you need to know the finished size of your quilt. And for those who are math challenged, use some graph paper. For the backing, use 4 squares for a yard (each square would represent 9" or a quarter yard). Mark off a 108" wide (3 yards) and draw a long way down (open ended). Use a pen or a sharpie and you can keep reusing this template. The with a pencil, mark in the size of your finished quilt. This will let you see how much you will need. You can also pencil in what you would need for binding if you are using it for that also. This is a bit cumbersome, but many people have an easier time when they can see it actually written down.

If your quilt is 71 by 71 and you are buying wide back at 108...
Buy 80 inches , which is exactly 2 1/4 yards....which gives you the extra 4 inches top and bottom if you are longarming...
you will also have over 30 inches by 80 inches leftover for one of the sides, which will cover binding...

if if you are buying 44 inch...buy 2 lengths...joining the two with one seam...this will give you at least 80 inches of width...so extra for longarming...that works out to 4 1/2 yards of backing only for the backing...
then add about 1/2 for binding...and more specifically...
you need 300 inches of binding...roughly...so that works out to 8 strips..if it is 2.5 inches wide you are using...you need 20 inches...just over 1/2 yard...as 18 inches is a yard....maybe best to get a little more in case of booboos... Hope this helps...

Ok I misunderstood...save the info I just sent for backing

if the pattern says 7 yards for BACKGROUND that works out to 9072 square inches...36 by 36 by 7

in the wide back one yard is 36 by 108 which I think works out to 3888 sq inches...so 9072 divided by 3888 is 2.3 yards of the wide fabric...I think this is correct...it works out to the same square "inchage"....I just made up a word I like...

Wow! All of you are amazing!!! Really good info for a math challenged person. Several suggestions for converting narrow to wider fabric ( and vice versa) make a lot of sense to me & I will be printing everything out & keeping near to hand as I progress to various projects. I did go to my local shop yesterday & ended up with 8 yds. of the narrower fabric. (The extra yard was an end of bolt reduced price.) They didn't have the wider that I was told they had in the black that I needed & as I wanted to get going....
I hate when my choices are reduced!!! Anyway- Thank you all so very much for your suggestions & guidance.
sewverybusy1

Calculating yardage for wide backing
 

If she was looking to change the current 44" wide to 36" wide, you'd be right. However, her preferred backing is 108" wide. She needs the length of her quilt plus 10-12" for insurance.

I have an app loaded on my phone from Robt. Kaufmann called "The Quilters Little Helper". I have used it in all kind of measurement issues such as you ask about. Try it! Comes in handy when you are sho p ping! Good luck!