A 20×20 inch insert is the standard fit for an 18×18 pillow cover, because the extra two inches fills the corners and gives the pillow a full, plump shape.
An 18×18 cover paired with a 20×20 insert is the pairing most upholstery and pillow shops recommend, and it is the one that produces the crisp, stuffed look people picture when they buy a decorative cover. The two-inch rule exists because fabric pulls inward at every seam; a cover measured flat is almost always smaller than its label claims, so the insert has to be bigger to compensate.
Here is how that sizing works, when a smaller insert makes sense, and how to measure a cover so you buy the right insert the first time.
Why A 20×20 Insert Fits An 18×18 Cover
A 20×20 insert works because a two-inch difference fills out the corners and sides of the cover without straining the seams or the zipper.
Pillow covers are sewn with a seam allowance on every edge, and once the cover is turned right-side out, those allowances eat into the usable space inside. A cover sold as 18×18 typically has an interior that measures closer to 17 to 18 inches across, and stuffing it with an insert of the same size leaves the corners loose and hollow.
Ariannabelle’s insert size guide puts it plainly: choose an insert about two inches larger than the cover for a full, tailored appearance. That recommendation is standard across the industry, and it covers the three things an undersized insert ruins — corner definition, surface fullness, and how the pillow photographs on a sofa or bed.
A Two-Inch Difference, Step By Step
- Cover: 18×18 inches.
- Insert for a full look: 20×20 inches.
- Insert for a soft, floppy look: 19×19 inches, if you can find one, or an 18×18 for a flat, slouchy finish.
How Firm You Want The Pillow Changes The Answer
A 20×20 insert gives the fullest look, a 19×19 insert gives a softer and slightly floppier one, and an 18×18 insert leaves the cover visibly flat.
Most sources treat the two-inch rule as the default and the one-inch oversize as the alternative for people who prefer a relaxed, lived-in cushion that sinks when you lean on it. There is no single correct answer — it depends on the finish you want.
| Insert Size | Finish You Get | Best For |
|---|---|---|
| 18×18 | Flat, loose corners | Slouchy, casual throws |
| 19×19 | Soft, slightly relaxed | A floppy, sink-into look |
| 20×20 | Full and plump | Most decorative 18×18 covers |
| 21×21 | Overstuffed, tight seams | Zipperless envelope covers only |
If you’re shopping for the cover itself, our roundup of tested 18 x 18 pillow covers covers the ones that hold up best with a 20×20 insert inside them.
How To Measure Your Cover Before You Buy An Insert
Measure the cover seam to seam across the middle, not corner to corner, so labeled sizing quirks from different makers don’t trip you up.
Finished cover dimensions vary more than most shoppers expect — a cover labeled 18×18 from one brand may measure fractionally smaller or larger than a cover with the same label from another. Land of Pillow’s measuring guide recommends laying the cover flat and measuring straight across from one seam to the opposite seam, then repeating on the other side, so you capture the true interior space the insert needs to fill. Aloriam’s sizing guide makes the same point: trust your own tape measure over the number on the tag.
One quick check before ordering: zip the empty cover closed and lay it flat. If the seams pull toward the middle or the fabric puckers, that cover is running small and a 20×20 insert is the safer pick. If it lies perfectly flat and smooth, either size will work, and the choice is just about how full you want the pillow to look.
FAQs
Is an 18×18 insert too small for an 18×18 cover?
Yes, in most cases an 18×18 insert leaves the cover flat with hollow corners, because seam allowances reduce usable interior space. The matching-size insert only works if you specifically want a floppy, underfilled cushion that sinks when leaned on. Most shoppers get a better result stepping up to a 20×20 insert.
Can I use a 22×22 insert in an 18×18 cover?
A 22×22 insert is generally too big for an 18×18 cover. The extra four inches strains the seams, distorts the shape, and can damage a zipper over time. If you want an overstuffed look, a 21×21 insert is the practical ceiling, and only for covers with envelope closures or generous seam allowances.
Do down and polyester inserts size the same?
Down and polyester inserts follow the same two-inch sizing rule, but they compress differently. Polyester rebounds to its full loft and holds a firm shape; down compresses under weight and needs fluffing. Either way, a 20×20 insert is the right pick for an 18×18 cover.
References & Sources
- Ariannabelle. “Insert Size Guide.” Supports the two-inch-larger insert rule for a full pillow look.
- Aloriam. “Sizing Guide.” Confirms sizing guidance and the need to measure covers rather than trust labels.
- Land of Pillow. “How to Measure a Pillow.” Explains seam-to-seam measuring to find true cover dimensions.
