In the US, a king bed is either 76x80 (standard king) or 72x84 (California king). The mattress can be anywhere from 5 to 20 inches deep, and a box spring can add to that, so a 10-inch drop can be too little, depending on the bed and how much of the mattress and box spring you want to cover. It would be best to get the precise measurements of the mattress and box spring, also whether there is a footboard, etc.