How this instrument works
A full circle skirt pattern is a single large circle of fabric with a smaller circle cut from its centre for the waist opening. Because the waist opening is a circle, its circumference has to equal your waist measurement, and the radius of any circle is its circumference divided by 2π — the same relationship that connects any circle's size to the distance around it. That waist radius, plus your desired skirt length added straight outward, gives the outer radius: the distance from the pattern's centre point out to the hem.
In practice, this pair of radii is what you'd set a compass, a pin-and-string, or a yardstick pivot to when drafting the pattern on paper or directly onto fabric: strike an arc at the inner radius for the waist cut line, then a second arc at the outer radius for the hem. The two radii share the same centre point, so the gap between the two arcs is constant all the way around and equals your entered skirt length.
This is the full-circle version specifically — a skirt cut from one complete circle of fabric, which gives the fullest possible swing and gather at the waist for a given waist measurement. Half-circle and quarter-circle variants use a smaller fraction of the same circle and a different waist-to-radius relationship, since less than a full circumference of fabric is being wrapped around the same waist.
- Enter Waist measurement (in) — your actual waist measurement, or your waist plus a small ease allowance if you want the finished waistband slightly looser.
- Enter Desired skirt length (in) — the distance from waist to hem you want the finished skirt to be.
- Read Inner (waist) radius (in) — the radius to mark for the waist cut-out.
- Read Outer (hem) radius (in) — the radius to mark for the hem edge, measured from the same centre point as the waist radius.
- Remember these are pattern radii before seam allowance — add your usual seam allowance to both cut lines separately when cutting fabric.
Worked example — a 28 in waist, 24 in skirt length
Enter 28 into Waist measurement (in) and 24 into Desired skirt length (in). Inner (waist) radius reads 28 ÷ (2 × π) = 4.4563 in, and Outer (hem) radius reads 4.4563 + 24 = 28.4563 in.
To draft the pattern, pin a string or set a compass to 4.4563 in from a centre point and mark the waist cut arc, then extend to 28.4563 in from the same centre point and mark the hem arc. The 24 in gap between the two arcs, measured along any straight line from the centre, is your finished skirt length before hemming.
Questions
Should I add ease to my waist measurement before entering it?
Most patterns add a small ease allowance — commonly around half an inch to an inch — so the waistband isn't skin-tight and there's room for a seam allowance and a zipper or elastic closure. Add whatever ease your chosen closure method needs to your actual waist measurement before entering it here; the formula itself doesn't add any ease on its own.
How much fabric does a full circle skirt actually need?
More than the length alone suggests, because fabric width has to cover the full outer diameter (twice the outer radius), not just the length. A 24 in skirt on a 28 in waist needs fabric roughly 57 in across (2 × 28.4563 in) to cut the full circle in one piece, which is why full circle skirts are often pieced from multiple panels or cut from extra-wide fabric.
What's the difference between a full circle and a half circle skirt?
A full circle skirt uses the complete circumference of fabric around the waist, giving maximum fullness and swing. A half circle uses half that fabric, so its waist radius is found by dividing the waist measurement by π instead of 2π — the same circle geometry, just wrapping a smaller arc of fabric around the same waist opening, which drapes flatter with less swing.
Why is the hem circumference so much bigger than my waist?
Because circumference grows with radius, and the hem radius is your waist radius plus the entire skirt length. A 24 in-long skirt on this 28 in waist example has a waist circumference of 28 in but a hem circumference of 2π × 28.4563 ≈ 178.8 in — nearly six and a half times around — which is exactly the dramatic flare a circle skirt is known for.
Can I use this for a skirt with a waistband instead of a raw waist edge?
Yes — just make sure your waist measurement entry already reflects the actual seam line where the skirt body attaches to the waistband, not your body measurement plus the waistband's own width, since the waistband is typically drafted and sewn as a separate pattern piece.