Learn how to make diagonal quilt backing! Use our diagonal backing calculator and sew the magic diagonally pieced backing for your quilt!
So youโve finished a lovely quilt top and now itโs time to make backing and baste your quilt. Yaaay!
No matter what quilt backing fabric youโre using, itโs very likely that youโll need to piece quilt backing one way or another.
Thatโs because (with the exception of very small quilts – think baby quilt sizes) the width of a quilt is usually wider than the width of backing fabric (measured from selvage to selvage).
There are many different ways you can piece quilt backing. Three of our go-tos include:
- The โbasicโ quilt backing calculation (which you can do for free with the free backing calculator in Quilt Geek)
- The โfabric-savingโ quilt backing calculation featured in Quilt Geek, our quilting calculator app
- Sometimes we also like to get a bit more creative with a modern pieced quilt backing
Well, today weโre looking into yet another method – the diagonally pieced quilt backing or diagonal backing for short.
Weโll look into how to calculate diagonally pieced quilt backing and of course, how to make diagonal backing. I.e. how to sew the pieces together.
This is one of those magic tricks in quilting that Iโm sure youโre going to love!
What is Diagonal Quilt Backing?
As the name suggests, diagonally pieced quilt backing is backing that has been sewn together along the diagonal.
The whole point of making diagonal quilt backing is to get more width from your fabric. If youโre using backing fabric with a WOF of 42โ and need backing that is 60โ wide – diagonal backing is one way to achieve this.
(I’ll show you HOW in a minute.)
When to Use Diagonally Piced Backing?
Although diagonal quilt backing is – as youโll soon see – a great method, itโs not suitable for any project.
It all depends on the dimensions of the quilt and the width of backing fabric youโre working with.
In general, the math behind all of this works when the required width of quilt backing is no more than 1.5 x width of fabric (WOF).
For example, if youโre using fabric with a WOF of 42โ this method will work if the required backing width is no more than 42 x 1.5 = 63โ.
The Logic Behind Diagonal Backing
This is one of those things where a picture is worth a thousand words, so Iโll explain this visually. First, weโll look into the logic of how the diagonal quilt backing works. Itโs actually easier than you probably think.
Start with a piece of fabric (weโll talk about the required dimensions in a bit) and cut it along the diagonal.

Then, shift the pieces to get more width as shown in the image.

Pretty simple, right? It really is!
The trickier thing here is getting the quilt math right and figuring out how much backing fabric (yardage) you need for your quilt. But weโve got a solution for that!
How to Calculate Diagonal Quilt Backing?
Here at DTQ, we do everything we can to make quilt math easier for you! That’s why we created Quilt Geek, the quilting calculator app.
In the upcoming update, Quilt Geek will also include a Diagonal Quilt backing calculator!
Until then, you can use the manual method of calculating diagonal backing.
Option A: Use the diagonal backing calculator in Quilt Geek
COMING SOON to Quilt Geek!
Option B: Calculate diagonal backing manually
The other option is to do the diagonal quilt backing math yourself. I am not going to explain how exactly we came up with this formula. There are other formulas youโll find on the www – the ones weโve tested produce similar results – but this one here is the only one that we know for sure gives you the results we need. (It did take some head-scratching to figure it out, I am not going to lie.)
First, youโll need to calculate the required quilt backing dimensions. You might think this is the same as the dimensions of your quilt, but itโs not. You need some extra fabric (overage) for quilting, but also for the construction i.e. sewing the triangular pieces together.
To get the required backing width (RBW) use the formula:
RBW = quilt width + overage (on both sides) + 1 inch (for seam allowance)
To get the required backing length (RBL) use the formula:
RBL = quilt length + overage (on both sides) + 1 inch (for construction)
Now that you have RBW and RBL you can calculate the required yardage (Y):
Y = ( RBL x WOF ) : (2 x WOF – RBW)
This is the number in inches. To convert it into yards, divide by 36 and round up to the nearest โ yards.
How to Sew Diagonally Pieced Backing?
Now that we have everything calculated, we can finally sew diagonal backing for our quilts.
Step 1: Cut the required yardage
First, make sure youโre starting off with the yardage you calculated following the directions above. You want to be working with a rectangle (so check for wonky sides etc.).
Step 2: Cut along the diagonal
Now we need to cut along the diagonal. You can do this by drawing a line from the top left corner to the bottom right corner and cutting along the line with a rotary cutter or scissors. This will only work if you have a very large surface to work on (the floor works well, too).
The other option is to fold the fabric along the diagonal and cut using fabric scissors.
Both of these can be a bit tricky, so take your time and go slow. The cut will be โhiddenโ in the seam so donโt worry if itโs not 100% perfect. But do your best!
Youโll get two triangular pieces.
Step 3: Shift the triangles
Now it’s time to shift the two triangles. Remember the diagrams in the beginning? Shift the right triangle downwards along the diagonal until you get the required backing width (RBW) if measured across both triangles.
Note: See the RBW formula in the calculations above.
Step 4: Pin, pin, pin
Fold the right triangle over to the left triangle along the diagonal. Pin the seam in place. We recommend pinning every couple of inches to make sure everything stays in place. Because this is a bias edge, fabric is very prone to distortion and pinning helps a ton.
Step 5: Sew
Sew the backing seam with a ยฝโ seam allowance. Go slow to prevent distortion. You might even want to use a walking foot to make sure both layers feed evenly through the machine.
Press the seam open.
Step 6: Cut away the โearsโ
Youโll be left with two small triangles – one at the top and one at the bottom. Align a ruler with the edge of the backing and cut the excess triangle away.
And thatโs it! You can now used your diagonal backing as you would any other type of backing. Baste your quilt sandwich, choose your preferred quilting motif, and get quilting!
Why use Diagonal Backing
After all this, you may be wondering why you would even use diagonally pieced backing. Honestly, itโs a matter of preference, but there are cases when diagonal quilt backing is the fastest and most fabric-saving way to piece your backing.
Because youโre just shifting the two pieces of fabric (and not rotating them), it also means that the pattern on the backing fabric runs in the same direction all over the quilt back.
Another fun โperkโ of using diagonally pieced backing is the fact that it reduces bulk around the backing seam. Usually, with traditionally pieced backing, youโll have seams running vertically or horizontally which means they might bulk up when the quilts are folded. With the diagonal quilt backing seam, the bulkiness is distributed, so it doesnโt happen (unless you like to fold your quilts diagonally, but I donโt think many people do thatโฆ)
And finally, if using busy prints, the diagonal seam is usually almost invisible!
There you go! You now know how to calculate diagonal quilt backing and how to sew diagonally pieced backing for your quilt! What do you think? Is this something you might try? Let us know in the comments below!