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3. Change the frame rate

How to change subtitles to another frame rate

The same film runs at 25 frames per second in some releases and 23.976 in others, so the 25 fps one is about 4% shorter. Subtitles made for one drift steadily on the other: right at the start, seconds off by the end.

Retime them in three steps

1. Open the video and the subtitles in Subwave.
2. Choose Tools → Change the frame rate…. Set Made for to the release the subtitles came from and This video to yours (Subwave fills it in from the video).
3. Check a line near the end, then save.

Not sure which rate the subtitles were made for? If they drift later as the film goes on, they were made for a slower rate (23.976 for a 25 fps video); earlier, a faster one. Or use two-point sync, which works without knowing.