In case your PHP skills are worse than mine and you can't figure out the simple math in the source to my tempo calculator, here's an explanation.
Note durations
Given a tempo in beats-per-minute (BPM), it is possible to determine the exact duration of a particular note, such as quarter notes, eighths, sixteenths, and the dotted and triplet variations thereof. This is handy for dialing in rhythmic delay times. The conversion is from beats-per-minute to seconds-per-beat. A quarter note at 60bpm will last exactly 1 second (1000ms).
Half note = 120 / BPM Quarter note = 60 / BPM Eighth note = 30 / BPM Sixteenth note = 15 / BPM Dotted-quarter note = 90 / BPM Dotted-eighth note = 45 / BPM Dotted-sixteenth note = 22.5 / BPM Triplet-quarter note = 40 / BPM Triplet-eighth note = 20 / BPM Triplet-sixteenth note = 10 / BPM
So, if you have a tempo of 98bpm, and want to hear delayed notes as triplet quarter notes, you would divide 40 by 98, giving a result of 0.408 seconds (408ms) to set your delay time to.
Hertz
On the other hand, many modulation effects such as chorus and tremolo operate in terms of Hertz - how many times the modulation cycles up and back down in one second - rather than in terms of how long a single cycle takes. If you want to dial in such an effect based on tempo, the math is strikingly similar. You simply turn beats-per-minute into beats-per-second. A quarter note at 60bpm equals 1 Hz.
Half note = BPM / 120 Quarter note = BPM / 60 Eighth note = BPM / 30 Sixteenth note = BPM / 15 Dotted-quarter note = BPM / 90 Dotted-eighth note = BPM / 45 Dotted-sixteenth note = BPM / 22.5 Triplet-quarter note = BPM / 40 Triplet-eighth note = BPM / 20 Triplet-sixteenth note = BPM / 10
In the same 98bpm example as above, if you wanted to dial in a phaser that pulses on eighth notes, you would divide 98 by 30, which equals 3.267 Hz, that being how many eighth-notes happen in one second at 98bpm.