Fabric Math Help Needed Please
 

I am trying to figure out how much fabric that I need to buy for a quilt backing that 115 X 115? The fabric is only 42".

I figure about 8 3/4 yards.

Using my Robert Kaufman app, it says 9 5/8 yds. That's with no allowance for overage. Allowing for 3" of overage on each side, 10 1/8 yds.

13 2/3 yards which is what my Quilter's Fabricalc came up with. I find this calc to be very generous with the backing.

To get the length you need add the extra fabric around the edge of your top, usually 4" for hand/domestic quilting. 6-8" LA.
DSM--- 115+4=119. Width: Divide 119/40=~3 (you will loose almost 2 inches in selvage/seams) for # repeats of fabric. Length: divide 119/36 = 3.33 or 3 1/3. Next multiply length x width. 3*3 1/3= 10 yards.
LA---115+8=123. W: 123/40=~3.02. L:123/36=~3.42. WxL=3.02x3.42= 10.33, I would round to 10 1/2 yds.

I calculated the same as giquilt, so I would definitely get at least 10.5 yards--and if the binding is going to be made out of the backing material, I would get an extra 3/4 yards for that.

I would figure 40" useable. That would mean 3 widths. Then you need 115 x 3 for the length. That is 345 inches, divided by 12 inches which gives you just under 30 feet divided by 3 feet to the yard is 10 yards. Is my math right??

You take the length you need and divide it by 36. I'd add 10 inches to your length to allow enough extra for quilting. So that's 125 divided by 36. That's 3.4. So I'd round up to 3.5 yards length. You'll need three widths of fabric to cover 115 inches but it won't be quite 10 inches extra.

The two base number are always 36 inches in a yard and 40 inches of useable fabric width.

I agreed with SissorQueen. I sell fabric almost everyday.

You will need 10.5 yards cut into 3 equals lengths of 3.5 yard each.