Why subtitles drift
The same film often exists at two speeds. Cinema and most streaming releases run at 23.976 frames per second; European television and many older releases run at 25, played about 4% faster. Subtitles carry clock times, so subtitles timed for one speed slowly fall out of sync on the other: right at the start, a little off after a few minutes, badly off by the end.
With subtitles made for 23.976 on a 25 fps video, a caption timed at 10 minutes belongs at 00:09:35,425, almost 25 seconds earlier. At 1 hour 30 minutes the error is 3 minutes 41 seconds.
Fix it from the two frame rates
The tool opens on this fix. Choose what the subtitles were made for and what your video plays at; every time is scaled by the ratio between the two. From 23.976 to 25, every caption comes 4.10% earlier. The other way round, from 25 to 23.976, every caption comes 4.27% later. The two numbers differ because each is a share of a different length.
| Subtitles made for | Video plays at | Change | A caption at 1:30:00 moves to |
|---|---|---|---|
| 23.976 | 25 | 4.10% earlier | 01:26:18,821 |
| 25 | 23.976 | 4.27% later | 01:33:50,625 |
| 24 | 25 | 4.00% earlier | 01:26:24,000 |
| 23.976 | 24 | 0.10% earlier | 01:29:54,605 |
| 29.97 | 30 | 0.10% earlier | 01:29:54,605 |
Not sure which rates apply? Try 23.976 to 25 first if the subtitles fall further behind the voice as the film goes on, and 25 to 23.976 if they run further ahead.
Fix it from two captions
When you don't know the frame rates, or the drift doesn't match any of them, use two captions instead. Pick one near the start and one near the end, find the moment each line is spoken in your video, and type those times. The tool stretches and moves the whole file so both land exactly, and every caption in between follows. This also fixes a drift and an offset at the same time.
Times can be typed as 00:01:02,500, 1:02.5 or plain seconds like 62.5.
If the error is the same everywhere
If every caption is off by the same amount, from the first line to the last, it is an offset, not drift: use the subtitle sync tool to shift them. For the timecode side of frame rate changes, see the timecode frame rate converter.
Questions
Why do my subtitles slowly go out of sync?
They were timed for a version of the video at another frame rate, usually 23.976 against 25 fps. The video runs about 4% faster or slower than the subtitles expect, so the error grows through the film.
How do I convert subtitles from 23.976 to 25 fps?
Choose 23.976 as the rate the subtitles were made for and 25 as the rate the video plays at. Every caption comes 4.10% earlier; download the result.
What if I don't know the frame rates?
Use two captions: give the correct start time of one near the beginning and one near the end. The tool stretches the file so both match and the rest follow.
Does this change the subtitle text?
No. Only the times change. Text, styling and the file format stay as they were.