If subtitles timed for 25 fps footage are used unchanged on 29.97 fps video, they fall progressively behind: the mismatch creates about 9.95 seconds of drift per minute. Correct it by multiplying every subtitle start and end time by 25 ÷ 29.97 ≈ 0.834168. A 10:00.000 cue becomes approximately 8:20.501. First confirm that the error grows over time; a constant displacement calls for a different fix.
Why does a 25 fps subtitle file drift on 29.97 fps video?
A subtitle cue is placed at a time corresponding to a frame on its original timeline. If the video plays at a different frame rate, that frame occurs at a different clock time. On a 29.97 fps timeline, the same frame number is reached sooner than on a 25 fps timeline, so unchanged 25 fps cue times arrive late relative to the picture.
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The conversion factor is the source frame rate divided by the destination frame rate:
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems25 ÷ 29.97 ≈ 0.834168
Multiply each cue’s start and end time by that factor. Since it is below one, the converted cues move earlier. The worked 25-to-29.97 example reports about 9.95 seconds of drift per minute; the factor and example are described by DEV Community author m_whale. The figure is a calculated example, not a measurement of how often this problem occurs.
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Example: convert a ten-minute cue
For a cue at 10:00.000, multiply 600 seconds by 0.834168. The result is approximately 500.501 seconds, or 8:20.501. Apply the same factor to both the start and end of every cue so that the cue duration scales consistently.
Is it drift or a constant subtitle offset?
Compare a dialogue event near the beginning with one near the end. If the subtitles are displaced by roughly the same amount at both points, the problem is likely a constant offset: move the cues together by the needed amount. If the opening is aligned but the error grows toward the end, suspect a rate mismatch and scale the full timeline instead. Telestream’s CaptionMaker User Guide 8.3 describes the start-aligned, end-misaligned pattern as captioning drift and documents a stretch/shrink timecode operation.
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- Similar error at start and end: investigate a fixed timing offset.
- Error increases over time: investigate a frame-rate or timeline-duration mismatch.
How to diagnose and correct the mismatch
- Confirm the video version and playback frame rate. Check the actual version being played; do not infer its rate from a timecode label alone.
- Compare cues near the beginning and end. Match the first cue to an opening dialogue event and the final cue to a late dialogue event. The change in error helps distinguish a constant offset from accumulating drift.
- Scale the cues if the source is 25 fps and the destination is 29.97 fps. Multiply every start and end timestamp by 25/29.97, approximately 0.834168. Use one consistent factor across the file.
- Validate the conversion. Check cues near the start, middle, and end against the actual picture and audio. If the error still grows, recheck the assumed rates and confirm that the subtitle file and video belong to the same edit.
If the video includes inserted material, edits, or a different cut, a single global scale may not correct every mismatch. Check the affected sections rather than assuming one factor can repair discontinuities.
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Frame rate and timecode mode are not the same
Frame rate describes how often the image updates. Timecode mode describes how frames are numbered or labeled. Telestream’s guide defines them as “Video frame rate—the rate at which the image is updated” and “Time code mode—the numbering or labeling system used to uniquely identify each video frame.” Because a timecode label can represent a different clock time under a different mode, do not treat the label alone as proof of playback rate.
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Drop-frame is not a fix for a 25-to-29.97 mismatch
Drop-frame (DF) and non-drop-frame (NDF) are distinct timecode modes for 29.97 footage. Drop-frame changes the numbering convention; it does not convert arbitrary 25 fps subtitle timestamps to the 29.97 fps playback timeline. If already-synchronized captions are only changing timecode representation, preserve their clock times. If they must match a version that plays faster or slower, adjust the clock times to match that change.
For scale, Telestream’s 2020 guide gives 3.6 seconds per hour for conversions between 29.97 and 30 fps, as well as analogous near-integer conversions. That is a different rate pair and should not be confused with the roughly 9.95 seconds per minute in the 25-to-29.97 example.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When one conversion factor may not be enough
The 0.834168 factor applies when the subtitle timeline is authored against 25 fps footage and must be retimed to match the corresponding 29.97 fps timeline. It assumes the timelines differ by that rate relationship. If the actual video is a different edit, contains inserted sections, or has another timing change, scaling the whole subtitle file may leave local errors. Recheck the source and destination versions and validate cues across the timeline before relying on the converted file.
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