We sell an insert for each of our pillow cover sizes and our insert guide is as follows. For the 18x18 pillow order the 20x20 insert. For the 20x20 pillow order the 22x22 insert. For the 22x22 pillow order the 24x24 insert. For the 12x24 pillow order the 12x25 insert. For the 12x38 pillow order the 12x39 insert. You can purchase our inserts here.