Fitted sheets, despite their name, may pop right off of that mattress if they do not fit well, even if you are certain you bought the right-size sheets for the bed. If your mattress seems taller or thicker than a basic old-fashioned mattress, it may require deep-pocket sheets. Measuring the mattress and carefully reading the measurements listed on sheet packages help ensure your next fitted-sheet purchase stays on the bed as it should.
Measuring Your Mattress
Looking at the mattress isn't enough to determine the sheet size. Instead, measure its complete height or thickness to know just how deep those fitted sheets must be to fit the bed. Since many mattresses billow upwards or pillow at the top, just measuring the side of the mattress may not do. Instead, place a large straightedge on the highest part of the mattress so it sticks beyond the edge of the bed. Press it down, without indenting the mattress, to ensure the straightedge is completely horizontal. Measure the distance from the bottom of the mattress to the straightedge using either a tape measure or a yardstick. If the distance is greater than 12 inches, you may need deep-pocket sheets.
Selecting Sheets: Mattress Height Specifications
Jot down your mattress height; then read through sheet set packages to find those that specify mattress height. Manufacturers may use the term "pocket depth" or other terms instead of mattress height. Packages that show mattress height indicate that the sheets fit any mattress of that size -- full sheets for a full mattress, for instance -- up to that particular height. As an example, if the package indicates a 12-inch-high mattress, the sheets fit any mattress 12 inches high or less.
Picking Pocket Depth
Some sheet packages show "pocket depth" instead of mattress height. This terminology covers the height or depth of the fitted sheet, but it is a bit trickier than using mattress height to make your purchase. Subtract 2 inches from the pocket depth to determine if the sheets fit your mattress. If the pocket depth is 16, the fitted sheet fits a mattress 14 inches high or less. Avoid purchasing sheets with a pocket depth exactly the same as your mattress height, as the fitted sheet will most likely not fit well, given the added plushness and build of the mattress. The sheet corners are likely to pop off if the pocket depth matches the mattress height.
Sometimes, sheet packaging does not mention the exact sheep depth or pocket measurement. If this is the case, look for depth terminology listed as "standard," "deep" or "extra deep." Standard-pocket sheets fit mattress up to 12 inches high, while deep-pocket sheets are designed for mattresses 13 to 17 inches high. Extra-deep fitted sheets are for mattresses 18 to 25 inches high. If you're unable to find your exact mattress size range, opt for a pocket depth larger than your mattress height; the excess sheet fabric can be tucked under the mattress.