I've seen lots of fun patchwork circle skirts here on Craftster. I'd love to try one out and I have lots of scraps from quilting. I made up the following table in Excel to help figure out the size.
Basically, I know that my hip measurement is 48.5, so that's my starting circumference. From that, it was easy to find out the radius (c / 2pi) and the diameter (r*2).
Since I want to use 4 inch patches (3.5 once sewn because of .25 seam allowances), I figure that the second row's diameter will increase by 7 inches. From that diameter, I can find the circumference of row 2.
...and so on and so on through 10 rows.
My second calculation was for yardage of fabric. Since I know the circumference of each row and the fact that each patch will be 3.5 inches wide, once sewn. I just took the circumference by 3.5 to arrive at the number of patches.
Then, I took the number of patches by the 16 square inches in a patch to arrive at the total number of square inches. A yard of 45" fabric contains 1,620 sq. inches (45*36). From there, I can divide the total number of square inches per row by 1620 to arrive at the percentage of a yard, needed. Of course, I'd round up to quarters, halves, wholes.
If you've gotten this far, you probably understand math. Is my logic pretty sound?