How to use the gathering fabric fullness calculator
- Enter the finished width of the gathered edge, such as the waistband or the rod.
- Enter the fullness. A value of 2 means the fabric is twice as wide as the finished edge. The 2 shown is an example.
- Enter the usable fabric width, the finished length and the hem and top allowance.
- Open More options to change the seam and side hem allowances, then read the widths to join and the yards to buy.
Formula
flat width = finished width x fullness + 2 x side hem
joined width of n widths = n x fabric width - (n - 1) x 2 x seam allowance
widths = the smallest n whose joined width is at least the flat width
yards = widths x (finished length + allowance) / 36
The fullness you actually get is the joined width less the side hems, divided by the finished width, and it is usually a little over what you asked for.
A gathered skirt
For a 30 inch waistband at twice the fullness, the flat width is 30 x 2 + 1 = 61 inches. One 44 inch width is too narrow, and two widths join to 88 - 1.25 = 86.75 inches. The cut length is 22 + 3 = 25 inches, so the fabric is 2 x 25 / 36 = 1.39 yards, and the actual fullness is (86.75 - 1) / 30 = 2.86.
For a wide curtain panel with a 100 inch finished width at 2.5 times fullness, a 1 inch side hem and 54 inch fabric, the flat width is 252 inches and 5 widths join to 266 inches. With an 84 inch length and 8 inches of allowance that is 5 x 92 / 36 = 12.78 yards.
Choosing a fullness
Fullness is a design choice. Light, drapey fabrics gather well at higher fullness, and thick fabrics look bulky when gathered too much. Common choices range from lightly gathered up to very full, but the right amount depends on the fabric and the look. Test a strip of your fabric gathered to the ratio before you cut.
Why more fabric than the minimum
Fabric comes in whole widths, so the joined width usually exceeds the flat width. The extra makes the gathers a little fuller. If that is too much, cut the last width narrower and press the seam allowance, which saves some fabric but not the yards you buy.
Pre-wash the fabric before you measure the usable width, since shrinkage changes it. Pattern matching and nap direction may call for more yardage.
Related sewing tools
Find curtain yardage with the Curtain Fabric Calculator, general yardage with the Fabric Yardage Calculator, the waistband with the Elastic Length Calculator, and edges with the Bias Tape Calculator.
Assumptions and limits
- Widths are joined edge to edge with the seam allowance you enter on each side of every seam.
- The fabric width is the usable width between the selvages.
- The allowance covers the hem and the seam at the gathered edge.
- Pattern repeats, nap and shrinkage are not modeled.
Frequently asked questions
How much fabric do I need for a gathered skirt?
Multiply the finished width by the fullness, add the side hems, find how many fabric widths that takes and multiply by the cut length in yards. A 30 inch waistband at twice the fullness needs 1.39 yards in the example.
What does 2 to 1 gathering mean?
The flat fabric is twice as wide as the finished gathered edge. A 30 inch waistband takes 60 inches of fabric, plus allowances.
Why is my actual fullness higher than I asked for?
Fabric is bought in whole widths, so the joined width usually passes the minimum. The page shows the fullness you will actually get.
Can I use this for curtains?
Yes, for a simple gathered panel. The Curtain Fabric Calculator adds rod width, pattern repeats and pairs of panels if you need those.