Measure around the quilt
Add up all four sides of the quilt, then add about 12" for joining the strips and turning corners. A 50" × 60" throw has a perimeter of 220", so you need about 232" of binding.
Count your strips
Most quilters cut binding 2 1/2" wide across the width of the fabric. After trimming selvages, each strip gives you about 40" of usable length.
Divide the length you need by 40 and round up. For the throw, 232" ÷ 40 = 5.8, so cut 6 strips.
Turn strips into yardage
Multiply the number of strips by the strip width: 6 × 2 1/2" = 15". Round up to the next quarter yard you can buy, which is 1/2 yard.
That half yard covers most throws. A queen needs closer to 3/4 yard.
Or skip the math
The Quilt Builder works out binding along with everything else, so the yardage list already includes it when you print your pattern.