My college-student brought home a pile of jeans that mostly fit her but gap at the waist. She asked if I could fix them. I asked the Internet and found a number of complicated solutions. Then I found a really easy solution involving elastic. My daughter was skeptical but agreed I could give it a try on one pair. I mostly followed the instructions and It worked so well she brought me several more pairs to de-gap. I have a few pairs of my own jeans that will likely get this treatment too.
The technique takes advantage of the fact that most pants waist bands are basically a tube that goes around the entire waist. There is an inside and an outside and empty space in between. I have previously improved the fit of several pairs of yoga pants by opening up the side seam on the the inside of the waistband and threading elastic all the way around, and sewing the two ends of the elastic together. For jeans, you just need a piece of elastic across the back. So the problem is what to attach it to. The solution is to sew the elastic to the waistband under a belt loop so it is hidden. (Even if you don’t sew it under a belt loop, if you use thread that matches the jeans it is not going to really show. And of course, if you wear a belt, it is not an issue at all. But one of the reasons to add the elastic is so that you do not need a belt to hold your jeans up.)
Most jeans do not have a convenient side seam as the inner waistband is a continuous piece (although one of the pairs of pants my daughter brought me were actually pants, not jeans, and they had a convenient seam that I popped right open with my seam ripper). So for the jeans, I just used a small sewing scissors to cut a little vertical slit through the inner waistband, positioned right under a belt loop on each side of the back.
Then I used a safety pin to thread elastic inside the waistband between the two slits.
I trimmed the elastic, leaving the waist bunched up a bit (it will smooth out when you wear the pants). I pinned the elastic a bit inside each slit so it wouldn’t pull out while I sewed. (Yes I know the photo below is from a different pair of jeans than the one above, but trust me, I did the same thing on all of them.)
Then I used my sewing machine to sew a wide zigzag over the slit, holding the belt loop out of the way so it didn’t get caught in the stitches. I did this for both slits and removed the pins.
Now her pants fit and the stitches holding the elastic in place are hidden under the belt loops.