The average height of a mattress is between 8 to 15 inches. Regardless of whether you choose a platform bed or a traditional frame with a box spring, you’ll want to choose a mattress thickness that makes it possible for you to sit comfortably at the edge of the bed with your knees at 90 degrees and your feet flat on the ground. Also take into account your level of mobility, as you’ll want to be able to get in and out of bed easily.