I always use 2 1/2" wide strips for binding my quilts.
Here's a formula to figure out how much fabric to purchase to bind your, well, any project that requires binding really!
(width of quilt in inches + length of quilt in inches) times 2 + 24" = amount of inches for total binding.
Divide the total by 40 and round up, that = the total amount of strips. Times that total by 2.5 and that = the amount of inches of fabric needed.
Make sense? Here's another way to look at it:
(width of quilt in inches + length of quilt in inches) x 2, + 24" = amount of inches for total binding.
Amount of inches for total binding divided by 40 = the total amount of strips to cut (round this # up if a decimal).
(Amount of strips to cut) x 2.5" = amount of inches of fabric needed