The dimensions of a Full XL box spring cover are 53x79x9. A Protect-A-Bed Box Spring Cover - FULL XL should not be used on a mattress at all. The material that the box spring cover is made from is not durable enough to use on a mattress. Please select the Full XL Mattress cover. We have covers that will fit from 8" deep to 15" deep. You should measure the depth or thickness of your mattress to determine which cover you should purchase.