Backing fabric calculations?
 

I need to calculate how much backing fabric I need for a quilt size 74x87 using 104" wide fabric?

According to the Robert Kaufman app, you will need 2 2/3 yard and that is giving you 4 inches extra on each side

I would use the 104" as the longer length of the quilt. Using that, I would get 2.25 yards giving you 81"x104". That would give you 3.5" on either side for your longarmer to hook on to. Of course, if the fabric is directional, then you would need the 2 2/3 yards for a 96"x104" backing

I'm with Krafty14. I wouldn't try to piece it to use less fabric. This will just give you some scrap fabric to use elsewhere.

I would get three yards - a few inches extra to allow for shrinkage (if you wash your fabric before using it)

But then, I'm a believer in "It's better to have it and not need it, than to need it and not have it"

90 inches is 2.5 yards - and I would allow an extra 1/2 yard to allow for shrinkage and for the long arm person (if you do the LA route)