Shift subtitle timing

Why subtitles go out of sync

There are two different problems that look the same at first, and they need different fixes. A constant offset means every line is early or late by the same amount. It happens when the subtitle file was timed against a cut of the film that has a different intro, a studio logo, or a trimmed start. A drift means the subtitles start in sync and slowly fall further out, so the error at the end is much larger than at the start. That is almost always a frame-rate mismatch: the file was timed for video playing at one rate and you are watching it at another.

Fixing a constant offset

Pick a line near the start, watch for when it is spoken, and compare it to the time in the file. The difference is your shift. Use a plus sign to push everything later and a minus sign to pull it earlier. The box takes seconds (-2.5), milliseconds (+1500ms) or a clock value (0:01.5). Here is a cue and what a shift of +2.5 does to it.

Before                              After +2.5 s
00:00:01,000 --> 00:00:03,500       00:00:03,500 --> 00:00:06,000
Welcome to the show.                Welcome to the show.

Fixing drift with two points

Choose Sync two points. Enter the time the first spoken line should start and the time the last spoken line should start. The tool finds the stretch factor and the offset that satisfy both. For example, if the first line is at 0:01:00 but should be at 0:01:03, and the last line is at 1:30:00 but should be at 1:30:12, the stretch is about 1.00169 and the offset about 2.9 seconds. A line that was at exactly one hour in the file lands at about 1:00:09. Measure both points against the video, not against another subtitle file, and pick lines near the very start and very end for the best result.

Frame-rate conversion

If you know the rates, choose Convert between frame rates. Subtitle times are in seconds, but a file timed for video at 25 frames per second, used with a copy of the film at 23.976, shows every line too early and more so as the film goes on, because the same frames take longer to play at the lower rate. The fix is to multiply every time by the ratio of the two rates, here 25 divided by 23.976, about 1.0427. Over a one-hour film that is about two and a half minutes of drift, which is why it is obvious by the end and invisible at the start. Common pairs are in the list: 23.976 and 25, 24 and 25, and 29.97 and 30. The rates apply the right way round when you read the label as “the subtitles were timed for, then the video you have.” If a pair does not remove the drift, try its reverse, or use the two-point sync, which needs no knowledge of the rates at all.

Notes on the output

Times are rounded to the nearest millisecond. You can write the result as SRT or VTT whatever you pasted. To change only the format, use the SRT to VTT or VTT to SRT converter. If you are making subtitles from scratch, the guide to making SRT subtitles from a video covers timing from the start, and transcript file formats explained explains what each file holds.