Subtitle Sync Corrector

The Subtitle Sync Corrector fixes captions that drift steadily further out of sync as a video plays. You paste the SubRip file into the SRT Content box, type the length of the video the subtitles were made for into Original video length (seconds) and the length of the video you actually have into New video length (seconds), and every cue is rescaled proportionally. The output is a complete SubRip file that downloads as .srt.

This is the classic mismatch between a subtitle and a different cut or transfer of the same programme. A PAL version runs about four percent shorter than its film source; a broadcast master carries adverts or a recap that the streaming version does not; a re-edit trims thirty seconds from the cold open. In all these cases the first caption may look almost right while the last one is a minute out, which is why a constant delay adjustment never quite fixes it. Subtitlers, fansubbers, archivists and anyone matching a downloaded subtitle to their own copy of a film hit this constantly.

The maths is deliberately simple and predictable. The tool computes a single scale factor of new length divided by original length, then multiplies both the start and the end of every cue by that factor, rounding to the nearest millisecond. A 630-second video with 600-second subtitles gives a factor of 1.05, so a cue at 10 minutes moves 30 seconds later while a cue at 10 seconds moves only half a second. Caption text, line breaks and cue order are untouched - only timestamps change - and both length fields must be filled in for the calculation to run.

The whole correction happens in your browser, so nothing is uploaded and the rescale is instant even on a long feature.

• Proportional rescaling: every start and end time is multiplied by the ratio of the two durations you enter, which corrects drift that grows across the programme rather than just nudging the whole file.
• Two-number interface: Original video length (seconds) and New video length (seconds) are the only settings, so there is no ratio to calculate yourself and nothing to misconfigure.
• Text left alone: only timestamps are recalculated, so translations, line breaks and cue order come through exactly as they went in.
• Clear failure mode: if either duration is missing the tool tells you to enter both rather than silently producing an unusable file.
• Millisecond rounding: rescaled times are rounded to whole milliseconds so the output is valid SubRip that any player and validator accepts.
• Handles stretch and squeeze: the same field pair works whether the new video is longer than the original or shorter, so trimmed cuts and extended editions are both covered.
• Local processing: the fix subtitle sync online workflow runs entirely in the page, with no upload and no waiting for a server.

1. Paste the out-of-sync SubRip file into the SRT Content box, or click Load sample to see how the rescale behaves on example cues.
2. Find the total running time in seconds of the video the subtitles were originally timed to and enter it in Original video length (seconds); a 10-minute source is 600.
3. Check the duration of your own copy of the video and enter that in New video length (seconds), for example 630.
4. Press Run to rescale every cue by the resulting factor and print the corrected SubRip.
5. Compare the very last cue in the output with the closing dialogue of your video - the end of the file is where a scaling error is most visible.
6. Spot-check a cue near the middle too, since a correct first and last cue with a wrong middle usually means the video has a cut rather than a uniform stretch.
7. Click Copy or Download to save the corrected .srt, then load it against the video to confirm.

Example 1 - subtitles for a shorter cut. Enter 600 as the original length and 630 as the new length, giving a scale factor of 1.05.

Before:
1
00:00:10,000 --> 00:00:14,000
The harbour was rebuilt in 1902.

42
00:09:00,000 --> 00:09:04,000
And that is where the tour ends.

After:
1
00:00:10,500 --> 00:00:14,700
The harbour was rebuilt in 1902.

42
00:09:27,000 --> 00:09:31,200
And that is where the tour ends.

The first cue barely moves, the late cue shifts by twenty-seven seconds - exactly the pattern a fixed delay cannot fix.

Example 2 - a PAL speed-up. A film subtitle timed to a 7200-second source played back on a 6910-second PAL transfer needs 7200 and 6910, a factor of about 0.96, which pulls every cue progressively earlier and restores lip sync through the final reel.

• Rescues subtitles you cannot re-time: a downloaded or archived caption file for a different cut becomes usable without transcribing the programme again.
• Fixes what a delay cannot: drift that grows over an hour is corrected properly, so captions stay locked through the closing scene rather than only at the start.
• Takes seconds, not an evening: two numbers replace what would otherwise be a manual re-time of hundreds of cues.
• Zero risk to your translation: because the text is never touched, a carefully worked translation cannot be corrupted by the timing fix.
• Improves the viewing experience: viewers who rely on captions get dialogue that matches the picture, which is the difference between accessible and merely present.
• Private by default: no upload means personal media libraries and client masters stay on your own machine.