How to sew an elastic waistband casing

To sew an elastic waistband casing, fold the top edge of the garment to the wrong side to make a tunnel slightly wider than the elastic, stitch along its lower edge leaving a 5 cm (2 in) gap, thread the elastic through with a safety pin, overlap the ends by 3 cm (1 1/4 in) and zigzag them together, then close the gap. Cut the elastic to roughly 90 % of your waist measurement plus the overlap, and try it on before you join it.

How long should the elastic be?

The finished elastic should hold the garment up without digging in when you sit down. A good starting point is about 90 % of your waist measurement for the finished loop, plus 3 cm (1 1/4 in) for the overlap. For a 76 cm (30 in) waist, that is about 68 cm (27 in) plus 3 cm, so cut 71 cm (28 in). Firm or wide elastic can be a little longer, soft or narrow elastic a little shorter. The elastic length calculator does the arithmetic for you.

Before joining the ends, always wrap the elastic around your waist where the garment will sit, pin it, and check that the stretched loop passes over your hips. That two-minute test beats any formula. Sewly works out this length from your waist measurement and lists it in the notions of its pull-on patterns.

Which elastic should I use?

Not all elastic behaves the same way:

Widths of 2.5 to 4 cm (1 to 1 1/2 in) suit most adult waists. Sewly's pull-on patterns default to 3 cm (1 1/4 in) and let you choose anything from 1 to 5 cm (3/8 to 2 in).

How wide should the casing be?

The casing must be a little wider than the elastic so it slides through without twisting: elastic width plus 5 to 10 mm (1/4 to 3/8 in). There are two common ways to fold it, and they need different allowances above the waistline:

• Single fold with a finished edge: allowance equals the elastic width plus 1 cm (3/8 in). Finish the raw edge with a zigzag or a serger, fold the whole allowance down once, and stitch close to the finished edge. Sewly's pull-on trousers, shorts and gathered skirts add exactly this allowance above the waistline, sized to the elastic width you pick.
• Double fold: allowance equals the elastic width plus 2 cm (3/4 in). Press the raw edge under 1 cm (3/8 in), then fold again. Neater inside, slightly bulkier.

If your pattern was drafted for a different elastic width than the one you bought, add or remove the difference at the top edge before cutting.

How to sew an elastic waistband casing

1. Sew and finish the seams that cross the waist (side seams, center seams), and press them. Inside the casing, stitch the seam allowances flat for 5 cm (2 in) so the safety pin does not catch on them.
2. Prepare the edge. For a single fold, zigzag or serge the top raw edge. For a double fold, press 1 cm (3/8 in) to the wrong side.
3. Fold and press the casing. Fold the top down to the wrong side along the waistline, measuring the depth from the fold all the way round. Press firmly and pin every 10 cm (4 in). Use the right heat: about 200 °C (390 °F) for cotton and linen, 150 °C (300 °F) for viscose and wool blends, 110 °C (230 °F) for polyester.
4. Optional: edgestitch the top fold 2 to 3 mm (1/8 in) from the edge. It gives a crisp top and stops the fabric rolling over the elastic.
5. Stitch the casing. Stitch 3 mm (1/8 in) from the lower edge of the casing, with a 2.5 to 3 mm stitch (8 to 10 stitches per inch), all the way round, leaving a 5 cm (2 in) gap at the center back or at a side seam. Backstitch at both ends of the gap.
6. Thread the elastic. Fix a large safety pin or a bodkin to one end. Pin the other end to the garment beside the gap so it cannot vanish inside. Push the pin into the casing, gather the fabric onto the elastic, and slide it along. Keep the elastic flat as it goes.
7. Check for twists. Run your fingers around the whole casing; the elastic should lie flat all the way.
8. Try it on. Pin the two ends together at the planned overlap and try the garment on. Adjust the length now, not later.
9. Join the ends. Overlap the ends by 3 cm (1 1/4 in) and stitch a square with a cross through it, or several rows of wide zigzag. Trim loose threads.
10. Close the gap. Let the elastic slip into the casing, stretch the waist so the gap lies flat, and stitch it closed in line with the rest of the casing stitching.
11. Spread and anchor. Spread the gathers evenly around the waist. Stitch in the ditch of each side seam through all layers, about 1 cm (3/8 in) long, so the elastic cannot twist or travel.

Tip: Sew a small loop of ribbon into the center back seam before closing the gap. You will always know which side is the back.

Channel stitching for a ready-to-wear look

For wide elastic, sew two or three rows of parallel topstitching through the casing and the elastic once it is in, stretching the waistband flat as you sew, with a 3 to 3.5 mm stitch (7 to 8 per inch). Use woven or knitted elastic for this; braided elastic loses its stretch when stitched through. The rows stop the elastic from rolling and look like the waistbands of shop-bought joggers.

Common mistakes and fixes

• Elastic twisted inside the casing. Pull it straight through the gap, or work the twist round to the gap with your fingers. Anchor it at the side seams afterwards.
• The waist is too tight or too loose. Unpick the gap, pull out the joined section, cut off the join and join it again at the right length. One more reason to try it on first.
• The elastic rolls over. Use non-roll woven elastic, make the casing only 5 mm (1/4 in) wider than the elastic, and add channel stitching.
• The safety pin will not pass a seam. The seam allowances inside the casing block it. Tack them flat, or ease the pin past the bump with your fingers from the outside.
• The stitching caught the elastic. You closed the gap with the elastic underneath. Unpick and sew again, pushing the elastic away from the needle with a pin.
• The casing is too narrow. The elastic will not pass. Unpick the casing stitching and sew it again lower, or use narrower elastic.

Elastic casings are the waist of many first projects: the pull-on trousers, the pull-on shorts and the gathered skirt all use one. The casing glossary entry covers the term and its variations.