Straight-grain binding length and fabric for any edge.
Adds ~10\u2033 for joins and corners; strips cut across a 40\u2033 usable width.
Straight grain binding is cut in strips across the width of the fabric and joined to make a long length. To find how many strips you need, divide the total edge length by the usable width of the fabric, which is a little less than the full width because of the selvages. For 200 inches of edge and 42 inches of usable fabric, you need five strips.
Multiply the number of strips by the strip width to get the length of fabric to buy, and add a little extra for the ends. A strip width of 2.25 to 2.5 inches is common for quilt binding folded in half, which gives a narrow finished edge on the front and back.
Allow about ten inches in total for joining the ends and for mitering corners. Join strips with a diagonal seam, which spreads the bulk of the seam and makes it less visible when it is folded over the edge.
Straight grain binding is a good choice for edges that are mostly straight, such as quilts, table runners, and placemats, where it is stable and easy to sew. For curved edges, use bias binding, which stretches around the curve.