HOW TO USE THE METRIC MODULATION CALCULATOR
Old BPM = Write the numeric value for the BPM on the bar before the metric modulation took place. For this example, the quarter note will equal 120bpm.
Figuring out OPN (Old Pivot Note Values) and NPN (New Pivot Note Values):
Take the equaled notes on top of the metric modulation.
Figure out the Greatest Common Factor between these two rhythmic figures. What is the biggest rhythmic figure that could divide both equaled notes? Express the equaled notes in this figure.
The number left of the equal sign is your OPN. Write it in the OPN blank field of the calculator.
The number right of the equal sign is your NPN. Write it in the NPN blank field of the calculator.
After following these steps, the New BPM Value should appear. The BPM shown is for the pulse after the metric modulation has occurred. If you did everything right, your calculator should look like this:
Before the metric modulation the quarter note equaled 120 BPM, after the metric modulation the quarter note will equal 60 BPM.
Note: Remember that on 3/8 the new pulse is an eighth note, while on 4/4 the pulse is a quarter note and so on...
If you'd like to learn the story behind this calculator, read the February 4th, 2015 post of the "Learn" section.