For an 18x18 pillow cover, most decorators recommend using a slightly larger insert so the pillow looks full, not flat.

Short answer

Use a 20x20 pillow insert for an 18x18 cover.

Why size up?

  • Inserts that match the cover size (18x18 in an 18x18 cover) tend to look a bit saggy and under-filled once you actually stuff them inside.
  • Many designers follow a simple rule: choose an insert about 2 inches larger than the cover for medium squares like 18x18.
  • Several sizing guides specifically say: “For an 18x18 pillow cover, use a 20x20 insert.”

What if I want it less firm?

If you prefer a softer, less “plump” look, you can:

  • Drop down to a 19x19 insert if you can find that size, which some sources suggest as a gentle oversize.
  • Choose a 20x20 insert but with a less densely filled (less stuffed) form, so it looks full without feeling overly firm.

Quick rule of thumb for future sizes

  • Small squares (under 18x18): go 1 inch larger.
  • Medium squares (18x18–20x20): go 2 inches larger.
  • Large squares (22x22 and up): 2–3 inches larger, depending on how overstuffed you like them.

TL;DR: For “what size pillow insert for 18x18 cover,” the go-to choice is a 20x20 insert for that full, designer-style look.

Information gathered from public forums or data available on the internet and portrayed here.