One jelly roll is 40 strips, approximately 40"x2.5". If you used the strips as whole pieces (as in the jelly roll race quilt), each strip would provide about 3200 square inches (40 strips *40" long *2" wide after seam allowances). Now if you know what size quilt you want, figure its square inches and divide that by 3200 to get how many jelly rolls you need. For example, to make a queen quilt that is 90x100, you need 3 jelly rolls (90x100/3200=2.8). If you cut the strips up, you will have more fabric in the seams and will therefore need more jelly rolls at some point.