Start with the quilt size
Enter the finished width and height of your quilt, then choose your strip width and usable fabric width. Usable width is what remains after you exclude the selvages and any damaged edges. The calculator adds a tail for joining the two ends of the binding; adjust it to suit the finishing method you use.
Allow for the strip joins
Each join uses some of the strip length. Here, a diagonal join uses one strip width, while a straight join uses two seam allowances. Choose the join you plan to sew. The result shows the length left after those joins, so you can check that the assembled binding will reach all the way around your quilt.
A 60 × 80-inch quilt, worked out
The perimeter is 280 inches. Add a 12-inch finishing tail and you need 292 inches of binding. With 40 inches of usable fabric width, 2½-inch strips and diagonal joins, eight strips give you 302½ inches of joined binding. Those strips take 20 inches of fabric. Rounded up to the nearest ⅛ yard, that's ⅝ yard to buy.
Check the amount before you shop
Extra fabric and purchase rounding are shown separately. Add extra if you want room for a practice join or a cutting mistake, then choose the increment your shop sells. This calculator plans straight-grain strips. If your quilt has curved edges and needs bias binding, you'll need a separate bias-cutting plan.