If I am using a pattern I usually buy the specified yardage or a little over to allow for shrinkage or errors. If I am making up my design, I go through the math to determine the amount of fabric needed for a block--then multiply that out based upon the size of the quilt I am going to make. It might help to make a sample block first to get the amount of fabric needed, then multiply that by the number of blocks. Then calculate the amount for borders (if any). I cut binding strips 2 1/4" wide and based upon the finished size you can calculate the amount of yardage based upon the number of strips needed. Lastly the backing. I don't usually buy backing until I am done with the quilt top and see what scraps and extra blocks I have left and piece those together first, then buy enough backing fabric to get the right size.
I guess there is no easy way without lots of figuring if you don't have a pattern to give you yardage. Most of us must over buy or we wouldn't have so much stash!!