What size should I cut a pillow cover for an 18 inch insert?

For a plump, store-bought look, cut the front 18 by 18 inches and each of the two back pieces 18 by 12 inches. For a looser cover the same size as the insert, cut the front 19 by 19 inches and the back pieces 19 by 12½ inches. Both use a ½ inch seam allowance, a 4 inch overlap and a 1 inch hem allowance.

Should a pillow cover be smaller than the insert?

Slightly smaller is better for most inserts. A cover about 1 inch smaller than the insert in each direction pulls the insert into the corners, so the cushion looks full instead of floppy. If the cover is the same size or larger, the cushion looks flat and the corners sag.

How much should the envelope back overlap?

Use 4 to 6 inches. Less than 4 inches can gap open and show the insert. More than 6 inches adds a thick ridge and uses more fabric. The overlap grows with the cover, so use 4 inches for covers up to 16 inches and 5 to 6 inches for 20 inches and bigger.

How much fabric do I need for an 18 inch pillow cover?

About ⅞ yard of 44 inch wide fabric, including 5 percent for shrinkage. The front is 18 by 18 inches and the two back pieces are 18 by 12 inches each, which take 30 inches of length when the two back pieces sit side by side. Two covers from one fabric take about 1¾ yards.

Can I make a cover for a rectangular or lumbar insert?

Yes. Enter both the width and the height of the insert. The calculator also tries the cover turned a quarter turn, so the opening can run along either side, and reports the layout that uses less fabric. A 12 by 20 inch lumbar insert with the plump fit needs a 12 by 20 inch front and two 12 by 13 inch back pieces, which all fit in one 20 inch row of 44 inch fabric.

Pillow Cover Calculator: Envelope Cushion Cut Sizes (Free)

Pillow cover calculator

How the pillow cover calculator works

An envelope cover has one front piece and two back pieces that overlap in the middle, so you need no zipper. The insert slides in through the overlap. The fit depends on one choice: whether you cut the front at the insert size or a little bigger. Here are the steps, in order:

Plump fit: front = insert sizeSame size fit: front = insert size + 2 × seam allowanceBack piece height = (front height + overlap) ÷ 2 + hem allowanceBack piece width = front widthFinished cover = front − 2 × seam allowance

Why the plump fit is the default. When you cut the front at exactly the insert size, the seams take away the seam allowance on every side, so a ½ in seam makes the cover 1 in smaller than the insert in each direction. The insert has to fill a space that is a little too small for it, so the corners are filled out and the cushion looks full. A cover cut to the insert size plus seam allowances ends up the same size as the insert. It slides in more easily and looks looser. That suits a firm foam insert or a quick-wash cover. For a soft polyester or feather insert, use the plump fit.

Worked example. An 18 in insert with the plump fit, a ½ in seam allowance, a 4 in overlap and a 1 in hem allowance. The front is cut 18 by 18 in (46 by 46 cm). Each back piece is 18 wide and (18 + 4) ÷ 2 + 1 = 12 in tall. The finished cover is 17 by 17 in. On 44 in fabric, which has 43 in of width after the selvages, two 18 in pieces fit across. The two back pieces share one row of 12 in and the front takes another row of 18 in, 30 in in all. Adding 5 percent for shrinkage gives 31½ in, so you need ⅞ yd.

Hem one long edge of each back piece first: fold the edge in ½ in, fold again ½ in, and stitch. Then lay the front face up, place the back pieces face down on top with the hemmed edges toward the middle and overlapping, pin, and sew all four sides. Clip the corners and turn the cover right side out. The seam allowance used here is the same one described in the seam allowance guide.

Cushion cover cut sizes for square inserts

Cut sizes with a ½ in seam allowance, a 4 in overlap and a 1 in hem allowance
Insert	Front, plump	2 back pieces, plump	Front, same size	2 back pieces, same size
12 in30 cm	12 by 12 in	12 by 9 in	13 by 13 in	13 by 9½ in
14 in36 cm	14 by 14 in	14 by 10 in	15 by 15 in	15 by 10½ in
16 in41 cm	16 by 16 in	16 by 11 in	17 by 17 in	17 by 11½ in
18 in46 cm	18 by 18 in	18 by 12 in	19 by 19 in	19 by 12½ in
20 in51 cm	20 by 20 in	20 by 13 in	21 by 21 in	21 by 13½ in
22 in56 cm	22 by 22 in	22 by 14 in	23 by 23 in	23 by 14½ in
24 in61 cm	24 by 24 in	24 by 15 in	25 by 25 in	25 by 15½ in
26 in66 cm	26 by 26 in	26 by 16 in	27 by 27 in	27 by 16½ in

Change the seam allowance, overlap or hem above if yours differ. For a rectangular insert, enter both sides.

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