The Mathemagical World of Quilting - Part Two

Welcome back for another exciting instalment of The Mathemagical World of Quilting. Last time we discussed the inch vs centimetre debacle and how to estimate the size of a very basic quilt top (The Mathemagical World of Quilting - Part One). 

This time, we will take a look at estimating backing and binding fabric requirements. 

Note: The methods and calculations I use below are simply to get you thinking more about how to use fabric sizes to estimate your needs. You may use different techniques for backing or binding and that's fine. This is just a guide to demonstrate the maths.

We will use our Beginner Quilt Along quilt for this example. The quilt top is approximately 36'' x 45'' (rounded to whole numbers).

Before we dive into the backing fabric, I'll quickly mention the batting. This should be two or more inches bigger, on all four sides, than the quilt top. This is to account for any movement during quilting. You may prefer a larger excess, but, I find this works for the size of quilt in our example.  

In our case, the batting would be 40'' x 49''. Batting will come either pre-cut, with measurements, or in yardage. Often it is extra wide and will fit a reasonable sized quilt top. If your quilt is bigger than the batting, then you may need to do a little of the jiggery pokery discussed below in Backing Fabric.

Backing Fabric
Your backing fabric should be a few inches larger than the batting, on all four sides. For a quilt this size, I make the backing fabric approximately two inches bigger, on each side, than the batting. Again, you may prefer a bigger excess, but I find this adequate.

If our batting is 40'' x 49'' the backing fabric should be at least 44'' x 53''.

How do we buy fabric for this? Well, in this instance it is quite easy. Assuming quilting fabric is 44'' wide, you would simply need to purchase the 53'' in length to cover the quilt top. If you remember the previous post, one yard equals 36''. This means that 53'' would be 1.5 yards, rounded to the nearest quarter of a yard. You might decide this is cutting it a bit fine and an extra quarter yard wouldn't hurt, but that's really up to you. Everyone has different allowances that they like to work with and you may find yours are much bigger or smaller than the next quilter.

That was pretty easy right? But what happens if the quilt top doesn't magically fit along the width of fabric? Let's say, our backing requirement was actually 80'' x 53''.

You could just go and buy wider fabric that is designed for just such an occasion, but let's assume you have your heart set on a good old 44'' wide print. How do you make it fit? The same way you get anything to fit in quilting...cut it.

Your fabric is 44'' wide, so two widths of fabric, side by side, would cover the width you need (2 x 44'' = 88''- minus seam allowance...you only need 80'').  This means that you need two times whatever length you're after. In our case, the quilt top requires 1.5 yards in length, so you would need to double this and purchase 3 yards.  You then just need to cut the fabric, at 1.5 yards, and sew it along the long edge. Instead of a piece 44'' x 3 yards, you have a piece 88'' (minus seam allowance) x 1.5 yards. Perfect.

There are other ways of working the maths or you may find you don't want a seam directly in the middle of the quilt. That's all something that you will need to think about at the time and adjust your calculations accordingly. This is purely to get you thinking a bit more about how to use fabric sizes to estimate your needs. I'm not necessarily saying that this is the best way to sew your backing.

Binding Fabric
Don't worry, this part is easy. The binding goes around the edge of the quilt, so it needs to be the distance around the quilt plus a generous amount extra to cater for mitred corners and general fiddling.

Our quilt is 36'' x 45''. The binding length would be 2 x (36'' + 45'') = 162'' + generous excess. I will add the excess later.

If our width of fabric is 44'' then we need to figure out how many strips of binding we need. 162''/44'' = 3.68. This means you need at least four strips. In this case, rounding up to four probably gives enough excess to work with but I like to add another strip just in case. You always want to factor in cutting or sewing errors. That takes us to five strips.

Now you just need to figure out how much length of fabric to buy. This is the easy part. It is just the number of strips multiplied by the width of the strip. In our case 5 x 2.5''= 12.5''. We need at least 12.5'' of fabric. You could probably round down to 12'' and just purchase 1/3 yard, but I always feel safer getting extra and tend to bump it up to the 1/2 yard. I would hate to reach this final step of my quilt and suddenly run out of fabric because I slip with the rotary cutter or something. Anything extra just goes in the stash, so it's not really wasted. You may feel differently though and not be quite so generous in your estimates.

There you have it. Now you have a better idea of how to estimate your backing and binding requirements. I hope this will be useful in your future quilting.