How to Measure Pillow Inserts for Covers | The 1–2 Inch Rule

Getting the right pillow insert for a cover comes down to one measurement: the finished size of the cover itself, taken seam-to-seam with the cover empty. From there, a simple oversizing rule gives you the plump, corner-filled look most people want. Here’s the method that works every time, plus the exceptions for lumbars and tricky fabrics.

The core of buying pillow inserts for covers is understanding that the insert should be larger than the cover. A 20×20 cover, for example, typically pairs with a 22×22 insert. That extra inch or two per side is what pushes the corners out and keeps the pillow from looking flat and sad. The sizing guide below walks through the measurement step by step.

Measuring the Cover: Seam-to-Seam Is the Only Number That Matters

Remove the insert and lay the cover completely flat. Pull the seams taut — you want the fabric fully extended, not bunched or folded — and measure from corner seam to corner seam with a flexible tape measure. This gives you the finished size, which is the number that matters.

Two common mistakes ruin this measurement. First, measuring across a stuffed pillow instead of an empty cover adds bulk and gives you a wrong number. Second, using the raw cut size of the fabric (before sewing) instead of the finished seam-to-seam size overestimates the cover. Always measure the empty, flattened cover with the seams pulled straight.

If the cover has a zipper that won’t open, or it’s permanently sewn shut, measure the outside corner-to-corner with the seam pulled as taut as you can get it. Then add roughly 0.5 to 1 inch to account for the fabric thickness you can’t flatten out.

What Size Insert Goes in a 20×20 Cover?

The standard rule is to go 1 to 2 inches larger in each dimension than the cover’s finished size. For most decorative squares 18×18 and larger, a 2-inch oversize insert is the most common choice. For smaller pillows or covers made of less-stretchy fabric, 1 inch larger is often the safer pick.

Here’s how that plays out in practice:

Some brands recommend going up to 4 inches larger depending on the pillow type and how much loft you want. The trade-off: an oversized insert strains seams and makes zippers hard to close, so don’t chase maximum plumpness at the expense of the cover’s integrity.

Lumbar Pillows and Fabric Exceptions

Lumbar pillows don’t always follow the same 2-inch rule. Because they’re long and narrow, a too-large insert can overstuff the ends and distort the shape. Some sources recommend matching the cover size exactly for lumbars to avoid that problem, while others stick with the 1-inch oversize. The safe route: start with 1 inch larger and see how it looks before going bigger.

Fabric matters too. Stretchy zippered covers tolerate a larger insert because the material gives. Thicker, stiffer fabrics like linen or velvet hold their shape more rigidly, so a 1-inch oversize is usually the wiser choice. The fill type also factors in — feather and down inserts compress more readily and can handle more oversizing, while polyfill holds its shape and can strain a tight cover.

If you’re measuring an existing insert to match it, here’s the most accurate trick: flatten the insert without compressing the fill, wrap a soft tape around its girth (the full circumference), then divide by 2. That gives you the flat size. It’s more reliable than measuring across a puffy insert that squishes unevenly.

When the insert is already in hand and sized correctly, the goal is a cover that looks full but not strained — seams lying flat, corners pushed out, zipper closing without a fight. The right insert makes the cover look expensive; the wrong one makes it look saggy or stretched.

If you’re ready to buy, the tested couch pillow insert recommendations on this site break down which fills and sizes hold up best for daily use.

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