Quick answer: how to calculate diagonal backing
Backing width equals top width plus 2 times overhang, and backing length equals top length plus 2 times overhang. Width in the formula equals backing width plus 2 times seam allowance. Flynn limit equals 1.5 times fabric width. Diagonal piecing with one piece works only when width in the formula is greater than fabric width and no greater than the Flynn limit. Fabric length equals backing length times fabric width divided by 2 times fabric width minus width in the formula. Yardage to buy is fabric length divided by 36, rounded up to the nearest 1/8 yard.
Example: a 50 by 60 inch top, overhang 4 inches, fabric width 44 inches, seam 0.5 inch. Backing is 58 by 68 inches, width in the formula is 59 inches, Flynn limit is 66 inches, so diagonal piecing applies. Fabric length is 103.17 inches, exact yardage is 2.87 yards, yardage to buy is 2.88 yards.
Near-limit example: a 54 by 72 inch top, overhang 4 inches, fabric width 44 inches, seam 0.5 inch. Backing is 62 by 80 inches, width in the formula is 63 inches, Flynn limit is 66 inches, so diagonal piecing still applies, but only just. Fabric length is 140.80 inches, exact yardage is 3.91 yards, yardage to buy is 4.00 yards. If width in the formula were above 66 inches, the calculator would refuse diagonal piecing, and straight piecing or wide backing would be the right choice, not a larger diagonal number.
Fabric width here means the width you enter in the calculator, and for a standard bolt, enter usable width after removing selvages, not bolt width, or the Flynn limit will be overstated. The two-piece diagonal variant has no formula in the sources found, and this calculator does not implement it, so do not expect a two-piece result from this page.
If you only need a number, use the Diagonal Pieced Backing Calculator. If you want to understand why the limit exists, and when diagonal saves fabric, read on.
What diagonally pieced backing is
Diagonally pieced backing is backing made from one fabric piece, cut diagonally, slid wider, and sewn back together along the diagonal. Instead of joining rectangular panels side by side, you cut one piece into two triangles, shift one triangle along the diagonal, and sew them together. The backing becomes wider than the fabric width, with one diagonal seam across the back.
This method was devised by John Flynn, and it is most efficient when backing width is a little wider than fabric width, but not too much wider. When backing width is much wider than fabric width, straight piecing is usually better. When backing width already fits in fabric width, diagonal piecing is not needed at all.
When diagonal piecing saves fabric, and when it does not
| Backing width, including seam allowance | What to do |
|---|---|
| Less than or equal to fabric width | Use one panel, straight backing. Diagonal piecing is not needed. |
| More than fabric width, up to 1.5 times fabric width | Diagonal piecing, by John Flynn's method, can save fabric compared with straight piecing. |
| More than 1.5 times fabric width | Use straight piecing. Single piece diagonal piecing is not the most efficient method for wider backs. |
For 44 inch fabric, the Flynn limit is 66 inches, including seam allowance. For 60 inch fabric, the limit is 90 inches. For 108 inch wide backing fabric, the limit is 162 inches, which covers almost every quilt, but wide backing fabric is usually bought as a single panel anyway, so diagonal piecing is rarely needed with wide fabric.
Step by step: how to calculate diagonal backing
Step 1: Get 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. Overhang is for quilting and longarm loading, not for decoration.
Step 2: Get 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
Compare width in the formula with 1.5 times fabric width. If width in the formula is greater than fabric width, but not greater than the limit, diagonal piecing is suitable. If width in the formula is less than or equal to fabric width, use one panel. If width in the formula is greater than the limit, use straight piecing.
Step 4: Work out 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.
Why the denominator matters: as width in the formula gets closer to twice fabric width, the denominator gets smaller, and fabric length needed grows quickly. That is why the method stops being efficient well before twice fabric width, at 1.5 times fabric width.
Step 5: Work out 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.
Try it with the calculator: Use the Diagonal Pieced Backing Calculator with your own top size, overhang and fabric width. It uses the same steps as this guide, it checks the Flynn limit, and it shows backing size, fabric length needed and yardage together.
Worked example: Flynn's published example
Take a quilt top 48 by 60 inches, overhang 3 inches on each side, fabric width 44 inches, seam allowance 0.5 inch.
Backing size needed: width 48 plus 6 equals 54 inches. Length 60 plus 6 equals 66 inches.
Width in the formula: 54 plus 2 times 0.5 equals 55 inches. Flynn limit: 1.5 times 44 equals 66 inches. Width in the formula, 55 inches, is within the limit, so diagonal piecing is suitable.
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 for this example.
When diagonal piecing is not suitable: take a twin top 68 by 88 inches, overhang 4 inches on each side, fabric width 44 inches, seam allowance 0.5 inch. Backing size needed is 76 by 96 inches. Width in the formula is 77 inches, which is greater than the Flynn limit of 66 inches. For this size, use straight piecing. Straight piecing yardage for this size is 5.38 yards, from the Backing Calculator chart.
Diagonal compared with straight and wide backing, our own computed table
This table uses the same quilt for all three methods, a 50 by 60 inch top, overhang 4 inches, fabric width 44 inches, seam 0.5 inch, except wide backing, which uses 108 inch fabric. Straight yardage is computed from the Backing Calculator rule, diagonal yardage from the Flynn formula, and wide yardage from one panel, not copied from another site.
| Method | Seams | Yardage to buy | When it suits |
|---|---|---|---|
| Diagonal piecing, one piece | 1 diagonal seam | 2.88 yards | Backing wider than fabric width, but within the Flynn limit, and one seam is wanted |
| Straight piecing | 1 straight seam, 2 panels | 3.88 yards | Any width, including above the Flynn limit, and when a straight seam is preferred |
| Wide backing, 108 inch fabric | 0 seams, 1 panel | 2.00 yards | Backing fits in wide fabric width, and fabric choice and price suit wide backing |
Diagonal saves fabric compared with straight piecing in this example, 2.88 yards against 3.88 yards, but wide backing saves more, 2.00 yards, when wide fabric is available and affordable. Choose by fabric availability and price, not by method alone.
How to make sure your result is correct
- Backing size check. Backing width must equal top width plus 2 times overhang, and backing length must equal top length plus 2 times overhang.
- Width in formula check. Width in the formula must equal backing width plus 2 times seam allowance, not backing width alone, or the limit test will be wrong.
- Limit check. Width in the formula must be greater than fabric width, for diagonal to be needed, and no greater than 1.5 times fabric width, for diagonal to be recommended. Outside that range, use straight piecing or wide backing.
- Formula check. Fabric length must equal backing length times fabric width divided by 2 times fabric width minus width in the formula. If the denominator is zero or negative, the formula does not apply.
- Buy versus exact check. Buy yardage must never be below exact yardage. If it is, you rounded the wrong way.
- Usable width check. Enter usable fabric width after removing selvages, not bolt width, or the Flynn limit will be overstated and a too-wide back can be accepted by mistake.
How to cut and sew diagonal backing, in brief
- Cut the fabric piece to the calculated length, and press it flat.
- Fold the fabric diagonally, from one corner to the opposite corner, and cut along the fold, to get two triangles. A rotary cutter and a long ruler help here, and a large flat surface helps more.
- Slide one triangle along the diagonal until the combined width equals width in the formula, allowing for seam allowance. Measure through the centre to check backing length as well.
- Pin carefully, every few inches, because the diagonal edge is on the bias and stretches easily. A walking foot helps when sewing, because it feeds both layers evenly.
- Sew the triangles together with the seam allowance used in the calculation, and press the seam open for a smoother back.
- Trim the top and bottom ears to square up the backing to backing size needed, and check backing size before basting.
This is a summary, not a full diagonal backing tutorial. If you have never used the diagonal method, practice on a small sample first, with the same seam allowance you plan to use, and check that the finished sample measures what you expect, before cutting into your backing fabric.
Common mistakes with diagonal backing
- Using diagonal piecing for a very wide backing. Diagonal piecing is not efficient for backs wider than the Flynn limit. Use straight piecing for wider backs.
- Forgetting seam allowance in width in the formula. Width in the formula must include seam allowance, or the finished backing will be narrower than planned.
- Stretching the bias diagonal edge. The diagonal cut is on the bias, so it stretches easily. Handle it gently, pin often, and do not pull while sewing.
- Rounding fabric length down. Rounding down gives backing that is too short. Always round yardage up.
- Skipping the centre measurement. Check backing length through the centre after sliding the triangles, not only at the edges, because the diagonal shape can be misleading at the corners.
Calculate your own diagonal backing now
Use your top size, overhang and fabric width in the calculator, and check the result against the steps in this guide.
Open the Diagonal Pieced Backing CalculatorFrequently asked questions
No. John Flynn's single piece diagonal method is most efficient when backing width, including seam allowance, is 1.5 times fabric width or less. For wider backs, use straight piecing.
Use straight piecing, with the Backing Calculator. For backs wider than the limit, straight piecing is more efficient, and for backs wider than twice fabric width, the diagonal formula does not work at all, because the denominator becomes zero or negative.
The diagonal seam is on the back of the quilt, pressed open. It is visible on the back, like any backing seam, but it does not show on the front. If the back will be seen a lot, choose a backing print that does not draw attention to the seam, or place the seam where it will be less noticeable.
Usually you do not need to. If backing width, including seam allowance, already fits in wide fabric width, use one panel. Diagonal piecing is for standard width fabric that is a little too narrow for the backing you need.
Because the diagonal cut is on the bias, and bias stretches easily. Pin often, sew slowly, use a walking foot if you have one, and press the seam open gently, without pulling the edge out of shape.
Related guides and calculators
Next, read How to Calculate Quilt Backing for straight piecing, and How to Piece a Quilt Backing for joining panels. For the back itself, use the Diagonal Pieced Backing Calculator.