The thickness of a futon mattress is determined by how it will be used and stored. If you consider a floor bed and rolling it up to store it in a closet, a thickness of 3 to 4 inches or less would be easy to roll up. A 6-inch futon is ideal if you look for a mattress for a sofa or bed that can be folded three times. If the bed or sofa can only be folded twice, a mattress larger than 8 inches is recommended.