For a full, plump decorative look, the best pillow insert for a 16×16 cover is 18×18 inches; for a softer, less stuffed appearance, choose 17×17.
The most common mistake people make with square throw pillows is matching the insert size exactly to the cover. This usually leaves you with a saggy, floppy pillow that looks underfilled. Decorative pillow covers are designed to be slightly smaller than the insert, creating tension that gives the pillow its tailored, plump shape. The general rule for square pillows under 18 inches is to size up by one to two inches, and a 16×16 cover gives you two excellent options depending on the look you want.
Why You Shouldn’t Use a 16×16 Insert
A 16×16 insert inside a 16×16 cover will almost always look disappointing. Because the cover fabric has its own thickness and seams, the same-size insert can’t fill the envelope completely, leaving gaps at the corners and a limp center. Decorative pillows are meant to look crisp and supportive, not like a sleeping pillow after a bad night. The only case where an exact match works is when you genuinely want a very soft, nearly flat pillow — for example, on a children’s bed or as a backrest where firmness is a drawback.
If you prefer a flatter, more casual look and the cover is a soft material like linen, a 16×16 or 17×17 insert works well. Some retailers explicitly recommend matching the cover or sizing up only one inch for small square pillows to avoid an overstuffed appearance. Ariannabelle and Chloe & Olive, for example, recommend a 17×17 insert for a 16×16 cover, which gives a moderate loft without being too tight.
17×17 vs 18×18: Choosing Your Fullness
These two sizes cover almost every preference you might have. An 18×18 insert gives the most dramatic transformation: the cover looks fully packed, the seams stand out cleanly, and the pillow stays plump even after someone leans on it. This is the standard choice for a “luxury hotel” look and is the size Eryn Home recommends for a 16×16 cover. Because the insert is two inches larger on each side, the fabric stretches slightly, which smooths out wrinkles and gives a crisp silhouette.
A 17×17 insert is the middle ground. It fills the cover well without the firm tension of the 18×18, so the pillow remains soft to the touch but still looks intentional rather than deflated. This size is ideal if the cover is made from a heavier or textured fabric like velvet, bouclé, or chunky knit, since those materials don’t need as much insert volume to look full. If you’re layering multiple pillows on a sofa or bed, a mix of 17×17 and 18×18 inserts creates a natural, inviting depth.
| Insert Size | Look and Feel | Best For |
|---|---|---|
| 16×16 | Flat, soft, underfilled | Backrest support, casual setting, children’s rooms |
| 17×17 | Moderate fullness, soft loft | Heavy fabric covers, layered arrangements, moderate firmness |
| 18×18 | Plump, tailored, firm | Standard decorative pillow, hotel look, smooth fabrics |
Fabric Thickness Changes the Fit
The same insert can look different inside a linen cover versus a velvet one. Heavily textured or thick fabrics eat up internal space, so an 18×18 insert inside a thick velvet 16×16 cover may look more like a 17×17 in linen. For loose-weave or stretchy covers, an 18×18 can fill them out beautifully; for tight-weave or lined covers, the same size can strain the seams or make the corners bulge. If your cover feels thick or has a separate liner, lean toward the larger 18×18 insert. If the cover is thin and smooth, 17×17 may be all you need.
When you’re ready to buy, check out our roundup of the best 16×16 pillow inserts for options that fit both looks, with reviews on how each insert fills a cover in real use.
FAQs
Can I use a 20×20 insert in a 16×16 cover?
No. A 20×20 insert is four inches larger on each side and will strain the seams, distort the shape, and likely damage the cover. The general rule is a maximum of two inches larger for square pillows under 18 inches. Stick with 17×17 or 18×18.
Does this sizing work for lumbar or rectangular pillows?
Not directly. Lumbar and rectangular pillows follow different sizing logic because their proportions are longer on one side. Many lumbar covers work best with an insert that matches exactly or sizes up only on the width, not the length. Check the manufacturer’s guide for rectangular shapes rather than applying the square rule.
What if my insert is from a different brand than the cover?
That’s fine, but check the insert brand’s own sizing guidance before ordering. Some brands cut their inserts generously or conservatively, so their 18×18 may actually measure 18.5 or 17.5. If you’re switching brands, read the product description for actual dimensions rather than relying on the label size alone.
References & Sources
- Chloe & Olive. “Pillow Insert Size Guide.” Recommends 17×17 insert for a 16×16 cover.
- Eryn Home. “Pillow Insert Size Guide.” Recommends 18×18 insert for a 16×16 cover for a fuller look.
- Ariannabelle. “Insert Size Guide.” Recommends 17×17 insert for a 16×16 cover.
