Remove the insert.
Measure the cover only—filled pillows can look smaller than their true size.

Turn the cover inside out.
This helps you measure along the seams accurately.

Lay the cover flat on a hard surface.

Measure seam to seam for the width.
Measure straight across from the left seam to the right seam.

Measure seam to seam for the height.
Measure straight down from the top seam to the bottom seam.

Use these measurements to choose your pillow size.
Your final size should match the seam-to-seam width and height.