Open Diagonal backing in the studio. The form on this page is that calculator.
Why a diagonal seam exists
Backing wants to be one piece, or as few seams as possible. When the quilt is only a little wider than the bolt, two straight panels waste a lot of the second width. John Flynn's method cuts one length on the diagonal and slides the two triangles until the pair is wide enough. You buy less cloth, and you still have a single seam.
The method has a limit. It is for a backing that is wider than the fabric and no more than about one and a half times that width. On 42 inch goods, that is a backing up to about 63 inches wide. Wider than that, the calculator tells you to buy straight panels. Even inside the limit, straight seams sometimes cost less, and the card shows both numbers so you can buy the smaller one.
How the Flynn length is calculated
Overhang is added on every side first. Four inches is a comfortable domestic-machine overhang. A longarmer often wants more, so type what your quilter asked for. One inch is then added to the width being stretched, for a half-inch seam pressed open. The formula solves for the length of fabric to cut. You cut that length from corner to corner, slide the triangles, sew, and trim the ears.
The seam is on the bias and it stretches. Pin it, sew with a walking foot if you have one, and press it open. When the diagonal method does not apply, use the straight backing calculator.
When the diagonal seam saves fabric
Overhang is included in the backing size. The last row is past one and a half widths, so the method stops.
| Quilt | Backing size | Fabric width | Diagonal | Straight seams |
|---|---|---|---|---|
| 36 × 52 in, 4 in overhang | 44 × 60 in | 42 in | 1 7/8 yards | 2 1/2 yards |
| 40 × 60 in, 4 in overhang | 48 × 68 in | 42 in | 2 3/8 yards | 2 3/4 yards |
| 48 × 60 in, 3 in overhang | 54 × 66 in | 44 in | 2 1/2 yards | 3 yards |
| 50 × 65 in, 4 in overhang | 58 × 73 in | 42 in | 3 1/2 yards (straight is cheaper) | 3 1/4 yards |
| 60 × 72 in, 4 in overhang | 68 × 80 in | 42 in | Not for this size | 3 7/8 yards |
How to use this calculator
- Enter the quilt top, then the overhang on each side. The backing is the quilt plus that overhang all the way around.
- Set the usable width of the backing fabric. Wide cloth, around 108 inches, often needs no seam at all, and this method is the wrong tool for it.
- Read both yardages. Buy the smaller one. If the card says a diagonal seam does not help, it has already chosen the straight panels.
Tips for a diagonal back
- Handle the bias edge gently. Stretching it while you sew makes a ridge you will feel through the quilt.
- Check the print. A directional fabric looks odd when the two halves of the back are rotated. A plain or all-over print is the easy choice.
- Measure the top you have, including any border that grew during piecing. The formula is only as good as the inches you type.
When the piece is part of a whole quilt. The back of a pieced quilt is still a rectangle. This page is that rectangle. The fabric calculator counts every piece in a design, then adds binding, backing, and batting. These pages answer one question at a time.