Free Online Subtitle Sync & Editor — Fix SRT, VTT, ASS Timing Offsets
Inspect & Preview SRT Subtitles with Video. Auto-detect subtitles, sync timing offsets with millisecond precision, edit cue text, and export synced SRT/VTT files.
What is Subtitle Viewer and what does it do?
The Subtitle Sync & Multi-Audio Track tool is a free online subtitle synchronizer that fixes out-of-sync subtitles with millisecond precision. It auto-detects subtitle streams, lets you shift timing offsets, edit cue text, and export corrected SRT, VTT, or ASS files.
It also detects and switches embedded multi-audio tracks in MKV files, so you can align the right dialogue track with the right subtitle — all locally, with zero upload.
- Shift subtitle timing forward or backward in ±100ms and ±500ms steps.
- Edit individual cue text, timestamps, and dialogue lines.
- Auto-detect embedded subtitles inside .mkv video files.
- Switch between multiple embedded audio tracks.
- Export corrected .srt, .vtt, or .ass files with one click.
How does Subtitle Viewer work?
The tool parses your subtitle file into timed cue blocks (start time, end time, text) and renders them against the video timeline. Adjusting the offset adds or subtracts milliseconds from every cue timestamp simultaneously, keeping durations intact.
When you are happy with the alignment, the corrected cues are serialized back into valid SRT/VTT/ASS syntax and downloaded to your device — the original file is never sent anywhere.
100% private: all processing happens locally in your browser using client-side WebAssembly, WebCrypto, WebAudio, and HTML5 Canvas. Your files are never uploaded to any server.
Who needs Subtitle Viewer?
Movie & anime fans
Fix subtitles that appear a second too early or too late without learning complex subtitle software.
Translators & subtitlers
Nudge timing and proofread cue text before delivering a final subtitle file.
Language learners
Attach matching subtitles to videos and correct them for accurate reading practice.
Video uploaders
Prepare perfectly timed SRT or VTT files for YouTube, Vimeo, or course platforms.
Supported file formats
Subtitle Track Viewer & Audio Sync opens and processes the following file extensions directly in your browser:
.srt.vtt.ass.mkvHow to use Subtitle Viewer — step by step
- 1
Load Video & Subtitles
Select your video file and matching .srt, .vtt, or .ass subtitle file.
- 2
Adjust Timing Offset
Use millisecond buttons (+/- 100ms, 500ms) to align dialogue perfectly.
- 3
Export Synced File
Download your corrected, perfectly synced .srt file with one click.
Frequently asked questions
How do I fix subtitles that appear too early or too late?
Simply load your video and subtitle, then adjust the timing slider or click + / - millisecond buttons until text matches the spoken audio.
Can I edit subtitle text directly?
Yes, you can click on any dialogue cue to fix typos, change timestamps, and re-export.
Which subtitle formats are supported?
SubRip (.srt), WebVTT (.vtt), and Advanced SubStation Alpha (.ass).
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