Measure the quilt top first
Lay the finished top flat and measure through the middle in both directions. Edges stretch as you handle them, so the center measurement is the one to use. Piecing can also make a top a little bigger or smaller than the pattern says, so work from your own numbers.
Standard or wide backing
Quilting cotton is sold 42 to 44 inches wide. Once the selvages are trimmed and 4 inches of extra backing is added on each side, any quilt top wider than about 32 inches needs the backing seamed from two or more pieces.
Wide backing fabric comes in 90, 108 and 118 inch widths. A 108 inch wide back covers a quilt top up to about 98 inches wide in a single piece, which is why it is popular for queen and king quilts. The quilt calculator compares both options for you.
Plan binding with the rest of the fabric
Binding usually needs well under a yard, even for a king quilt. Buying it at the same time as the backing makes it easy to match colors, and the calculator gives the strip count and yardage together.