A song’s tempo is usually expressed in terms of beats per minute (bpm). But figuring out a song’s tempo by counting beats for 60 seconds isn’t especially practical. A more efficient way is to find a smaller multiple of 60 and
use that as the basis for your calculations. For example, you
can count how many beats go by in 15 seconds and multi-ply that by 4 ( 15 x 4 = 60 seconds or one minute) or you
can count how many beats go by in 10 seconds then multi-ply that by 6. It can be hard to get a really accurate reading
with this approach, but it will get you in the ballpark, if
you’re in a pinch.
However, while most musicians think of tempo in terms
of bpm, most delay units represent delay time as milliseconds (ms). Studio guitarists used to carry conversion charts
in their gigbags to make sure they could always lock in with
tempos at a session, and we’re including one for you below.
If you ever find yourself in need of a beat-matched delay
but don’t have this chart handy, you can use some basic
formulas to convert the desired bpm into ms and set your
delay accordingly. The basis for the formulas is the number
60,000—the number of milliseconds in a minute. To convert bpm to ms, the formula is:
60,000/bpm = quarter-note ms
For example, 60,000/100 bpm = 600 ms. If you’re playing
to a track that is 100 bpm, you’ll need to set your delay at
600 ms to get quarter-note repeats.
To get smaller subdivisions of the quarter note, there are
two approaches. You can divide the quarter-note ms read-
ing proportionately, as needed. For repeats in eighth-notes,
divide the quarter-note ms by 2, and for repeats in 16ths,
divide the quarter-note ms by 4. At a tempo of 100 bpm,
quarter-note repeats are 600 ms, eighth-note repeats are 300
ms, and 16th-note repeats are 150 ms.
30,000/bpm = eighth-note ms
15,000/bpm = 16th-note ms
For triplet-based music, the formula is:
40,000/bpm = quarter-note-triplet ms
For example, 40,000 divided by 100 bpm = 400 ms. You
can divide the quarter-note-triplet ms reading to get values
for eighth- and 16th-note triplets. To get eighth-note-triplet
repeats, divide the quarter-note-triplet ms by 2, and to get
16th-note-triplet repeats, divide the quarter-note-triplet ms
by 4. At a tempo of 100 bpm, quarter-note-triplet repeats are
400 ms, eighth-note-triplet repeats are 200 ms, and 16th-
note-triplet repeats are 100 ms.
Or you can use these formulas:
20,000/bpm = eighth-note-triplet ms
10,000/bpm = 16th-note-triplet ms
To calculate ms for dotted-eighth-note rhythmic repeats,
the formula is:
45,000/bpm = dotted-eighth-note ms
For example at 100 bpm, you will need a delay time of
450 ms to get repeats in dotted-eighth-notes.