Circle Skirt Calculator

How the calculation works

A circle skirt is a ring of fabric. The waist is a small circle cut out of the middle, and the hem is the large circle around the outside. The only real calculation is the radius of that inner circle, so that its circumference equals your waist.

The circumference of a circle is 2 × π × radius. For a full circle skirt the whole inner circle becomes the waist, so:

waist radius = waist ÷ (2 × π)

A half circle skirt only uses half the circle for the waist, so the radius doubles: waist ÷ π. A quarter circle uses a quarter, so the radius is waist × 2 ÷ π. A three-quarter circle sits in between. The less of a circle you use, the larger the waist radius and the less flare the skirt has at the hem.

Then you add the skirt length to the waist radius to get the hem radius, and add the hem allowance on top. Finally you take the waist seam allowance off the waist radius. Cutting on the seamline would make the waist too small once it's sewn, so the cutting line has to sit inside it.

Worked example

A full circle skirt for a 28" waist, 22" long, with a 1" hem allowance and 5/8" waist seam allowance:

• Waist radius = 28 ÷ 6.283 = 4.46", about 4 1/2".
• Cut the waist at 4.46 − 0.625 = 3.83", about 3 7/8".
• Cut the hem at 4.46 + 22 + 1 = 27.46", about 27 1/2".

The hem radius is 27 1/2", so a one-piece full circle would need fabric 55" wide and 55" long. That fits on 60" fabric, but not on 45" fabric, where you'd cut two half circles instead.

Drawing the circles

For a full circle in one piece, fold the fabric in half and then in half again, so the folded corner is the centre of the circle and you are cutting one quarter through four layers. Then:

1. Hold the end of a tape measure at the folded corner (pin through it, or have someone hold it).
2. Swing the tape in an arc, marking the waist cutting radius with chalk every inch or two.
3. Do the same at the hem cutting radius.
4. Join the marks into smooth curves and cut through all the layers.

For half and quarter circles, the folded corner is replaced by the corner where your two straight edges meet. A paper pattern drawn the same way is worth making if you'll use the skirt again. A quarter-circle template can be cut out of large paper and reused for full, half or quarter skirts.

The bias stretches

Around a circle, the waist edge runs through every grain direction: straight grain, then bias, then cross grain and so on. The bias sections stretch while you handle them, so a circle skirt waist cut to exactly the right size often grows. Two habits help:

• Staystitch the waist as soon as it's cut, and stabilise it with a strip of lightweight fusible interfacing or stay tape cut to your waist measurement.
• Make the waist slightly smaller in fabrics that drape and stretch a lot, such as rayon, crepe and jersey. A waist adjustment of −1" to −2" (−2.5 to −5 cm) in the calculator is common practice. Firm cottons usually need no adjustment.

The hem drops for the same reason. Hang the finished skirt for at least a day before hemming, then level the hem from the floor up. It's normal for the bias sections to have dropped noticeably more than the straight-grain sections. Keep the hem narrow, or finish it with bias binding, because a deep hem on a full curve has far too much fabric to turn up neatly.

About the fabric estimate

The calculator checks your fabric width against the hem radius to choose a layout. It picks one piece if the circle fits across the width, otherwise halves (or a half plus a quarter), and adds a little extra length for straightening the cut edge and shrinkage. It doesn't nest pieces into each other's waste areas, which can save fabric on a careful layout, and it assumes the fabric can be turned in any direction. A one-way print, a napped fabric such as velvet or corduroy, or a big pattern repeat will need more. Remember the waistband or facing too.

For a skirt with less flare and fewer layout headaches, an A-line skirt drafted from your measurements is the gentler alternative.