What this calculator does
A circle skirt is cut from a ring of fabric: the inner edge, at some radius, forms the waist opening, and the outer edge, further out by the skirt length, forms the hem. The circle skirt formula behind it is the same one that links any circle's circumference to its radius, applied to whatever fraction of a full circle the pattern uses, full, half, or something in between.
This calculator works out that inner (waist) radius and outer (hem) radius from a waist measurement, an ease allowance for comfort and seams, a skirt length, and a choice of circle fraction. A full circle skirt uses the whole circle for maximum fullness; a half circle skirt calculator or quarter circle setting uses only that fraction of a circle's edge for the same waist measurement, which needs a larger radius to fit the same waist opening into a smaller arc.
The formula
The waist opening, plus ease, has to equal the length of the cut arc at the inner radius: waist + ease = 2π × radius × fraction. Solving for radius gives radius = (waist + ease) ÷ (2π × fraction). The outer, hem-edge radius is simply that inner radius plus the skirt length, since the panel is cut as a ring between the two.
| Term | Meaning |
|---|---|
| Ease | Extra room added to the waist measurement, for comfort and to allow for a waistband or seam. |
| Circle fraction | How much of a full circle the pattern uses: 1 for a full circle skirt, 0.5 for a half circle skirt, and so on. |
| Inner (waist) radius | The radius at which the waist opening is cut, sized so its arc length matches the waist plus ease. |
| Outer (hem) radius | The inner radius plus the skirt length: the radius at which the hem is cut. |
The inputs explained
| Field | What to enter |
|---|---|
| Waist measurement (cm) | The waist measurement the skirt needs to fit, without ease added. |
| Ease allowance (extra room + seam) (cm) | Extra allowance added to the waist for comfort, a waistband and seams. |
| Skirt length (cm) | The finished length of the skirt, from waist to hem. |
| Circle | How much of a full circle the pattern uses: full, three-quarter, half or quarter. |
When to use it
Cutting a full circle skirt pattern
A full circle skirt gives the most fullness and swing for a given waist size, but needs the largest piece of fabric, since the fabric radius needed is largest at fraction = 1.
Working from a half circle skirt calculator result to conserve fabric
A half circle skirt uses less fabric width for a shorter hem circumference than a full circle at the same waist, at the cost of less fullness, useful when fabric is narrow or limited.
Adjusting for a stretch or woven fabric
A smaller ease allowance suits a fabric with some stretch, while a firmly woven fabric with no give usually needs a slightly larger ease to move comfortably.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Fabric radius needed as waist size increases
A full circle skirt at a range of waist measurements, with ease and length held fixed.
| Waist | Waist (inner) radius to cut | Outer (hem) radius to cut |
|---|---|---|
| 60 cm | 9.87 cm | 69.87 cm |
| 65 cm | 10.66 cm | 70.66 cm |
| 70 cm | 11.46 cm | 71.46 cm |
| 75 cm | 12.25 cm | 72.25 cm |
| 80 cm | 13.05 cm | 73.05 cm |
| 90 cm | 14.64 cm | 74.64 cm |
Circle skirt formula compared across full, three-quarter, half and quarter circle
The same waist, ease and length, cut as a full, three-quarter, half or quarter circle.
| Circle fraction | Waist (inner) radius to cut | Minimum fabric width needed per panel |
|---|---|---|
| Full circle | 11.46 cm | 142.92 cm, before allowing for seams between panels |
| Three-quarter circle | 15.28 cm | 150.56 cm, before allowing for seams between panels |
| Half circle | 22.92 cm | 165.84 cm, before allowing for seams between panels |
| Quarter circle | 45.84 cm | 211.67 cm, before allowing for seams between panels |
Questions
What is the circle skirt formula?
The inner (waist) radius equals the waist measurement plus ease, divided by 2π times the circle fraction: radius = (waist + ease) ÷ (2π × fraction). The outer (hem) radius is that inner radius plus the skirt length.
How much fabric does a full circle skirt need?
Roughly a square of fabric with sides equal to twice the outer (hem) radius, since the whole circle has to fit within that width and height. For a 70 cm waist with 2 cm ease and a 60 cm length, that comes to a panel around 143 cm across.
Why does a half circle skirt need a bigger radius than a full circle for the same waist?
A half circle only uses half the circumference of a circle at a given radius for its waist edge, so to fit the same waist measurement into half the arc length, the radius has to be roughly twice as large.
How much ease should I add to my waist measurement?
A common starting point is 1 to 3 cm for comfort and seam allowance, more for a firmly woven fabric with no stretch and less for a fabric with give. It comes down to the specific fabric and how snug the waistband is meant to sit, so treat it as a starting figure to adjust on a test piece.
For the area and circumference of a plain circle at any radius, see the circle calculator. For a wedge-shaped arc and chord instead of a full ring, see the circle sector and arc calculator.