If you are trying to measure your pillows at home and your covers have zippers and removable inserts, take the inserts out before you measure your pillow. If your pillow is completely sewn up and you cannot remove the insert, pinch the two top corners of the pillow and pull the top seam as tight as you can while holding it next to a tape measure. If you cannot pull it completely straight, then add a little length to your measurement (usually 1/2 inch) to get an approximate size. You'll probably be pretty close.