How to Choose the Right Size Pillow Insert for Your Decorative Pillow Cover? | The 2-Inch Rule

That flat, lifeless look on your sofa isn’t the cover’s fault — it’s almost always an insert that’s too small. The fix for choosing the right size pillow insert for a decorative pillow cover is simple: size up. When you buy an insert that matches the cover exactly, it can’t fill the seams, so it reads as saggy and cheap. Go larger instead, and the pillow looks plump and intentional, the way it does in catalogs.

This guide covers the exact sizing for squares and lumbars, how fill type changes the math, and the one measuring mistake that throws everything off.

Why Buy A Pillow Insert Bigger Than The Cover?

Decorative pillow covers are sewn an inch or two wider than the insert dimension they’re designed to hold. If the insert and cover are the same size, the cover has no excess fabric to stretch over the fill, which leaves the pillow looking flat and wrinkled. The oversized insert pushes the cover taut.

There’s no need to worry about the cover being too small for the insert — the extra fabric is built to take it. Oversizing also reduces the risk of the insert “smiling” or creasing at the center over time.

Sizing Square Pillows: The 1–2–3 Rule

The most common rule is 2 inches larger for medium squares. But for small and large squares, the math shifts. Here’s the simple guide that covers most sizes:

• Small squares (14×14 to 16×16): go 1 inch larger (15×15 or 17×17 insert)
• Medium squares (18×18 to 22×22): go 2 inches larger (20×20 to 24×24 insert)
• Large squares (24×24 and up): go 3 inches larger (27×27 or 28×28 insert)

This “1–2–3 rule” is the most reliable takeaway across sources. For an 18×18 cover, a 20×20 insert is the sweet spot. A 16×16 cover gets an 18×18 insert. A 24×24 cover works well with a 26×26 or 28×28, depending on how plush you want it.

How To Size A Lumbar Pillow Insert

Lumbar pillows are the exception to the oversizing rule. Because they’re long and narrow, an oversized insert can strain the zipper and corners, and most lumbars are meant to sit firm rather than pillow-soft. For most lumbar shapes, match the insert to the cover’s exact dimensions. If the lumbar is a rectangle, the same 1–2 inch rule can apply, but it’s safer to start at the exact size and go up only if it looks under-filled.

Fill type also changes the fit. Down and feather inserts compress much more than polyfill, so a down insert can go up 2–3 inches while a polyfill one stays at 1–2 inches. For a down insert in a 20×20 cover, a 22×22 works well; for the same cover with polyfill, 22×22 may feel overly firm.

The Right Way To Measure A Pillow Insert

Measuring an insert corner-to-corner is the fastest way to make the wrong call — the corners can add 1–2 inches of error. Instead, measure the girth (the distance around the insert all the way around, both directions), divide by 2, and use that as your width and height. That gives the actual usable fill size.

Before ordering, measure your cover’s seam-to-seam dimensions rather than trusting the listed size. Covers can run a half-inch off. And when choosing an insert for a cover you already own, check whether the zipper sits on the bottom edge — overstuffed inserts strain zipper teeth and corner seams.

If you’re ready to buy, our tested roundup of the best decorative pillow inserts covers the top picks for every fill type.

Common Sizing Mistakes To Avoid

The two mistakes that cause the most repeat purchases:

• Matching the insert to the cover: this produces the flat, lifeless look. The whole point of sizing up is fullness.
• Oversizing too aggressively: going 4+ inches on a small or lumbar cover strains zippers, seams, and corners.

A quick reference for the most common sizes: 14×14 cover → 15×15 insert; 16×16 cover → 18×18; 18×18 cover → 20×20; 20×20 cover → 22×22; 22×22 cover → 24×24; 24×24 cover → 26×26 or 28×28. For a fuller, plush look on any square, you can push one size beyond the standard — but don’t exceed 4 inches on any cover.