Circle Skirt Calculator (waist radius, cutting radius & fabric for full, ¾, half, quarter)

Understanding the circle skirt calculator — concept and principle

This calculator drafts a circle skirt: the waist radius, the cutting radius with seam and hem allowances, the hem circumference and how much fabric to buy. It covers full, three-quarter, half and quarter circles, and is used by home sewists and costume makers who want a pattern without drafting paper.

A skirt is a ring: with the default inputs the result is 146 cm fabric (one piece, 150 cm wide).

How it is calculated

For a fraction f of a circle (1, ¾, ½ or ¼):

Waist radius r = waist ÷ (2π × f), because the waist seam is an arc of length f × 2πr.
Cut waist radius = r − seam allowance (the hole is cut smaller so the seam lands on the waist).
Outer cut radius R = r + skirt length + hem allowance.
Hem circumference = 2π × f × (r + length).
Fabric: a full or ¾ circle fits in a 2R × 2R square if the fabric is at least 2R wide; otherwise full = two half circles (4R long), ¾ = half + quarter (3R). A half circle needs R of fabric 2R wide, or 2R of fabric at least R wide; a quarter needs R.

The principle

A circle skirt is a ring of fabric. The inner edge becomes the waist and the outer edge the hem, so all the geometry comes from the circle formula C = 2πr. A smaller fraction of a circle must use a larger radius to give the same waist length, which makes the skirt less full. Taking off the seam allowance from the radius is important: if you cut the hole at radius r and then sew a seam inside it, the waist ends up too large. The skirt length and hem are added outwards from the waist line.

Worked example

Default inputs: waist 70 cm, length 60 cm, full circle, hem 2 cm, seam 1.5 cm, fabric 150 cm wide.

r = 70 ÷ (2π × 1) = 11.14 cm, shown as 11.1 cm.
Cut waist radius = 11.14 − 1.5 = 9.6 cm.
R = 11.14 + 60 + 2 = 73.1 cm; 2R = 146.3 cm.
Fabric 150 cm ≥ 146.3 cm, so one piece across the width, needing 2R = 146 cm (1.46 m).
Hem circumference = 2π × 71.14 = 447 cm.

Each point is the calculator re-run with only “Skirt length (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Skirt length (cm)	Fabric length to buy (cm)
15	56
30	86
45	116
60	146
75	353
90	413
120	533

How to read the result

Buy a little more than the fabric length to allow for straightening the cut end and shrinkage after washing. If your fabric is narrower than 2R, the layout switches to pieces with seams, and the fabric length grows. A long hem circumference means a lot of hemming, so a narrow rolled hem is usually easiest.

Common mistakes

Cutting the waist at r without removing the seam allowance, so the skirt slides down.
Using a directional print or nap in a one-piece layout, so half the skirt is upside down.
Hemming straight away; bias areas drop, so let the skirt hang for a day first.
Measuring the waist over thick clothes.

Keep in mind: The fabric estimate assumes non-directional fabric with no shrinkage and no pattern matching. Add extra for prints, nap, a waistband or a zip seam.

Frequently asked questions

Why is the waist radius so small?

A full circle spreads the waist over 360°, so a 70 cm waist needs a radius of only about 11 cm.

How much fabric do I need for a full circle skirt?

About twice the outer radius if the fabric is at least that wide; otherwise about four times the radius for two half circles.

Does a circle skirt need a zip?

Usually yes in woven fabric, unless the waist is elasticated. Cut the full circle as two halves if you want a seam for the zip.

Sources: Circle geometry: circumference C = 2πr and arc length = angle × radius (any geometry textbook)

Checked 9 Nov 2026. Circle geometry (arc length = angle × radius; circumference 2πr) applied to the skirt pattern; allowances are example values you can change.

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