Bias Binding Calculator

How a square becomes yards of binding

The continuous bias method sews a square into a tube with an offset, then cuts a single spiral strip round it. Because the whole square is consumed by the spiral, the length is simply the square's area divided by the strip width.

That makes the yield scale with the square of the side. Doubling the square from 18 to 36 inches does not double the binding, it quadruples it: 3.6 yards becomes 14.4. It also means narrower strips give proportionally more — the same 18 inch square yields 4.5 yards at 2 inches wide rather than 3.6 at 2.5.

In practice the joins take a little, so treat the figure as an upper bound and cut a slightly larger square than the arithmetic demands.

A worked example

An 18 inch square is 324 square inches. At a 2.5 inch strip width that is 129.6 inches of binding, or 3.6 yards — enough to bind a 30 inch square cushion with a foot to spare.

Working backwards is more useful. To bind a 60 by 80 quilt you need about 292 inches at 2.5 inches wide, so the square must be at least the square root of 292 times 2.5, which is 27 inches. A 28 inch square gives a comfortable margin.

Where the answer misleads

Bias binding stretches, which is the reason to use it and also a hazard. It will happily ease round an outer curve and can be persuaded round an inner one, but it will also stretch out of shape while you handle it, leaving a wavy edge. Handling it as little as possible and pressing without dragging both help.

Straight-grain binding is fine, cheaper and more stable on anything with square corners, which includes most quilts. Bias is needed for curves and wears slightly better on an edge that gets handled, because the fold does not fall repeatedly on the same threads.

The strip width depends on the fold. Double-fold binding is cut at four times the finished width; single-fold at twice. Cutting for one and sewing the other is a common and irreversible mistake.