If your pattern calls for fifteen 8 inch by 10 inch squares how much fabric will you need?

If your fabric is 45" wide then I would go for a yard (36"): cut three (3) 10" strips from selvage edge to selvage edge; then cut each strip into five (5) 8" lengths. This all depends upon if your fabric has a nap or pattern. If you need to reverse the lay out, then: cut four (4) 8" strips from selvage edge to selvage ddge; then cut each strip into rour (4) 10" lengths, leaving one extra.