Measure the perimeter of your quilt. Add 12-18 (corners, etc). Divide by 40. Round up.
Eg - quilt is 180" (after adding 12-18) -> Divide by 40 = 4.5 -> 5.
This is the # of strips you need. Multiply times how wide your binding is (2.25-2.5" usually). That's how much material you need.
5 strips x 2.5" = 12.5" (x wof) will be enough to do the binding
Make sense