How this calculator works
A circle skirt is a fraction of a circle with a smaller circle removed for the waist. The waist seam is an arc of the same fraction, so the radius of the waist circle is the waist divided by the fraction of the full circle. A full circle has the whole 2 pi of circumference available, a half circle has half of it, and so on. The skirt length is added to that radius, and the outer arc is the hem. The fabric piece is the rectangle that encloses the pattern: twice the outer radius for a full circle, and a smaller box for a half or quarter circle. When the box is wider than the fabric, the pattern needs more than one width.
Formula
waist radius = waist / (2 x pi x fraction), with fraction 1 for full, 0.75 for three-quarter, 0.5 for half and 0.25 for quarter. outer radius = waist radius + skirt length. hem = 2 x pi x fraction x outer radius. fabric length = pieces x box height, where pieces = box width / fabric width rounded up.
Worked example
A 70 cm waist in a full circle skirt has a waist radius of 70 / (2 x pi) = 11.1 cm. With a 60 cm skirt and 2 cm allowance, the outer radius is 73.1 cm and the hem is 459 cm. The pattern needs a 146.3 cm square. On 150 cm fabric it fits in one width, so the fabric length is 146.3 cm.
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Frequently asked questions
Why is the hem so much longer than the waist?
Every point of the hem is the outer radius away from the centre, so the circumference grows with the distance from the waist. A full circle skirt adds a full ring of fabric, and the hem can be several times the waist measure.
How do I add seam allowance?
The waist seam is a circle, so the simplest approach is to add the seam allowance to the waist radius and the hem allowance to the skirt length. This calculator adds one combined allowance to the length and leaves the radius unfitted, which is slightly generous at the waist. For a precise fit, draw the radius and add the seam allowance to the pattern edge.
Can I make a circle skirt without a pattern?
Yes. Fold the fabric into the quarters or eighths the style needs, mark the waist radius from the folded corner and the outer radius from the same corner, and cut both arcs. The fold lines become the side seams or the centre front and back.
What if my fabric is narrower than the pattern?
The pieces figure tells you how many widths are needed. Join them with a seam, usually at a side or centre back, and add the seam allowance for the join. A quarter circle skirt often fits on a narrow width; a full circle skirt rarely does.