The throw size on Quiltler is 50 by 65 inches — sofa scale, not a mattress. Four inches of overhang makes a 58 by 73 inch back. That is the same pair of numbers already on how much fabric for a throw quilt, isolated here because searchers ask for backing alone. Two widths of 42 inch cloth seamed along the 73 inch side is the usual shop cut. Wide backing at 108 inches covers the 58 inch side as a single panel.
If the throw is pieced, design it in the throw quilt designer so the top yardage is not a rectangle guess. Binding for the same size is how much binding for a throw quilt. Fat-quarter shopping for a throw layout is how many fat quarters for a throw quilt.