My son is blessed with a thin waist and has been able to wear the same swimming trunks for 3 years. Finally chlorine rotted the stretch right out of the elastic and he couldn’t hold his trunks up any more. My first thought was to pitch them and buy a new pair, but finding time to go to the store was harder than just fixing them.
I use this same method to add elastic to the back of garage sale found jeans and slacks too. It helps the kids wear a wider variety of sizes.
Note 1: Since this was a real mending job, my thread matches perfectly and may be hard to see. You can click on the pictures to enlarge them and see more details.
Note 2: I keep a stash of basic mending supplies that I purchase when the thread and notion walls are half off. My stash includes hanks of elastic in all sizes ($.50-$1 each) basic colors of thread: red, black, khaki, navy, white, pink etc., interfacing, and sewing machine needles in all sizes and types. This allows me to do quick sewing and mending without a trip to the store and I never have to pay full price for notions!
Step 1. Use a seam ripper to cut a slit in the inside of the waistband at each side seam.
Step 2. Measure the width of the opening and choose elastic that is about 1/8 in narrower. Slip a pin in one end and pull it through the waistband. (Wait to cut the length of elastic until the end.)
Step 3. Set your machine to a wide satin stitch (a zigzag with a very short stitch, so the stitches lay right beside each other.) Satin stitch over the hole you cut, catching the end of the elastic to secure.
Pin the elastic in place before satin stitching
Step 4. Adjust the gathers in the back to be even and trim the elastic to the desired length. Satin stitch over the other end to secure and repair the hole.
Step 5. Straight stitch through the center back of the elastic to keep it from twisting.
Confession: After the mending, he wore them once and we plan to donate them to Diabled Veterans instead of throwing them in the trash since the fabric is still good and the elastic gave them years of new life. I thought he deserved a bigger pair of trunks and this time of year is the perfect time to buy a swimming suit. They are on clearance for less than thrift store prices!