How to Choose the Right 20×20 Pillow Insert | The 2-Inch Rule

For a 20×20 pillow cover, choose a 22×22 insert to get that full, plump, structured look.

Picking the right 20×20 pillow insert comes down to one simple rule: buy it bigger than the cover. A 20×20 insert inside a 20×20 cover almost always leaves you with a flat, saggy pillow that looks deflated before you even sit down. The fix is a 22×22 insert, which fills the cover completely and gives you that crisp, designer-showroom appearance.

The Standard Sizing Rule for Square Pillows

For square throw pillows, the standard pairing is an insert about 2 inches larger than the cover in each dimension. That makes a 22×22 insert the right match for a 20×20 cover. This oversizing is what gives the pillow its structure and prevents the corners from looking empty or the middle from collapsing.

The reason the larger insert works so well is simple: a pillow cover needs fabric tension to hold its shape. When the insert is the same size, the cover hangs loose and the fill settles into the bottom. When the insert is 2 inches larger, the fabric stretches just enough to keep everything in place.

When the 1-Inch Rule Makes Sense

Not every insert needs to be a full 2 inches larger. Some retailers recommend a 1-inch oversize for polyfill inserts, especially for pillows 18 inches and larger, because polyester fill compresses and molds more easily than down or feather fill. A 21×21 polyfill insert can work well if you prefer a softer, more relaxed look that you can fluff and shape by hand.

The 2-inch rule is the safer default for down and feather inserts. Feathers need more room to expand and settle, and a larger insert gives them the space to loft fully. If you like a firm, structured pillow you can prop against the headboard, stick with 22×22 for your 20×20 cover.

Measuring Your Cover the Right Way

Before you order anything, measure your pillow cover seam-to-seam on a flat surface. The label on the cover tells you what size it was designed to be, not necessarily what it measures after sewing. Fabric thickness, seam allowances, and even the zipper placement can all affect the finished size by half an inch or more.

Lay the cover flat, smooth out any wrinkles, and measure from one seam to the opposite seam across the middle. Do this for both width and height. If your cover measures closer to 19.5 inches than 20, a 21×21 insert might serve you better than a 22×22.

Fabric weight matters here too. A thick, heavyweight fabric like linen or velvet consumes more internal space, so it often needs a larger insert to fill out. A thin, stretchy cotton cover, on the other hand, can overfill at 2 inches oversize and strain the seams. When in doubt, check the seam-to-seam measurement against the insert size and choose accordingly.

If you’re comparing pillows for a specific spot, our tested roundup of the best 20 x 20 pillows covers covers and full pillows that hold their shape.

Size Exceptions: Lumbar and Specialty Shapes

Everything above assumes a square throw pillow. Lumbar pillows follow different rules — they often use an insert the same size as the cover or even slightly smaller, because a lumbar pillow is meant to be softer and more yielding behind your back. The 2-inch oversize rule simply doesn’t apply the same way to a 12×20 or 14×24 lumbar cover.

You should also consider the corner issue. A very thick, stiff, or densely woven cover can underfill at the corners even with a 22×22 insert, leaving little empty triangles at each corner. A thin or stretchy cover can overfill and bulge. If you notice either problem, adjust your insert size up or down by an inch rather than fighting the fabric.

The table above condenses the sizing guidance: for a standard square throw pillow cover, the 2-inch oversize is the safe default, and the 1-inch oversize is a reasonable preference pick for softer polyfill.

FAQs

Will a 22×22 insert fit inside a 20×20 cover?

Yes, a 22×22 insert fits a 20×20 cover, and this combination is the standard recommendation for throw pillows. The cover stretches slightly around the larger insert, which creates the tension that keeps the pillow looking full and prevents sagging. If the cover is very thick or heavy fabric, the fit can feel tight at first.

What size insert do I need for a 20×20 lumbar pillow?

Lumbar pillows typically use an insert matching the cover size rather than one that is larger. For a 20×20 lumbar cover, a 20×20 insert provides the softer, more flexible feel that lumbar pillows are designed for. The stiffer square-pillow look isn’t usually what you want behind your back.

Why does my 20×20 pillow look flat with a 20×20 insert?

A same-size insert leaves the cover loose and lets the fill settle flat, especially with down or feather fill. The fabric has no tension to hold the pillow upright. Switching to a 21×21 or 22×22 insert tightens the cover and gives the pillow the volume and structure it needs to keep its shape.