The partial subtitle shifter fixes progressive drift — subtitles that start in sync but grow increasingly out of sync over time. Provide two sync points (a correct one and an incorrect one), and the tool applies linear interpolation to smoothly correct the timing across your entire file.
Input Subtitle File
Sync Points
Sync Point A — Correct reference point
Sync Point B — Drifted reference point
How to Fix Drifting Subtitle Sync
Progressive subtitle drift is usually caused by a frame rate mismatch. For example, a subtitle file timed for 23.976fps video will be 4.2% off when played on a 25fps version — starting milliseconds late but ending minutes late. The partial subtitle shifter corrects this by applying different adjustments to different parts of the file using linear interpolation.
Step 1: Find Two Sync Points
Find an early moment where the subtitles are correct (Sync Point A) and a later moment where they are clearly wrong (Sync Point B). For each point, note the cue number in the subtitle file and the actual video timestamp when that moment occurs.
Step 2: Enter the Sync Data
For Point A: enter the cue number and its correct timestamp (it's already correct, so just enter what's in the file). For Point B: enter the cue number, the wrong timestamp from the file, and what the correct timestamp should be (from your video).
Step 3: Apply and Download
Click "Apply Interpolation." The tool calculates the growing offset between A and B and distributes corrections proportionally. Cues before A are unchanged. Cues after B get the full correction from B applied uniformly. Download the fixed file in the same format as your input.
Frequently Asked Questions
What is subtitle drift and why does it happen?
Drift occurs when subtitles start in sync but gradually become more out of sync over time. The most common cause is a frame rate mismatch — the subtitle was timed for a 23.976fps video but you're playing a 25fps version. Each second of video causes a tiny desync that accumulates into seconds of delay by the end.
How do I find my two sync points?
Find an early moment where subtitles are correct (Sync Point A) and a later moment where they are clearly wrong (Sync Point B). Note the subtitle cue number for each point, the correct timestamp for Point A (it's already right), and both the wrong and correct timestamps for Point B (what the video shows vs. what the subtitle says).
What happens to subtitles before Point A?
Cues before Sync Point A are not modified — they are assumed to be in sync. Only cues between A and B are interpolated, and cues after B have the full B correction applied uniformly.
Does this work for VTT files too?
Yes. The tool auto-detects whether the input is SRT or VTT and outputs in the same format.
Is this free and private?
Yes — free with no signup. All processing happens in your browser. Your files are never sent to any server.