Help! Not enough backing fabric!
 

I normally don't purchase fabric.. the only fabric I buy is backing and I usually buy the very wide fabric, and when I don't, my fabric store figures out how much i need and that's what I buy and they are always spot on!
However, I was at a Discount Foam and Fabric store and found some $1 fabric that I thought "would do" for this big quilt. They didn't know how to figure out how much I needed and neither did I, so I bought what I thought was enough. Obviously, it's not! If I cut it into 3 pieces, and sew them together, it's too short, but there is excess on the sides.

I'm not in love with the quilt top or the backing fabric, so I kinda want to use it for this quilt. It's a purple/pink with little flowers on it and the top is pink and white, so they don't even really match. I don't think I can do the diagonal pieced back method, since this quilt top is so wide.

Other than taking a row off the quilt top (I'm willing, but I'd like to see if there's anything else I can do), is there any easy way to make it work without looking too bad?

How much did you buy and what is the size of quilt? That will help us to know if there is a fix with the fabric you have.
See if you could cut one of the three pieces length wise and add it to the bottom to make it long enough. I have done that before.

I would piece the back with that fabric and some of the fabric used in the front to make it big enough. It is ok to use more than one fabric on the back.

if you have extra width why not cut the fabric again, turn & add to top or bottom?
- cut the length of fabric into the width you need it to be- then one left over piece you may need to cut in half (at the fold) & stitch the 2 short ends together to make it the correct width to add- so your seams are horizontal instead of vertical???with a small print chances are the seams will not show when the quilt is quilted.

You said, "If I cut it into 3 pieces, and sew them together, it's too short, but there is excess on the sides."
so the backing is wide enough. What you can do is add a strip of another fabric between each of the 3 big wide pieces and the backing should be long enough.
wide piece #1
strip of another fabric
wide piece #2
strip of another fabric
wide piece #3

What are the sizes of quilt and fabric? If you have excess width, I bet you can use the diagonal method. Cut the fabric in half and sew together into an 80" wide piece. Then use that for your diagonal backing calculations.

Ok, measurements:

Top is 115x100 and the backing is 7 yds, 21 inches x 46 inches wide - I haven't cut into it yet

And I have no fabric from the top of the quilt. It was given to me. And as I said, they don't really go together

You have enough...just. Cut two full length pieces, then cut the remainder into thirds, Should be about 14" x 43" each, but cut actual thirds and make sure the grain runs the same way as the full strips before you cut. Seam the short sides to make the third strip and put that one in the middle between the two long ones. Measure twice, or more, there's not much wiggle room once you figure in a bit extra all around for quilting.

You'll have two full length vertical seams and two short horizontal seams in the center panel...looking kind of like a ladder.

This will work but may sound confusing. Maybe you should draw a diagram. I only gave you a bit on each side, but you might be able to work out more once you see where I'm going. Cut two strips 101.5 inches the full width of fabric. This should leave you with one piece that is full width and 70" long. BTW, my method doesn't end up with everything going the same direction. Then cut this third piece the full 70" by 27". That leaves a 4th piece that is 70" by about 18". This one can be cut in two 35" by 18" pieces. Sew these two back together to form a 35" by about 35" square. That square should go onto the end of the third piece. That should give you 3 strips to sew together that are 101.5" wide (you'll have to trim the square to match) and will cover the 100" part. Got it?

And you can always run a stripe of a different fabric inserted in this fabric. I do that all the time.