Why do subtitles drift after a frame-rate conversion?

When a film is converted from, say, 25 fps to 23.976 fps, its real-time duration changes, but subtitle timestamps made for the original speed are not. The result is a small error that grows steadily: captions start almost right, then slip further out of sync as the video plays.

How is the scale factor calculated?

The factor is source fps divided by target fps. A subtitle authored for 23.976 fps played over a 25 fps video must be sped up by 23.976 ÷ 25 ≈ 0.959, so every timestamp is multiplied by that ratio. Reversing the source and target reverses the correction.

Which frame rates are common?

23.976 and 24 fps are typical for film and streaming, 25 fps is PAL television, and 29.97, 30, 50, 59.94 and 60 fps appear in NTSC and high-frame-rate video. The classic mismatch is the 23.976-versus-25 PAL/NTSC pair.

When should I use offset instead of FPS conversion?

Use a constant offset when the whole file is late or early by a fixed amount and never drifts. Use FPS conversion when captions are roughly right at the start but slip progressively, which is the signature of a frame-rate mismatch rather than a simple delay.

Are my subtitle files uploaded?

No. Parsing, rescaling and reformatting all happen in your browser using JavaScript. Nothing is sent to a server, so your subtitle files stay private.

Subtitle FPS Converter

The subtitle FPS converter resyncs captions after a video has been converted between frame rates. The most common case is the 23.976-versus-25 fps PAL/NTSC mismatch: a subtitle made for one speed will drift when played over the other. This tool multiplies every timestamp by the correct ratio so the captions stay aligned for the entire runtime, not just at the start.

How it works

When a subtitle authored for a source frame rate is played over a video running at a target frame rate, real time scales by sourceFps ÷ targetFps. The tool multiplies every cue timestamp by that factor. For example, converting a 23.976 fps subtitle to match a 25 fps video uses a factor of 23.976 ÷ 25 ≈ 0.959, slightly compressing the timeline; the reverse direction stretches it.

Because the error from a frame-rate mismatch is multiplicative rather than a fixed offset, it grows the further you get into the video — a few milliseconds early on, but potentially a second or more by the end. Scaling every timestamp by the ratio corrects the drift uniformly across the whole file. The tool detects SubRip or WebVTT, rewrites only the timestamps, and leaves the cue text and numbering untouched.

Example and notes

A cue at 00:10:00,000 in a 23.976 fps subtitle, reframed to 25 fps, becomes about 00:09:35,424 — and the offset keeps growing in proportion to the time. That progressive correction is exactly what a single constant offset cannot achieve.

Note: if your captions are off by the same fixed amount everywhere and never drift, you want a plain time offset instead. The convenient swap button reverses the source and target frame rates in one click. Everything runs locally in your browser.