Extra quilt backing fabric
A reader writes...
How many extra inches of backing fabric do I need if I am going to stitch in the ditch?
The general answer if I don't know how I'll quilt the quilt
is to add about 6-8 inches to both the length and width of the quilt top and then cut the batting and backing to that size. So for a quilt top that is 60"x80", I would cut the batting and backing 66-68" x 86-88".
However, in your case since the only quilting you'll be adding is ditch quilting, I believe you could add a total of 4 inches to each dimension and be just fine...a bit less if the quilt is small like a table runner or small wall hanging. Ditch quilting won't 'take up' or 'draw in' as much of the quilt as, say, a tight stipple would.
You may want to do a line of stitching all the way around the perimeter of the quilt as shown in the picture below. This was a quilt my Mom was working on.
The red arrows point to the scant quarter inch edge stitches
Some quilters add this extra line of stitching, some don't. It's really a personal preference.
If you use it just make sure to stitch less than a quarter inch away from the edge of your quilt top so that the stitching will be hidden once you apply the binding and you won't have to rip it out.
For more on this subject, please visit my page Quilt Batting: How much extra do I need
Thank you for your question. I hope this information is helpful to you.
PS I do tend to cut 'extremely' generous batting and backings for my quilts.
But I didn't always.
What I've found over the years, is that if I cut a 'large enough' backing, then when it comes time to trim off the excess, I've still got a usable strip of fabric. For the scrap quilts I like to make, those 'trims' become a bonus source of fabric and a good reason to put something neat on the back...