Now, I'm planning on a second (or third) post about backings, but this one is a quick one to help you figure out just how much yardage you need.
If you're having your quilt finished on a long-arm, usually the rule of thumb is you need at least 4"-6" of extra backing per side. Let me repeat that, 4"-6" PER SIDE. This is where I personally have the most issues when receiving backings to quilt. We need that extra fabric, it's not an option.
To make this as easy as possible, I usually figure on adding 10" to both the length and width of the quilt to find out the size I'll need. (I'll expain later why exactly you need so much).
So, how do you figure it out?
Seams can also go vertical or horizontal and if the quilt is large enough, it may require more than one seam.
To figure out how much you need, start with the quilt size. If you're not exactly sure, always round up, never down. As I mentioned before, I usually add 10" to both the length and the width to figure out the minimum backing size I need:
Quilt is 53" x 70"
Figure backing at 63" x 80"
Now, I always start with the larger number first to see if I can fit it within the width of fabric. Most fabrics come 44"-45" or so they say. By the time you add in the seam and factor out the selvages, I use 40" as a usable width which leaves some wiggle room.
So the larger number will fit within the larger number twice (80/40=2) So I need two pieces of backing. Still with me?
Now how wide do both of those pieces need to be? We figured the backing needs to be 63" wide at least. This happens to be 1 3/4 yards wide. SO I will need two pieces 1 3/4 yards. Times two pieces I need 3 1/2 yards of backing.
But what is the exception to the rule? Say you have a quilt 70" x 90". Figure the backing then at 80" x 100". The width now is close to the 80" mark and you can get two widths of fabric to cover this span. This time you can run the seam vertical and save some yardage. here's how:
Two widths of fabric for the width, now figure the length of the quilt: 100" you'd need around 2 7/8 yards or you can round up to three to make it easier. Times two pieces equals 6 yards of backing. (or 5 3/4 if you didn't round up)
If you wanted to piece horizontally, you'd need 3 widths of fabric to cover the length. to figure out the width, you'd need three pieces 80" wide. Now that's 2 1/4 yards per piece. Times three pieces equals 6 3/4 yards. That's an extra 3/4 of a yard and an additional seam that would need pieced. Yuck!
If all of this math gives you a headache, first off I'm sorry, and any quilt shop should be able to help you figure out exactly how much you need. I will be back soon with the reasons why you need that much backing and how to properly prep your backing for quilting. Stay tuned!