Subtitle frame rate and speed correction
If the subtitles line up at the start but are half a minute out by the end, the frame rate is wrong. Pick the source and target fps and every timestamp is rescaled by the right ratio.
Why the drift grows
Film runs at 24 fps (23.976 in practice) while the European PAL standard runs at 25. Playing 24 fps material at 25 fps makes it roughly 4% faster — the PAL speed-up — which is more than two and a half minutes of drift per hour. No fixed offset can fix that; the timeline has to be scaled.
Common conversions
23.976 to 25 (making a PAL copy), 25 to 23.976 (bringing a PAL rip back to film speed) and 29.97 to 23.976 (NTSC telecine) cover most cases. Anything else can be entered as a plain multiplier.
Choosing an anchor
Leave the anchor at zero when the opening is already in sync. If instead a point in the middle is correct, set that timecode as the anchor and the timeline stretches around it.
Frequently asked questions
How do I find the factor if I do not know the frame rates?
Take the first and last line of dialogue. Divide the observed time span by the span the subtitle file claims, and use that as the multiplier.
Should I rescale or shift first?
Rescale first, then shift away whatever constant offset remains. Doing it the other way scales the offset too.
Are my files uploaded to a server?
No. Everything runs as JavaScript inside your browser and the file contents never leave your machine, which makes it safe for unreleased scripts and confidential recordings.
Is it free? Do I need to sign up?
It is free, needs no account and nothing to install, and there are no usage limits.