ummm...the basic formula is to measure the width and length of the mattress, then add the amount of "drop" you want for the sides. Also need to decide if you want enough length to cover the pillows, with or without a tuck under the front of the pillows. I purchased a copy of "all-in-one Quilter's Reference Tool" by Harriet Hargrave, Sharyn Craig, Alex Anderson, Liz Aneloski which is nice cuz there is a chart inside that list common mattress sizes along with various "drops" and length options depending if you intend it to be used as a comforter, bedspread, etc. I find large squarish quilts work fine on a Queen or King if you just want another quilt on the bed otherwise I prefer to use the rectangular ones for smaller beds and for bedspreads. Oh the reference tool also has a lot of other useful measurement type info in it.