Insert manufacturers sell inserts by their height x width *before* putting in filling. Therefore, the most accurate way to measure them is along a seam line from corner to corner, using a flexible tape measure, while pulling the seam straight. Never measure an insert across the belly because you could get a measurement 1 to 2 inches too small, especially if you are using a rigid ruler. Additionally, make sure you are measuring your bare insert without any cover. A cover squeezes the insert into a smaller shape, resulting in too small a measurement. I assume that you are using an average insert with normal compressible fill such as feather/down, synthetic down or polyfil (not memory foam, upholstery foam, latex, overfilled, or in a non-removable pillow cover). Please notify me if your insert does not meet these conditions.