The amount of fabric needed for a decorative pillow depends on its size and shape. As a general rule of thumb, you can calculate it by adding twice the dimensions of your pillow plus an extra inch on each side to account for seam allowance. For example, if you want to make an 18x18 inch square decorative pillow, you would need two pieces of fabric that are at least 19x19 inches each (to include seam allowance). However, this estimation may vary depending on your sewing preferences and how plump or firm you want your pillow to be.