How to use the pillow cover size calculator
- Enter the width and height of the pillow insert in inches.
- Set how much smaller the cover should be than the insert. A cover a little smaller than the insert looks fuller. Use 0 for the same size.
- Set the overlap of the two back panels. Open More options for the hem at the opening, the seam allowance and the fabric width.
- Cut one front and two backs to the sizes shown. The page also gives the fabric to buy.
Formula
finished cover = insert - reduction
front = finished size + 2 x seam on each direction
each back width = (finished width + overlap) / 2 + seam + 2 x hem, height as the front
check: 2 x (back width - seam - 2 x hem) - overlap = finished width
The hem is folded twice, so it uses twice its finished width in fabric. Each back panel has a seam on its outer edge and a hemmed edge at the opening, and the two hemmed edges overlap by the overlap amount. The fabric is packed in rows across its width, as drawn and turned, and rounded up to the next eighth of a yard.
An 18 inch insert in a 17 inch cover
For an 18 by 18 inch insert, a reduction of 1, an overlap of 4, a hem of 0.5 and a seam of 0.5, the finished cover is 17 by 17. Cut the front 18 by 18 and two backs 12 by 18, because (17 + 4) / 2 + 0.5 + 2 x 0.5 = 12. The check works: 2 x (12 - 0.5 - 1) - 4 = 17 inches across.
On 54 inch fabric the three pieces fit in one row, 18 + 12 + 12 = 42 inches wide, so the length is 18 inches and about 0.5 yard. A 20 by 12 inch insert with a reduction of 2 and an overlap of 5 on 44 inch fabric gives a front of 19 by 11 and backs of 13 by 11, and turning the pieces needs only 0.625 yard.
Why the back is two pieces
An envelope back lets you take the insert out for washing without a zipper or buttons. The two back panels overlap in the middle like the flaps of an envelope, and the insert is held in by the overlap. A longer overlap holds the insert more firmly, and a short one is easier to stuff but can gape.
The hem on each panel is folded twice, which hides the raw edge. Sew the panels to the front with right sides together, turn the cover right side out through the opening, and press.
Fabric and extras
The yardage covers only the three pieces. Add extra for a directional print, pattern matching, piping or shrinkage if you wash the fabric first. For other sewing sizes, the Fabric Yardage Calculator estimates larger projects, the Hem Allowance Calculator works out turned-up edges and the Gathering Fabric Fullness Calculator sizes ruffles.
More layout tools
For other layouts and cutting sizes, try the Regular Polygon Calculator, Ellipse String Layout Calculator and Dovetail Layout Calculator.
Assumptions and limits
- The cover is rectangular, with a seam all around, and the insert is stuffed in after sewing.
- The overlap runs across the width. For a taller cover, swap width and height first.
- All sizes are in inches. Defaults for the reduction, overlap, hem, seam and fabric width are examples.
- The fabric layout packs the three pieces by rows and does not match prints.
Frequently asked questions
How much smaller should the cover be than the insert?
It is a matter of taste. Many people make the cover an inch or two smaller than the insert for a fuller look, and the same size for a softer one. Use the field to try both.
What is a good overlap?
Enough to hold the insert in, usually a few inches. The default is an example. Too little overlap can let the insert show through, and too much wastes fabric.
Why is the back piece width not simply half the front?
Each back panel also carries the overlap, the hem folded twice and a seam on its outer edge. The check line on the page shows that the two panels still make the finished width.
Can I make a rectangular lumbar cover?
Yes. Enter the different width and height. If the overlap would need to run along the long side instead, swap the two numbers.