Quilt Diagonal Pieced Backing Calculator: John Flynn Method

Work out diagonal pieced backing before you cut. Enter quilt top size, overhang, fabric width and seam allowance, and see backing size, formula width and fabric length, with the formula in steps.

Calculate your diagonal backing

Quilt top width, across fabric width in

Quilt top length, along fabric length in

Overhang on each side in

Overhang is added on each side of the top, so backing is larger than the top. Four inches on each side is a common starting point for home quilting.

Fabric width

Seam allowance for joined strips in

Seam allowance is the diagonal seam allowance, pressed open. John Flynn uses 0.5 inch. Width in the formula equals backing width plus 2 times seam allowance.

This calculator works out single piece diagonal backing, by John Flynn's method, only. It is most efficient when backing width, including seam allowance, is 1.5 times fabric width or less. For wider backing, or for straight panels, use the Backing Calculator. Enter width as the side that will run across fabric width. If your quilt is wider than it is long, rotate it so width is the shorter side.

Your result

Enter your measurements, then calculate.

How the calculator works, by John Flynn's method

Step 1: Backing size needed

Backing width equals top width plus 2 times overhang. Backing length equals top length plus 2 times overhang. Overhang is added on each side, so it is counted twice.

Step 2: Width in the formula

Width in the formula equals backing width plus 2 times seam allowance. John Flynn uses a 0.5 inch seam, pressed open, so width in the formula is backing width plus 1 inch when seam allowance is 0.5 inch. This width is the width the diagonal method must achieve, including seam loss.

Step 3: Check the Flynn limit

John Flynn's single piece diagonal method is most efficient when width in the formula is 1.5 times fabric width or less. For 44 inch fabric, that limit is 66 inches. If width in the formula is greater than fabric width, but not greater than the limit, diagonal piecing can save fabric compared with straight piecing. If width in the formula is less than or equal to fabric width, diagonal piecing is not needed, because one panel already covers backing width. If width in the formula is greater than the limit, the calculator does not recommend diagonal piecing with one piece, and it recommends straight piecing instead.

Step 4: Fabric length needed, by Flynn's formula

Fabric length needed equals backing length times fabric width, divided by two times fabric width minus width in the formula. In symbols, LF equals LQ times WF divided by two times WF minus WQ, where LQ is backing length, WF is fabric width, and WQ is width in the formula. This is the same formula published by John Flynn, and used by Quilter's Paradise for its diagonally pieced backing calculator.

Step 5: Yardage

Exact yardage equals fabric length needed divided by 36 in inch mode. Yardage to buy is exact yardage rounded up to the nearest 1/8 yard. In centimetre mode, metres to buy are rounded up to the nearest 0.1 metre. The unit toggle changes all inputs, outputs and rounding together.

How diagonal piecing works, in brief

Cut the fabric piece to the calculated length. Fold the fabric diagonally, from one corner to the opposite corner, and cut along the fold, to get two triangles. Slide one triangle along the diagonal until the combined width equals width in the formula, allowing for seam allowance. Pin carefully, because the diagonal edge is on the bias and stretches easily. Sew the triangles together with the seam allowance entered above, and press the seam open. Trim the top and bottom ears to square up the backing to backing size needed. This is a summary, not a full diagonal backing tutorial. Practice on a small sample first, if you have never used the diagonal method.

Assumptions, shown next to the result

Fabric width is the nominal width entered above. Seam allowance is the diagonal seam allowance, pressed open. No extra is added for cutting mistakes or shrinkage, beyond rounding yardage up. This calculator is for single piece diagonal backing only. For backs wider than the Flynn limit, use straight piecing, or the two piece diagonal method, which is a different method with a different calculation, and is not covered by this calculator.

Yardage chart by quilt size

This chart uses the same formula as the calculator: 4 inch overhang on each side, 44 inch fabric, 0.5 inch seam allowance, yardage rounded up to the nearest 1/8 yard. Backing width including seam allowance must be 66 inches or less for diagonal piecing to be recommended. Where it is wider, the chart shows straight piecing yardage instead, for comparison.

Quilt size	Top size	Backing size needed	Diagonal yardage	Straight yardage, for comparison
Baby	36 by 48 in	44 by 56 in	1.63 yards	1.63 yards
Lap	50 by 60 in	58 by 68 in	2.88 yards	3.88 yards
Twin	68 by 88 in	76 by 96 in	Not recommended, width over limit	5.38 yards
Full	80 by 92 in	88 by 100 in	Not recommended, width over limit	8.38 yards
Queen	90 by 108 in	98 by 116 in	Not recommended, width over limit	9.75 yards
King	108 by 108 in	116 by 116 in	Not recommended, width over limit	9.75 yards

Chart yardage is a guide for the sizes shown. Diagonal yardage saves 1.00 yard for Lap in this chart, compared with straight piecing. For Baby, diagonal and straight yardage match in this chart, because strip count is the same. Your exact top size, overhang and fabric width may give a different result. Use the calculator above for your own measurements.

Worked example, checked against the calculator

Top size 50 by 60 inches, overhang 4 inches on each side, fabric width 44 inches, seam allowance 0.5 inch.

Backing size needed: width 50 plus 8 equals 58 inches. Length 60 plus 8 equals 68 inches.

Width in the formula: 58 plus 2 times 0.5 equals 59 inches. Flynn limit: 1.5 times 44 equals 66 inches. Width in the formula, 59 inches, is within the limit, so diagonal piecing is recommended.

Fabric length needed: 68 times 44 divided by two times 44 minus 59, which is 2992 divided by 29, equals 103.17 inches.

Exact yardage: 103.17 divided by 36 equals 2.87 yards. Yardage to buy, rounded up to the nearest 1/8 yard, equals 2.88 yards.

Straight piecing comparison for the same size: 2 panels, total length 136 inches, yardage to buy 3.88 yards. Diagonal piecing saves 1.00 yard in this example.

Flynn's published example, reproduced: quilt top 48 by 60 inches, backing wanted 55 by 66 inches, fabric width 44 inches. Fabric length needed: 66 times 44 divided by two times 44 minus 55, which is 2904 divided by 33, equals 88 inches. Exact yardage: 88 divided by 36 equals 2.44 yards. Yardage to buy, rounded up to the nearest 1/8 yard, equals 2.50 yards. This matches Flynn's published length of 88 inches.

Frequently asked questions

How do I calculate diagonally pieced quilt backing?

Add overhang to both sides of the quilt top to get backing size. Add seam allowance to backing width to get width in the formula. Fabric length needed equals backing length times fabric width, divided by two times fabric width minus width in the formula. Convert fabric length to yardage, rounded up to the nearest eighth yard.

When does the diagonal backing method work?

John Flynn's single piece diagonal method is most efficient when backing width, including seam allowance, is one and a half times fabric width or less. For 44 inch fabric, that means backing width including seam allowance of 66 inches or less. If backing width is wider, use straight piecing instead.

How much fabric do I need for diagonally pieced backing?

It depends on quilt size, overhang and fabric width, and on whether backing width is within the Flynn limit. Use the calculator above with your top size, overhang and fabric width. The result shows backing size, fabric length needed and yardage to buy, or a warning if diagonal piecing is not suitable.

What is the difference between diagonal and straight pieced backing?

Straight pieced backing joins rectangular panels side by side. Diagonal pieced backing cuts one fabric piece diagonally, slides the two triangles, and sews them together along the diagonal, so backing becomes wider than fabric width with less fabric, when backing width is within the Flynn limit.

Do I need extra backing for longarm quilting?

Yes, if a longarm quilter will quilt your top, ask how much overhang they need, and enter that number as overhang in the calculator. Do not guess, because longarm loading needs more room than home quilting.

What if my backing is wider than the Flynn limit?

Use straight piecing, with the Backing Calculator. The single piece diagonal method is not the most efficient method for wider backs, and for backs wider than twice fabric width, the formula does not work at all, because the denominator becomes zero or negative.

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