My quilt is 100 x 100. I was looking at the wide fabric for backing that is 108 inches wide. How do I calculate this?

A yard has 36 inches in it so the fabric will be 36x108 per yard. So to do a quilt that is 100x100 you would need 3 yards. That will give you a piece that is 108x108 inches total.

I would get 3 yds.

You would need at least 3.25 yards of the wide backing fabric. Personally, I'd get 3.5 yards.

Sometime a longarmer (if you were sending it out be quilted) requires 6" extra on all sides, but I think the extra 4"-5" inches/per side should be enough and workable.

On my King Size quilts which are in that same 108" range, I always purchase 3 1/2 yards of backing fabric. It's not that expensive to add that little bit on. I'd rather have a little extra than not enough.

Of course that is ONLY if you are buying the 108 inch fabric,if you are buying regular fabric then its different

great info thanks for clearing it up