Gathering ratio calculator
Enter the length the gathers must fit and a gathering ratio: you get the length of fabric to gather, how many panels to cut across your fabric's width and how much fabric to buy.
The edge the gathers are sewn to: waistband, bodice or tier.
1.5 soft, 2 classic, 2.5 to 3 very full.
The cut height of each panel, allowances included.
At each end of the panels you join.
Fabric to gather
Cutting 2 full widths without trimming gives a ratio of 2.69.
To gather fabric into a seam, cut it longer than the seam by the gathering ratio: fabric length = finished length × ratio. A ratio of 1.5 gives soft gathers, 2 a classic gathered skirt and 2.5 to 3 a very full look in light fabric. For a 76 cm (30 in) waistband gathered at 2, you need 152 cm (60 in) of fabric before seam allowances.
What gathering ratio should I use?
Gathering looks fuller in light fabric, and thick fabric bunches up into a bulky seam, so the ratio goes down as the fabric gets heavier.
| Fabric | Examples | Ratio | Look |
|---|---|---|---|
| Light | voile, chiffon, cotton lawn, double gauze | 2.5 to 3 | Full and floaty |
| Medium | poplin, linen, chambray, viscose | 2 | Classic gathered skirt |
| Heavy | denim, corduroy, wool, canvas | 1.3 to 1.5 | Soft fullness without bulk |
The presets in the calculator use 2.5, 2 and 1.5. A ruffle follows the same rule: 1.5 to 2 times the edge it is sewn to is common, and each tier of a tiered skirt is often about 1.5 times the one above it.
How many panels do I cut?
Fabric comes 110, 140 or 150 cm (45, 55 or 60 in) wide, so a long gathered piece is cut in panels across the width and seamed together. The calculator divides the length to gather by the fabric width less the seam allowances, rounds up, and gives each panel's cutting width.
- Metric: 152 cm to gather on 140 cm fabric with 1.5 cm seam allowances makes 2 panels, each cut 79 cm wide.
- Imperial: 60 in on 55 in fabric with 5/8 in seam allowances makes 2 panels, each cut 31 1/4 in wide.
If you would rather not trim the panels, use full widths: the calculator shows the ratio that gives. Enter the height of each panel (the finished length plus the waist seam and hem allowances) and it also gives the fabric to buy: panels × height, rounded up to 10 cm or 1/8 yd. Add a little if you pre-wash.
Tips for even gathers
- Sew two rows of long machine stitches inside the seam allowance and pull the two bobbin threads together; gathering fabric evenly walks through it.
- Mark the gathered edge and the edge it joins into quarters, pin the marks together, then spread the fullness between them.
- For heavy fabric or a long edge, zigzag over a strong thread or dental floss and pull that instead: it will not snap.
- Sew with the gathered side up so you can keep the folds from twisting under the needle.
For an elastic waist instead of a waistband, the elastic length calculator gives the elastic to cut. Sewly's gathered skirt pattern lets you set the ratio between 1.3 and 3 (1.8 by default) and works out the panels for you.
Frequently asked questions
What does a 2:1 gathering ratio mean?
The fabric is twice as long as the edge it is gathered into: 200 cm of fabric for a 100 cm edge, or 80 in for a 40 in edge. It is the usual ratio for a gathered skirt in medium-weight cotton.
Do I apply the ratio before or after seam allowances?
Before. Multiply the finished length by the ratio, then add a seam allowance at each end of every panel. The calculator adds them when it gives the panel widths.
How much fabric do I need for a gathered skirt?
Multiply your waistband length by the ratio, divide by the fabric width to get the number of panels, and multiply by the skirt length plus allowances. Sewly's gathered skirt in EU 38 (US 6), midi length, takes 2.2 m (2 1/2 yd) of 140 cm (55 in) fabric.
Why are my gathers uneven?
Usually because the fullness was pulled up from one end. Divide both edges into quarters, pin the marks together and slide the gathers along the threads until each section is even before stitching.