Method and boundaries
Keep the stitched lines and cut edges separate.
Skirtmark models one full circular ring of fabric. Every radius starts at the same centre, but the waist cut edge, stitched opening, finished hem and outer cut edge are different circles.
Choose the opening before calculating
Enter the circumference you want at the stitched opening line, including any ease your construction needs. The calculator does not infer it from body size or recommend ease. An unbroken opening may need to pass over the hips; a smaller opening may require a closure that this tool does not design. Do not treat the example as a fitting recommendation or assume a waist measurement alone guarantees that the panel can be put on.
Derive the seamline radius
A circle's circumference is C = 2πr. Therefore the stitched opening radius is r = C/(2π). The tool uses finite numerical arithmetic and Math.PI, so the radii are approximations to the ideal circle geometry.
Move the cut edge inward at the waist
If s is the chosen waist seam allowance, the cut opening radius is rᵢ = r − s. The cut circle is smaller because the seamline sits farther out into the panel. If the allowance consumes the entire radius, there is no positive circular cut opening in this model and the input is rejected. The allowance is not added to the circumference, and it is not subtracted from the intended finished length.
Measure the finished length from the stitched opening
If L is the chosen finished radial length, the intended finished hem radius is r𝒻 = r + L. Add the outward hem allowance h to obtain the outer cut radius R = r + L + h. The distance from the inner cut edge to the outer cut edge is then R − rᵢ = L + s + h. This check makes both allowance directions visible.
Understand the square and area
A full flat circle of outer radius R fits geometrically inside a square of side 2R. Skirtmark reports that minimum unfolded square side. It does not add working clearance or selvage losses, decide whether a fabric bolt is wide enough, nest pieces, split the panel into joined sectors or tell you how much fabric to buy.
The flat ring area is π(R² − rᵢ²). The calculation uses the equivalent factorization π(R − rᵢ)(R + rᵢ), with the radial thickness computed as L + s + h. Area is square centimetres and is not interchangeable with a square's side or a length to buy. The intended finished hem circumference is 2π(r + L).
The diagram is for understanding
Solid lines show the cut circles; dashed lines show the stitched opening and intended finished hem. The enclosing square represents the flat blank. The drawing is scaled to the screen, so narrow allowances or a very small opening may be hard to distinguish visually. Read the labelled numerical radii. Do not print the diagram expecting a measured pattern: no print scale, page tiling or calibration is provided.
Strict centimetre inputs
- Chosen stitched opening circumference: more than 0 through 1,000 cm.
- Finished radial length: more than 0 through 500 cm.
- Inward waist seam allowance: 0 through 10 cm, while keeping the cut opening radius positive.
- Outward hem allowance: 0 through 50 cm.
- Each field accepts a plain unsigned decimal with digits before the decimal point and at most three fractional places. Leading zeroes and ASCII outer spaces are allowed. There is a 32-character limit including those spaces.
- Blank values, signs, exponents, fractions, units, commas, tabs, non-ASCII spaces and trailing punctuation are rejected. No unit guessing or silent clamping is performed.
Approximation and current-only notes
The display keeps up to 12 significant digits and uses scientific notation where needed to avoid turning a small positive radius into a displayed zero. These digits do not imply corresponding cutting precision. Text and JSON retain raw numerical approximations and the entered decimal fields, so display rounding never feeds back into the calculation. Editing clears the previous output, diagram, exports and manual-copy fallback. A clipboard request already sent cannot be recalled, but late replies cannot restore stale notes.
Primary references and scope
OpenStax's circle-geometry lesson supports the circumference, diameter and area formulas. The allowance directions here follow this explicitly chosen concentric-circle model. BERNINA's circle-skirt maker tutorial provides practical inner/outer-circle context and highlights the opening needed for the chosen construction. Its hemming guide explains that bias sections can grow unevenly. This calculator does not copy either pattern's allowance recommendations or compensate for fabric behaviour.
No fit, closure, waistband, joined-panel, bias-growth, shrinkage, grain or fabric-buying guarantee is part of the calculation. A flat geometric draft is one input to your own sewing plan.