Duration × FPS gives an estimate
For constant frame rate (CFR), multiply seconds by frames per second. Ten seconds at 30 FPS gives 300 frame periods. The calculator displays the product and rounds up a fractional result to estimate the picture periods covering your duration. An already whole-number product is unchanged. This is an estimate from your inputs, not a count obtained by decoding footage.
Fractional rates and rounded durations
The 23.976, 29.97 and 59.94 presets use 24000/1001, 30000/1001 and 60000/1001 internally. Rounded container durations can make the formula differ from actual picture count. Exactly one second at the 29.97 preset gives about 29.97003 periods, rounded up to 30. Inspect source metadata or decode the stream when an exact count matters.
Variable frame rate needs another check
Phone recordings and screen captures can use variable frame rate. Average FPS multiplied by duration is only an approximation. This page does not inspect files or read metadata. A dedicated offline decoder can count pictures. For example, ffprobe can inspect a video stream with -count_frames and report nb_read_frames; processing can take time.
Plan a safe extraction
A 60-second 30-FPS clip may have about 1800 pictures, far beyond the extractor's 120-picture gallery. For an overview, sample once per second or choose 12 evenly spaced pictures. Use a contact sheet to keep the overview in one file. Inputs accept seconds, MM:SS or HH:MM:SS, FPS above zero through 240, and results up to one billion frame periods.