Pillow Cover Calculator for Any Insert Size

Should a pillow cover be smaller than the insert?

Yes, in most cases. A cover that finishes about 1 in smaller than its insert will look full and plump, with neat corners. A cover the same size as its insert can look a bit flat and baggy, particularly after the insert has settled over time. This tool defaults to the plump fit, so an 18 in insert gets a cover that finishes at 17 in, cut at 18 in with 1/2 in seams.

That rule works for typical polyester and feather inserts, which compress easily. A firm foam insert will not squash in the same way, so choose the exact fit for those. For a large floor cushion, you may want the cover to be even snugger. If you are not sure, measure across the insert before you buy fabric, as labels are often rounded, and one 18 in insert can be slightly larger than another.

How an envelope pillow cover works

An envelope cover has a single front and two back pieces that overlap across the middle, like the flap on an envelope. Each back piece is as wide as the front. Its height is half the finished height, plus half of the overlap, plus a seam allowance on the outer edge, plus a hem on the inner edge. The overlap is what keeps the insert out of sight, so do not skimp on it.

For a 17 in finished cover with a 5 in overlap, each back piece is 8 1/2 + 2 1/2 + 1/2 + 1, which gives 12 1/2 in. A cover that is hemmed with a double fold of 1/2 in uses 1 in of height. Many patterns use a larger overlap, up to about 10 in, for a more secure hold. If you plan to use the pillow a lot, or if the fabric is slippery, make the overlap bigger, and the tool will change the back size to match.

How to sew an envelope cover

Hem one long edge on each back piece by folding it over 1/2 in twice, then stitching close to the fold. Lay the front right side up, then place one back on top with its right side down, lining up the raw edges and keeping the hem toward the center. Add the second back on the other side, so the two hems overlap in the middle. Pin all around.

Sew around all four sides with a 1/2 in seam, going back and forth over the overlap for extra strength. Clip the corners at an angle, being careful not to cut the stitching. Then turn the cover right side out through the opening, and push out the corners gently with a blunt tool. Give it a good press before you put in the insert. If you want to finish the seams, use a zigzag or pinking shears, as this will help the cover survive washing.

Using a zipper instead of an envelope

A zipper gives a smart, tailored look, with no opening on show, and this is a good choice for cushions that get a lot of use. Choose the closed option in the tool, and you will get a single back of the same size as your front. Allow an extra 1 in or so in the height if you are putting the zipper in a seam across the back, since you lose some to the seam allowance.

An invisible zipper in the bottom seam gives the cleanest result. Choose a zipper about 2 in shorter than the width of the cover. A regular zipper can work too, with a small flap sewn over it. A cover sewn fully closed, with no opening at all, is an option for a decorative pillow that you do not expect to wash. Hand stitch the gap after you put in the insert.

Choosing fabric and making a set

Medium-weight fabrics hold their shape best on a pillow. Cotton, linen, canvas and home decor fabrics all work well. A quilting cotton can make a good cover too, and a layer of fusible interfacing or batting behind it will add body. A large or quilted patchwork block makes a lovely pillow front, and you can make one from a leftover block. A fat quarter gives you an 18 in front if your insert is on the small side.

Sets of pillows look best with a mix of scale and pattern. A common combination is a larger print, a smaller print and a solid, in related colors. For an odd number of pillows, use sizes that step down, such as 22 in, 20 in and 18 in. To plan a set, add the covers one at a time in the calculator, and add the yardage together. Watch out for directional prints, which may not make the most efficient use of your fabric, and allow extra to match large motifs.

Common pillow cover mistakes to avoid

The most common mistake is measuring the insert by its label instead of with a tape. Inserts are often sold by a nominal size, and they can be a little larger or smaller than stated, so measure across the center before you cut. The second is using too small an overlap, which makes the back gape open and shows the insert. If your envelope looks loose, make the overlap larger on your next cover. Remember that a plump insert pushes the opening apart, so a fuller pillow needs a bigger overlap.

Fabric direction is a further trap. On an envelope back, the two pieces lie one above the other, so a directional print must be cut the right way up on each of them. It is easy to cut one upside down. Another mistake is skipping the reinforcing stitches at the ends of the overlap, since this is the spot that takes the most strain as you push the insert in. Sew back and forth a few times there. Prewash any fabric you plan to wash later too, or the cover may shrink and leave the insert squeezed tightly inside.