BPM counts beats per minute. TempoCalc uses the common quarter-note beat contract, so one minute supplies the entire calculation: divide 60,000 milliseconds by BPM, then apply the written note and rhythm multiplier.
Start with the quarter note
At 120 BPM, 60,000/120 equals exactly 500 ms. At 128 BPM it equals exactly 468.75 ms. The decimal BPM input is represented as a fraction before division, so a displayed rounded value never becomes the next input.
Scale written note values
A whole note is four quarter notes, a half is two, an eighth is one half, and each smaller binary division halves again. These are exact rational multipliers, independent of the displayed decimal precision.
Invert duration to get hertz
A cycle lasting t milliseconds repeats 1000/t times per second. Therefore a 500 ms quarter note is 2 Hz and a 250 ms eighth note is 4 Hz. This is cycle frequency, not audio pitch.
Keep the tempo contract visible
A score can define another beat unit, and a DAW can automate tempo. TempoCalc reports constant quarter-note timing. Time signature groups beats but does not change the duration formula by itself.