If your mattress is showing its age and giving you a less than comfortable night's sleep, then you may be thinking about getting a replacement. This usually doesn't mean that you need a whole new bed — replacing the old mattress with a new one often does the trick. Before you decide which mattress to choose, you need to know what size of bed you're dealing with. If you've had your bed for years and can't remember what kind it is or inherited it from the previous owners of your home, then you need to check this out before you pick a mattress.