Quilt software will help you design your own quilts, but I have used it to layout blocks and sashing and the software will tell you how much fabric you will need, for top and backing. It will even print out templates in what ever size block you need. You can change the block size with one click. It will tell you the size of the quilt and you can add or subtract borders to make your quilt the size you need.
It can be very helpful.