A worked answer
A king top at 110 by 95 inches needs overhang on the back. Four inches each side is the default here — change it in the card if your longarmer asks for more.
With 4 inches of overhang the back is 118" × 103". On 42 inch fabric that is 8 5/8 yards. On 108 inch wide backing it is 3 3/8 yards.
The calculator tries both ways of laying panels and keeps the cheaper cut. Seam 3 widths across the length of the quilt. See also how much fabric for a king quilt for binding and batting in the same size, and the general backing calculator for any inches.
A 110 by 95 inch top with 4 inches of overhang needs a back about 118" × 103". On 42 inch fabric that is 3 panels (8 5/8 yards). Wide 108 inch backing is usually 3 3/8 yards.
Wide backing removes seams when one panel covers the quilt. Compare the two yardages on this page before you buy.