifteen focused utilities for subtitle quality control, conversion, audio sync, and workflow automation. All tools execute locally in your browser with zero server uploads.
Subtitle Validator & QC
Comprehensive quality control. Checks timecodes, overlaps, reading speed (CPS) and line length against customizable presets.
Batch Subtitle QC & Repair
Audit and repair whole folders of subtitle files in one run, with a single-click ZIP archive download.
SRT ↔ WebVTT Converter
Convert between SubRip and W3C WebVTT with correct headers, cues and timestamps.
YouTube SBV to SRT
Convert YouTube caption downloads into standard SubRip for Premiere, Resolve, and Final Cut.
ASS / SSA to SRT
Convert stylized anime and Aegisub subtitles into clean SubRip for Premiere and Resolve.
CSV & Excel Subtitle Converter
Convert CSV and Excel spreadsheets into SRT subtitles, or export subtitle cues into spreadsheet tables.
TTML / DFXP XML Converter
Convert Netflix and broadcast TTML XML subtitles into SubRip, or export SRT files into TTML 1.0 XML.
Cleaner & SDH Stripper
Strip HTML tags and sound-effect brackets, remove hearing impaired annotations, and renumber cues sequentially.
Case Converter & ALL CAPS
Convert broadcast ALL CAPS closed captions to clean sentence case without touching timecodes.
SRT to TXT Transcript
Strip timecodes and sound effects to turn subtitles into readable blog posts, summaries, and show notes.
Time Shifter
Move every cue forward or backward with millisecond precision to fix audio sync offsets.
Framerate Drift Converter
Fix progressive timecode drift between 23.976, 24, 25 PAL, and 29.97 fps mathematically.
Bilingual Subtitle Merger
Combine two language subtitle tracks into stacked dual-language captions with synchronized timing.
Sequential Subtitle Joiner
Merge multi-part video subtitle files sequentially with customizable silence gaps and automatic offsets.
SRT to YouTube Chapters
Convert subtitle dialogue into YouTube video description chapters with interactive title editing.
Encoding & Mojibake Repair
Repair subtitles that show garbled accents, re-save Windows-1252 and UTF-16 files as UTF-8, and remove a byte order mark that breaks the first cue.
Line Break & Reading Speed Reflow
Rebalance over-long subtitle lines at natural phrasing pauses, keep every line inside the character limit, and see which cues are too fast to read. Timings never change.
Find & Replace in Subtitles
Find and replace text across every cue at once, with whole-word and case toggles, optional pattern mode, and a before-and-after preview of every cue it touches.
CapCut Draft to SRT Converter
Extract auto-captions from CapCut Desktop draft_content.json files into SubRip SRT, WebVTT or plain text transcript without paying for CapCut Pro.
Timeline Markers ↔ Subtitles Converter
Convert timeline markers from DaVinci Resolve and Premiere Pro into SRT subtitles, or export subtitle cues into timeline markers.
SRT to ASS / SSA Stylizer
Convert plain SubRip and WebVTT subtitles into stylized ASS format with custom fonts, colors, outline borders, and drop shadows.
Subtitle Diff & Version Compare
Compare two subtitle revisions side by side. Track added, deleted, modified text, and retimed cues with millisecond accuracy.
YouTube Transcript to SRT Converter
Convert copied YouTube transcript text, TimedText XML, or JSON captions into standard SubRip (SRT), WebVTT, and clean plain text.
9:16 Vertical Video Subtitle Formatter
Format subtitles for TikTok, Instagram Reels, and YouTube Shorts. Splits long cues into 18–24 CPL mobile lines with safe-zone margin positioning.
Structure fixed. Dialogue untouched.
Structural problems get fixed automatically. Anything that changes meaning or timing is flagged for your review.
- Automatic BOMs, line endings, comma and padding errors, trailing whitespace, blank cues, cue numbering
- Review required overlaps, cue merging or splitting, timestamp shifts, tag stripping
2 00:00:04.100 --> 00:00:06.500 This cue uses periods instead of commas for milliseconds.
2 00:00:04,100 --> 00:00:06,500 This cue uses periods instead of commas for milliseconds.
Subtitle specifications and troubleshooting
Technical reference for video creators, editors, and localization freelancers.
YouTube Subtitle Guidelines & Character Limits
42 CPL and 21 CPS thresholds, safe zone boundaries for player controls, and upload error fixes.
SRT vs WebVTT: Full Specification Breakdown
Timestamp syntax differences, styling tag support, HTML5 video integration, and converter edge cases.
Fixing Subtitle Timing Drift & Framerate Mismatch
Why subtitles drift out of sync over 10 minutes when converting between 23.976 fps and 25 fps.
Repairing Corrupted Subtitles & UTF-8 BOM Errors
Diagnosing inverted timecodes, CRLF line endings, and byte order marks that break NLE imports.
Your subtitles never leave your browser
SubPreflight processes everything client-side in Web Workers. No files are uploaded, no account is required, and no dialogue is tracked.
100% Browser-Local
Parsing, validation, and auto-repair execute directly in your browser memory via Web Workers. Zero bytes ever leave your device.
No Accounts or Tracking
No signup, no login walls, and no analytics collecting dialogue. Ads fund the site, but your files are never uploaded or tracked.
Safe Structural Repairs
Structural bugs like CRLF, BOMs, and timestamp commas are repaired safely. Dialogue, wording, and timing are never altered without review.