If you figure on a 40 inch piece of fabric from selvage to selvage here is the formula:
A = Square size
B = Total numbers of squares needed for quilt
C = # of blocks you can cut selvage to selvage, round down
D = Number of rows you need along the width, round up
E = The yardage you need in inches, divide by 36 and you have the total yards, round up
40/A = C
B/C = D
D*A = E
Example
A = 5"
B = 120 squares needed
40/5 = 8 (C)
120/8 = 15 (D)
15*5 = 75 (E)
75/36 = 2.08 or 2.25 yards of fabric needed
HTH!
Rachel