Square inches help in estimating whether you have enough fabric, but it is only an estimate because the shapes you want don't always go into the fabric evenly. In this case, you can get 8 5" squares in one WOF, assuming that 40" of the width is actually usable. (It may not be - remember you can't use the selvages.) In one yard you can hopefully get 7 strips, but again, if the fabric is not cut straight that may not be possible. So, if you can do the above, then you will get 8*7=56 squares MAXIMUM in each yard of fabric.
Now, do you really want 5" squares, or is that the finished size? If it's the finished size, then you want to cut 5.5" squares, and that reduces the number you will get.