Fabric for envelope-back throw pillows.
Covers front and back with seam and overlap. For very large pillows check against your fabric width.
An envelope back pillow cover has a plain front panel and two overlapping panels at the back that close without a zipper. The calculator works out the fabric for the front and the back panels and multiplies by the number of pillows. Each back panel is a little more than half the width of the front, so that they overlap in the middle.
Many makers cut the cover about one inch smaller than the pillow insert, so the pillow looks full and plump instead of loose. Add the seam allowance on each side to the cut size, and a hem allowance on the back panel edges where they overlap.
Fabric for several pillows adds up, so check the width of the fabric and how many pieces you can cut from each length. Directional prints need the front and back to be cut the same way, and stripes and plaids need extra for matching.
Choose a fabric that suits how the pillow will be used. Canvas, linen, and upholstery weights wear well, while delicate fabrics are better for decorative pillows. Pre wash the fabric if the cover will be washed, so it does not shrink around the insert.