Waist cut-out radius and fabric for a circle skirt.
Cut a hole of this radius at the waist, then measure the length outward for the hem.
A circle skirt is cut from a circle of fabric with a smaller circle removed for the waist. This calculator finds the waist radius and the fabric size from your waist measurement and the skirt length. For a full circle, the waist radius is the waist measurement divided by about 6.28, which is two times pi, and half and quarter circles have larger waist radii.
The fullness changes the look and the fabric use. A full circle skirt swings widely and uses the most fabric, a half circle is more modest, and a quarter circle hangs closer to the body. Longer skirts need much more fabric, and the width of the fabric may require joining panels.
Cut the waist opening slightly larger than your measurement, since you need to get the skirt over your hips. Because parts of the hem are cut on the bias, they stretch as the skirt hangs, so hang the skirt for a day or so before you level and sew the hem.
Use a drapey fabric for the most graceful movement, and cut on a fold to keep the cuts accurate. Directional prints will need careful planning, since the pattern will point in different directions around the skirt.