Extra Deep Fitted Sheets are available in either 15″ (38cm) or 18″ (46cm). These are for deeper orthopaedic mattresses or if you have a mattress topper. Measure you mattress (including any topper and add 2″ (5cm.). e.g. if your mattress is 13″ you need a fitted sheet 15″ deep so it wont ‘pop’ off the corners.