Sync Subtitles (Fix Drift)
Subtitles drift out of sync over time? Fix framerate mismatch and progressive drift with two-point linear correction, in your browser. Free, no upload.
Drift is the sync problem a simple shift cannot fix: subtitles match at the start, then run ahead or behind by more each minute — about four seconds per hour for the classic 25fps-subtitles-on-23.976fps-video mismatch. The cure is linear correction. Anchor two moments (one early, one late): tell Subtitle Doctor where a line currently sits and where it should sit, and every timestamp in between is stretched proportionally. If you know the framerates involved, the presets compute the stretch for you.
Worth knowing
- Diagnose first: check sync at the start AND near the end. Same error at both ends → constant shift. Growing error → drift, and this is the right tool.
- PAL 25fps subtitles on a 23.976fps release drift ~4.3 seconds per hour — the single most common cause, covered by a one-click preset.
- Two-point correction handles any linear error, including combined offset + stretch, which pure framerate conversion cannot.
- Pick anchors far apart — first minute and last ten minutes — for the most accurate interpolation.
- ASS styling, VTT settings and cue text are untouched; only timestamps change.
FAQ
What causes subtitles to drift out of sync?
Almost always a framerate mismatch: the subtitles were timed against a video running at a different speed (25fps PAL vs 23.976fps film transfers being the classic pair). A different cut of the film — extended scenes, trimmed logos — causes stepwise jumps instead, which two anchors inside one continuous section can still fix.
How is this different from the shift tool?
Shift adds the same offset to every cue. Sync applies a linear transform — offset plus stretch — computed from your two anchor points, so the correction is exact at both ends and interpolated in between.
Do I need to know the framerates?
No. Framerate presets are a convenience; the two-anchor method needs nothing but the video and your eyes. Automatic audio-based sync is on the roadmap.