I just make sure they're deep enough for the pillow top mattress we have and the mattress pad. I am amazed at people who don't use mattress pads. When we bought our last mattress, they asked if they could weigh the current one. The new one weighs more than the old one did. They were shocked. Usually old ones weigh more because of dust and gross. Told the guys we use mattress pads that are also water proof. We perspire heavily and I also wash them at least once a month. Also use one for the foundation. They just look at us. I know there are patterns for fitted sheets. Just Google.