Size of fabric square to cut for continuous bias binding.
A square of this size yields roughly the bias length needed.
Continuous bias binding is made from a square of fabric cut on the diagonal, which gives a long strip without many seams. The total length of strip you can make equals the area of the square divided by the width of the strip. Enter the length of edge to bind and the strip width to find the size of square to cut.
Add some extra length for joining the ends and for mitering the corners, usually around ten inches or so. Bias strips stretch, which lets them curve smoothly around necklines, armholes, and rounded edges, so they work well where straight grain binding would pucker. Quilt binding is often cut on the straight grain instead, since the edges are mostly straight.
Press the strip carefully before you sew, being gentle so it does not stretch out of shape. Cut accurately, since a slightly off square gives crooked strips, and test the finished width of the folded binding on a scrap so it covers the seam the way you want.
For example, to bind 120 inches of edge with strips 2.5 inches wide, you need 300 square inches of fabric, so a square about 17.3 inches on each side is enough. Round up to 17.5 inches and use the spare for joining.