Allow room around the top
Backing is prepared larger than the quilt top so there is extra fabric around the edges during quilting. Use the amount your quilter or quilting method requires.
For example, a 60 × 72 inch top with 4 inches extra on every side needs a backing at least 68 × 80 inches: add 8 inches to each dimension. The extra is trimmed away when finishing.
One piece or joined panels
You can use a single piece of sufficiently wide fabric or join narrower panels. Joined panels need a seam allowance, and their layout depends on the fabric’s usable width.
The backing calculator compares straight-panel layouts. Allow separately for washing, squaring up, or matching a directional print. Binding is the edge finish and needs its own fabric calculation.