If you know what you want to make, and if it's not a pre-done pattern with yardage already figured for you, this website is a great help. You just enter the shape of the piece, the measurements of the pieces you are going to cut from each fabric, how many you need, and if the measurements are with or without the seam allowance. It does the computations for you. It's quite accurateand very easy. The only caution I would add, is to make sure to check the width of your fabric after washing if you prewash.
http://vrya.net/quilt/classic.php