When you put a 3 dimensional object (namely your insert) inside a 2-dimensional flat cover, some of the width/height went towards wrapping around the depth of your insert. For example, if your insert is 20x20 along the side seams, the cover is made 19x19 across the middle, and after stuffing your pillow might appear roughly 16x16x3 (assuming you have a 3 inch thick insert).  The plumper your insert, the smaller the finished pillow will appear versus the flat seam to seam measurement.The flat pillow cover dimensions will never translate inch for inch into the height or width of the finished pillow.  Think of this another way: As your eyes look at a pillow straight on, the sides and the top and bottom curve away from you, so you will perceive the pillow to be smaller because your eyes can't see around the curves.  Standard sizing by insert manufacturers is a major source of this confusion. Pillow inserts are typically made in the knife edge style, meaning there is no gusset to accommodate for the thickness of the stuffing. The shell of the insert is cut as a square or rectangle and the insert is sold by the length of the seams before filling is added. However, once filling is added turning the flat shell into a 3 dimensional pillow insert, the flat dimensions is no longer an accurate guide for how tall or wide the 3 dimensional pillow will look to you. This is why I urge many of you to buy bigger inserts than you think your furniture needs. I also advise you to purchase your inserts first, before buying the covers, and arrange the inserts on your furniture be sure they are roughly the right size relative to the scale of your furniture. Once you put my covers on your inserts, they will look more trim and tailored, but if you follow my suggestion above, you should not make the mistake of purchasing a size that is drastically too small for your furniture.Take a look at these photos where I have photographed various insert sizes on an average 2 seat and 3 seat sofas, so you can see approximately how each size looks in reality:12x18, 12x24, 14x20, 16x26, 16x16, 18x18, 20x20, 22x22, 24x24 to 26x26 (Euro sham)For example, if your sofa has a seat to back height of 20 inches high, do not make the mistake of using a 20x20 insert!  As you can see from the photos, if you want the finished pillow to provide full back support, you will actually need a 24x24 insert and a cover made to fit it.