Standard full-size fitted sheets are 54 inches wide and 75 inches long. The measurements are important only for the fitted sheet because it must fit exactly. The flat sheet can be a little long or a little short without making much of a difference as long as it is within a few inches of the standard size. Sheets and sheet sets are typically marked as full size, so it is not usually necessary to know the measurements of a full-size sheet. For a pillow-top mattress, however, it is important to know the depth of the mattress to ensure that the fitted sheet is as deep as the mattress.
Measuring the depth of the pillow-top mattress is the key to finding sheets that will fit. Because the measurement will not be accurate if the top is compressed, and the edges of the top of a pillow-top mattress usually slope, using two pieces of cardboard or stiff paper is the best way to get an accurate measurement. Place one piece of cardboard between the mattress and the box spring with about 1 or 2 inches sticking out. Set the other piece on top of the mattress, with 1 or 2 inches sticking out. Measure the between the gap between the two pieces of cardboard. This is the depth of the mattress.
Fitted sheets come in three different depths, referred to as pockets, in order to accommodate pillow-top mattresses. A full-size fitted sheet with standard pockets, or depth, will fit mattresses 7 to 14 inches deep. A full-size fitted sheet with deep pockets will fit pillow-top mattresses 12 to 15 inches deep. A full-size fitted sheet with extra-deep pockets will fit mattresses 16 to 22 inches deep. An extra-deep fitted sheet can be used on mattresses with less depth but will not fit quite as snugly. A fitted sheet with standard pockets will not fit an extra-deep mattress. Match the depth measurements of the pillow-top mattress to the measurements of the fitted sheet to get the right size.
Deep and extra-deep pocket sheets can sometimes be difficult to find in the right color, style and thread count. Because standard fitted sheets won't fit properly, one option is to use a larger flat sheet and elastic straps to keep it in place. For example, use a queen or king size flat sheet as the bottom sheet, tuck the corners under and secure them with elastic straps made for keeping sheets in place. This method will also sometimes work with a deep fitted sheet on an extra-deep bed or with a standard fitted sheet on a deep bed.