Frame rate calculator
NTSC video runs at 29.97 fps rather than 30, a legacy of fitting colour into the black-and-white broadcast signal in 1953. Over an hour that 0.1% difference accumulates to 3.6 seconds of drift between timecode and real time. Drop-frame timecode fixes it by skipping frame numbers — not actual frames — at the start of most minutes, which is why professional timecode uses semicolons and why the numbers occasionally jump.
Duration is frame count divided by frame rate. Fourteen hundred and forty frames at 24 fps is exactly 60 seconds, or timecode 00:01:00:00. The same frames at 25 fps run 57.6 seconds, which is why conforming between rates changes the length.
How to convert frames to time
Conforming 24 fps film to 25 fps for European broadcast simply plays it 4% fast — the classic PAL speedup, which shortens a two-hour film by nearly five minutes and raises the pitch of the audio by most of a semitone unless corrected. Going the other way, 3:2 pulldown spreads 24 frames across 30 by alternating how many fields each frame occupies. Neither is lossless, and knowing which was applied explains most of the odd durations in an archive.
Questions
Divide the frame count by the frame rate. 1440 frames at 24 fps is 60 seconds.