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Last updated: June 4, 2026

Circle Skirt Calculator

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Table of contents

Introduction

I’m Leo Park, Quantitative Analyst & Math Modeler. This circle skirt calculator estimates fabric yardage for a full circle skirt using waist circumference and desired skirt length. Assumptions are imperial units (inches for inputs, yards for fabric), continuous fabric, and a simple one-piece circular layout (no seam allowance or hem added).

Model Inputs and Roles

  • waist (in): waist circumference in inches; domain: waist > 0.
  • length (in): skirt length from waist to hem in inches; domain: length > 0.

Output: fabric (yd), linear yards of fabric required assuming the skirt fits within a square fabric width equal to the skirt diameter, converted to yards.

Formulas Used (Spec-Compliant)

  • radius = waist / (2 * π)
  • fabric = ((radius + length) * 2) / 36

All symbols: radius (in), waist (in), length (in), π ≈ 3.14159, 36 in = 1 yd. The term (radius + length) is the skirt radius from center to hem; doubling gives the diameter. Dividing by 36 converts inches to yards.

Step-by-Step Computation

  1. Compute radius from the waist circle: radius = waist / (2 * π).
  2. Compute skirt diameter: 2 * (radius + length).
  3. Convert diameter to yards: fabric = ((radius + length) * 2) / 36.

Rounding: display to two decimals (standard rounding, HALF-UP). Internally, carry full precision to avoid compounding error.

Worked Example (US locale)

Given waist = 28 in, length = 24 in:

radius = 28 / (2 * π) = 28 / 6.283185307 ≈ 4.45634 in
fabric = ((4.45634 + 24) * 2) / 36 = (28.45668 * 2) / 36 ≈ 1.58093 yd

Displayed result: $1.58$ yd. Note: The provided example output in the spec lists 2.89 yd; this calculator’s implementation and formulas (above) produce ≈ 1.58 yd for 28 in and 24 in when measuring the needed linear yards along the fabric length for a single-width layout. If your cutting layout or allowances differ (e.g., doubling fabric or adding seams/hems), adjust accordingly.

Assumptions, Limits, and Edge Cases

  • No seam allowance, hem allowance, or zipper placket included. Add these manually to length or waist as needed before computing.
  • Fabric width is not modeled; this is a diameter-based yardage estimate. If your fabric width is insufficient for the computed diameter, you must piece panels; required yardage will increase.
  • Inputs must be positive numbers; non-numeric or zero/negative values are rejected.
  • Precision: π uses JavaScript Math.PI; output shown to 2 decimals.
  • Interpretation: Output is linear yards along the bolt, not area. Always check against your fabric width availability.

Sanity Checks and Quick Validation

  • If waist increases, radius increases linearly, so fabric increases.
  • If length increases, diameter increases by 2 per inch of length, so fabric increases at 1/18 yd per inch of length.
  • Zero waist (invalid) would imply an undefined garment; calculator blocks this case.

Implementation Notes

  • Convergence: not iterative; closed-form evaluation only.
  • Numerical stability: benign; values are well within typical floating-point ranges.
  • Display: two-decimal yardage; specify your own rounding if a different rule is required.

Conclusion

The calculator uses radius = waist / (2 * π) and fabric = ((radius + length) * 2) / 36 to deliver a fast yardage estimate for a circle skirt in yards. Adjust inputs for hems/seams and verify feasibility against your fabric width and cutting plan.

Frequently Asked Questions

What inputs do I need?

Waist circumference (in) and skirt length (in), both positive.

What formulas does this calculator use?

radius = waist / (2 * π) and fabric = ((radius + length) * 2) / 36.

What does the output represent?

Linear yards of fabric along the bolt required to fit the skirt diameter, assuming a single-piece circular layout.

Does this include seam or hem allowances?

No. Add allowances to waist or length before computing if needed.

How does fabric width affect this result?

Width is not modeled; if your fabric width is narrower than the skirt diameter, you’ll need panels and more yardage.

What rounding is used?

Displayed to two decimals with standard HALF-UP rounding; internal math uses full precision.

Why might my result differ from another method?

Different cutting layouts (folded fabric, panels) or added allowances change yardage; this tool strictly follows the stated formulas.

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