Elastic Waistband Calculator
Get the elastic cut length and casing width for a waistband from your waist measurement and elastic width.
This assumes 2 inches of negative ease so the elastic stays up on its own, plus 1 inch of overlap to sew the two ends together into a loop. Try the loop on before you sew the casing shut.
How it works
Type in the waist measurement the garment needs to fit, pick the width of elastic you're using, and the calculator gives you two numbers: how long to cut the elastic, and how wide to sew the casing that holds it. The elastic length subtracts 2 inches of negative ease from the waist measurement, since elastic needs to be a little smaller than the body to stay up without slipping, then adds 1 inch back so you have enough overlap to stitch the two ends into a closed loop. The casing width adds a half inch to the elastic width, a quarter inch of wiggle room so the elastic isn't fighting the fabric, and a quarter inch of seam allowance to fold and stitch.
Worked example: a 30 inch waist with 1 inch wide elastic. Subtract 2 inches for ease to get 28, add 1 inch of overlap and you're cutting 29 inches of elastic. For the casing, 1 inch of elastic plus a half inch comes to 1.5 inches wide. A bigger 38 inch waist with the same 1 inch elastic just changes the cut length to 37 inches; the casing width stays 1.5 inches because it only depends on how wide the elastic is, not the waist.
FAQ
Where exactly should I measure my waist for this?
Measure where the garment will actually sit on your body, not necessarily the smallest point of your waist. Pants that sit at the hip need a different measurement than a skirt that sits higher up. Wrap the tape snugly but don't pull it tight; you want the number the fabric will rest at, not the number you can squeeze into.
Should I use wide or narrow elastic?
Wider elastic, an inch or more, distributes pressure over more fabric and tends to roll less inside the casing, so it reads a bit sportier and holds its shape longer. Narrow elastic works fine for lighter fabrics and kids' clothes and takes up less bulk at the waist. Either way, match the elastic width to the casing width this calculator gives you.
Why does it say to try the loop on before sewing the casing shut?
The 2 inch ease figure is a solid starting point, but bodies and fabrics vary. Overlap and pin the elastic ends, slip the loop over your hips or waist, and check it stays up comfortably without digging in before you commit to sewing the casing closed. It's a lot easier to adjust a pinned loop than to unpick a finished waistband.
Does the fabric matter for how much ease I need?
Yes, a little. A stiffer woven fabric holds an elastic waistband looser than a stretchy knit does, since the knit itself adds some give. If you're working with a heavier woven, you can trim a little less ease, closer to 1.5 inches instead of 2, and it will still stay up.
See this in action on how to sew a simple elastic waist skirt, which uses a casing sewn the same way as a double fold hem, and if the stitching itself is new to you, start with how to choose the right stitch length and stitch type.