Deep fitted sheets usually have a depth of around 12 inches, or 30 cm, which tends to be ideal for deeper mattresses. Deep fitted bed sheets are also well suited if you have a standard mattress with a mattress topper, to make sure that the sheets stay in place all night. The elasticated edges also make these fitted bed sheets really easy to put onto your bed. If your mattress is much deeper than 30cm, then you may need special deep fitted sheets made to measure.