If you are unsure of what size of pillows to order, here is a brief explanation of pillow measurements that may help.
The most important thing to remember is that pillow dimensions always refer to the pillow cover's width and length while lying flat unstuffed. The measurements are taken along the seams from corner to corner and are almost always rounded up to the nearest whole inch.
Looking at the example below of this elegant 20”x20” Sunbrella outdoor fabric pillow, you will notice that the actual measurements of the cover lying flat unstuffed are 19.3” x19.3”. The dimensions of this pillow are rounded up and it is therefore classified as a 20”x20” pillow.
Now, if you look at the same pillow cover stuffed with a polyfill insert (image on the right), you will notice that if you were to take a straight measurement from corner to corner, the width and height would be closer to 18”. Furthermore, if you were to take a straight measurement across the center of the pillow (as shown by the distance between the red lines), the width would be closer to 16.2”.