Mattrasses are getting deeper. So I usually take the size of the mattress - say queen is 60"x80" and add 15" overhang on side and bottom and then whatever needs to be added to the top for the pillow tuck, if it is going to go over a pillow. If not then 15" on all four sides. This is a start because the depth of the mattress may require additional or less inches. I may my queens 90"x108" if that helps. Kings are about 110"x110"