The best way to measure for rings is to use a tailor's tape-measure (cloth), and measure the Outer Circumference. Then it doesn't matter if the drum is out of round. This is the preferred method. If using a metal tape measure or a rule to measure the diameter, you must take at least 4 Outer Diameter measurements: top to bottom (North/South) and side to side (East/West). This will check for roundness. Note: the inner diameter measurement is not helpful.
Formula for Rope
This formula is based on drum that has 1/4" thick rings and 26 loops or less. If the drum has a thicker ring and/or more than 26 loops the formula will change.
Loops on the upper ring: Measure 4 times the circumference of the ring. Loops on the bottom ring: Measure 5 1/4 times the circumference of the ring. Verticals:
Measure the length between the top rim of the drum to where the bottom ring sits.
Add 2" This makes an extended vertical.
Count the loops on one ring (the number will be the same for both rings) and multiply by 2
This gives you the number of verticals
Multiply the extended vertical by the number of verticals (2 x the loops)
Divide by 12" This will tell you how many feet you need.
For one drum you will need the amount for the top and bottom loops and the verticals.
Android APP for calculating how much rope to use for verticals. Go to the Android Market place and look for Djembe Rope Calculator