Circle Skirt Calculator

Why a half circle needs a bigger radius

The waist opening of a circle skirt is an arc, and its length has to equal the waist measurement. For a full circle the arc is the entire circumference, so waist = 2*pi*r and r = waist / (2*pi). For a half circle the pattern only spans half a turn, so the same waist measurement has to be accommodated by an arc of pi*r, giving r = waist / pi — twice as large.

A quarter circle halves the arc again, so r = 2*waist / pi, four times the full-circle radius. The relationship runs the opposite way to intuition: less skirt means a bigger hole.

This is the single most common drafting error in a circle skirt, and it is unforgiving in one direction. A waist radius that is too large cannot be fixed; the skirt simply falls off.

A worked example

A 28 inch waist. Full circle: 28 / 6.283 = 4.46 inches. Half circle: 28 / 3.142 = 8.91 inches. Quarter circle: 56 / 3.142 = 17.83 inches.

For a 20 inch skirt length, the full circle's outer radius is 24.46 inches, so the pattern needs a square about 51 inches on a side once the hem allowance is added. Most fabric is 45 to 60 inches wide, which means a full circle skirt in anything under 54 inch fabric needs a seam — usually two half-circle panels joined at the sides.

Where the answer misleads

The waist is cut smaller than calculated, by the seam allowance, so the finished opening comes out at the measurement. The hem gets its allowance added in the other direction. Adding both to the same edge, or forgetting the waist one, are the two ways a correctly calculated skirt still comes out wrong.

A circle skirt also hangs unevenly by design. Parts of the hem fall on the true bias and drop over the first few days as the fabric relaxes, so a circle skirt should hang for at least 24 hours before the hem is levelled and cut. Hemming immediately produces a skirt that is longer at four points a week later.

Finally, the waist measurement should be where the skirt actually sits. A skirt worn on the hip needs the hip measurement, not the natural waist, and the difference is several inches of radius.