If the drum is not perfectly round, you must also measure from top to bottom and from side to side.
To check for roundness, take at least 4 Outer Diameter measurements across different points of the drum.

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