Two conversions with different meanings
Keep actual duration: target frames = round(source frames × source denominator ÷ source numerator × target numerator ÷ target denominator). Keep frame count: target frames = source frames. Target timecode is then labeled at the selected output rate.
Worked example
240 frames at 24 fps is 00:00:10:00. At 25 fps, keeping duration gives 250 frames and 00:00:10:00. Keeping frames gives 240 frames and 00:00:09:15, lasting 9.6 seconds.
Assumptions and limits
This converts numbers only; it does not retime video or resample audio. Duration mode rounds to the nearest whole target frame, ties away from zero, and reports the actual difference. Frame-count mode deliberately changes playback duration.
For supported rates and input formats, see the frame-rate reference on the calculator home page.