Let's take a step back and take a good hard look at the matter and be completely honest with ourselves: Figuring and buying backing fabrics sucks. It does. There is a lot to consider for such a simple concept and it is unavoidable because quilts must have backing. So whether you're just beginning or you're just looking for a little refresher, here is how to make figuring and buying backing a little less blah.
Calculating your Backing Needs
Y1 and Y2 should measure roughly the same, give or take .5", and same for Z1 and Z2. Yes, it is important to measure both sides, because sometimes our quilts just don't come out square.
If the sides are not the same as one another, take the average by adding the two together and dividing by 2. Ex.) Y1 + Y2 = ___. ____/2 = Width measurement. Use same method for length.
For the sake of example, let's say this quilt measures 55" x 70"
2. Add for the Quilter
IF you are taking your quilt out to be quilted by a professional longarmer, you need to add a few extra inches all the way around so the quilt can attach to their machine. If you are not, skip this step if you want, though, it is helpful even if you are quilting the quilt yourself, to have a little extra around the edges.
Determine how much extra fabric you need around the quilt. Most quilters ask for 4" extra all the way around, but be sure to ask.
Double the amount needed around the edges to account for the extra on both sides, and add to the width and length. Let's say the example quilt needs 4" extra. 4 x 2 = 8. 55 + 8 = 63 and 70 + 8 = 78.
With the amount needed for quilting added in, the quilt's new size is 63" x 78"
3. Do the Math
Do the math for both types of backing fabric - regular width and wide width. When you go shopping for your backing it'll help you estimate cost and guide your decision. I'm only going to do an example for regular width (~45") fabric. Oh, and when I describe the width of the fabric, I'm describing the selvedge to selvedge measurement...I'm sure you knew that all ready but I thought I'd make that distinction :) Also, as fabric costs rise, the selvedge to selvedge measurement will begin to shrink. We're all ready seeing widths closer to ~40" than ~45". Be sure to check the width of your backing fabric (listed on the bolt) before calculating for the average. If you are unsure of where to find it, ask your local quilt shop for help.
Now, if your quilt measures less than 40" either wide or long, you only need to account for the remaining size measurement and buy the yardage, no further calculation needed. Let's say your quilt is 38" x 38". Well, 38" fits one width selvedge to selvedge, which averages ~45". So you only need to calculate yardage for the remaining 38", which is just oiver a yard. Your local store will probably only sell in particular yard increments, so a 1 1/8yds would fit the bill. But what if your quilt is larger than one selvedge to selvedge width?
The example quilt measures 63" x 78" and we are backing it with ~45" wide fabric. Well, neither the width nor the length fit in to ~45", so we know now that we will need 2 widths of the fabric to cover the back, like so:
With ~45", you have ~90" total to work with, so choose the side that best fits the 90" with the least waste and that determines how your backing will be pieced. Generally horizonal piecing, like the second graphic, consumes less yardage because the longest side is accounted for with the selvedge to selvedge length and not the purchased yardage length, but as the quilt grows longer than 90" it ceases to be as cost effective. As I'll discuss later, the math works differently with wide backing. With wide backing you need to make sure that either the width or the length (either one) fits inside 108" and then calculate yardage for the remaining measurement.
But back to the math. Our quilt measures 63" x 78" and we have decided to piece the backing horizontally, like in the second picture above. Well, our length, 78" is accounted for in the ~90" we get out of pieceing the two 45" wide pieces. Done! Now all we need to do is determine our width yardage. So, our quilt is 63" wide and one yard is 36" long.
Divide the width by one yard: 63/36 = 1.75yd.
Remember, we need TWO equal pieces for this backing to cover the length of the quilt, so multiply your answer by 2: 1.75 x 2 = 3.5yd.
To back this quilt, we need 3.5 yards of standard width backing. If we choose to go with wide width, we need only 1.75 yards (enough for the width). How easy was that?
Choosing your Backing Fabric
Now that you have your backing requirements in your hot little hands, it's time to choose the fabric. I'm not going to get into color choice and pattern choice becausr that is so individually determined that I couldn't possibly begin to give any real pointers outside of what I do. Some people want their backing to match perfectly with the binding and with the front of the quilt, and others don't mind if it doesn't match 100%. Some people like to piece their backings so it's like a reversable quilt. That is all up to you, so I'll stick with the technicality.
This is the part that sucks, because yes...unless you are doing a baby quilt or smaller, it is spendy. You can choose bewteen regular width and wide backings like I mentioned before. Here are the pros and cons of each:
Regular Width Backing
Pros: A wide variety of color and pattern to match your quilt - high quality pieces available - regular price and Clearance priced options widely available - cost effective for full sized quilts and smaller - cheaper per yard than wide width - with quilt shop quality fabrics, there is no need to test for colorfastness
Cons: Unless your heart is set on a piece this is not cost effective for large quilts as you may need more than two selvedge to selvedge widths to account for even the shortest side - can produce a lot of waste - if your quilt is large you will have a seam running down the back of your quilt - piecing your backing can be an inconvenience or cumbersome for some people to manage for reasons of space or physical ability - if you choose a directional print, more yardage is required than usual depending n the direction the print is running
Wide Width Backing
Pros: Less yardage is required because of the larger selvedge to selvedge measurement - most cost efficient option for large quilts - blender style color schemes complement quilts nicely - no need to piece backing
Cons: Are not available everywhere - limited range of colors, really basics only - lower quality greigh goods than regular width are used so they tend to be rougher - notably more expensive than regular width - if used on a non-queen/king quilt, there is a very significant amount of waste - colorfastness can be an issue with red pieces - seldom found on Clearance
You also may want a different fiber other than cotton, which is what I have been talking about. The math and measuring is all the same, but consider your function:
Summer quilts tend to do better with cotton backings, and winter quilts tend to be more suited towards flannel. Kids like soft things so flannel or minkee are great for those quilts, but maybe not so much for an heirloom quilt. As your quilter before using Minkee, because it stretches and sheds they may decline to quilt your quilt with that backing. If it needs to be durable and long lasting, first and second run fabrics like those found at Walmart and JoAnn's aren't your best bet.
Buying backing is the pits, so buy backings during sales. Even if you don't know what size you need, it's cheaper overall to shell out the discounted money to get several yards of a good general backing to have in your stash for a future project. The Cotton Patch has a Backing Sale on the 30th of every month (in-store only...apologies to non-local readers!) that is mentioned with rules in our newsletters so we try and help with that (because we hate buying backings too). Save scraps to piece a backing to reduce cost too!
Everyone has their own way of doing things, so you may find a figuring methodology that works for you better than this, but I hope that helped clear up some of the backing problems and make the math easier, for beginners and experienced quilters alike!