I’ve done it before. Depending on the construction of the pants, the technique varies. I’m assuming, because you mentioned belt loops, that the pants have a separate waistband. One you remove the back and side seam belt loops, you can open the waistband from the inside for about an inch or so from each side seam. Thread in your elastic cut to your desired measurement plus a little extra, and stitch through each end of elastic through the waistband close to the side seam. Reclose the waistband. I’ve even cut slits in the waistband (inside) instead of opening it, to thread the elastic through. You can then hand stitch the slits closed, but I usually leave them open, as they seem to stay closed.

Yes, everything has a separate waistband. Some do not have belt loops, all I meant by mentioning those was that I could just get rid of any on the back, if it made the elastic contract more smoothly. I am wondering if it is hard to get this to look right: bunch up, twist, etc. Do I need to stitch the elastic down anywhere except at the ends.

In desperation I put a drawstring into a pair of shorts last year. Though I used fray-check on the openings, I think they are still going to start fraying quickly.

I have tried to do this procedure, and found that the waistband now fits, but, it is bunched up and will not lay nicely. I think the only solution, is to cut away the excess fabric in the waist band, and sew a seam in the waistband. A tapering of the pants, at the waistband, will have to be done, so there is minimal, if any, gathering.

I have done one so far and it turned out perfectly. I've now ordered elastic in other widths to do more. FWIW, this is not a fast procedure.

Foggyj2, I have altered most of my pants as you describe. But have had a lot of trouble tapering from the narrowed waist into the above-derriere area. Cannot get it to lay flat. I think the transition is too sharp. But there is not much space to make a transition from a smaller size at the waist to a fuller size over the rear.

And since my weight fluctuates by as much as 10 lbs., I feel like the elastic is more forgiving.

I would test other types of elastic. Mine is not bunching. I happened to have a lot when I started and chose a non-roll.

Maybe I should try it with another type of elastic. I have a "sway back," so it's hard to find pants that fit well. I'm glad you could make it work.

I have done a very simple version on jeans. Slit the waistband on each side of of the rear belt loops and feed elastic thru. Pin in order to determine the right “tightness”, then machine in hand stitch the elastic in place. It is helpful for my hourglass shape and basically free.

Google resize waist of jeans to see u tube

That gaping at the back waistband is a result of the popularity of a badly drafted pattern. Somewhere along back a few years there emerged a badly drafted factory pattern that was calculated not to make a more flattering garment, rather to speed and make easier cutting the garment in the factory. Your waistband may not be cut on the lengthwise grain. That is going to affect how it conforms and fits and puts it at odds with the grain line of the rest of the pants.

This badly fitting pattern gave us camel toe and that awful low waistband that caused so much muffin top and pants that are hard to be active in and that ride down our bodies all the time to where we are constantly hiking them back up.

This poor draft and bad fit became the fashion norm. Go figure!!!

You can add a sewn on casing to the inside of the back waistband and thread some wide elastic through it. Don't be surprised if you find the pants pulling awkwardly in some other direction if you do that. It is only the fabric of the waistband that is in excess and the top of the back seam of the pants has no excess. So it has to pull from somewhere, probably along the back seam. If it is a loose and roomy fit that you are dealing with, you might not notice it much. But if the pants are tighter you may notice it and you may get no benefit from doing so.

I assume that you are talking about ready to wear? Sometimes the details and the threads and such make it quite difficult to do too much altering in most ready to wear. You know that quality mens' pants are sewn with a back seam that is continuous with a back seam in the waistband so that they can be tailored to fit easily. Of course, not at kohl's, but in a quality men's store. Most women' clothes have no such option sewn in to them and you are on your own with it.

If you really wanted, you could remove the waistband and sew a waist facing instead. But, that is a lot of work.

Thank yo, dallasannie. Yes, ready to wear. Interesting observations on the badly drafted pattern! So it's not just me that finds most pants in my size may fit well all over: just not in the waist.

I cannot stand low rise pants. Some years years back it was almost impossible to find ones that came to the natural waist. It's better now, though.

As you say, "It is only the fabric of the waistband that is in excess and the top of the back seam of the pants has no excess. So it has to pull from somewhere, probably along the back seam." This is where I have the problem with just taking tucks or darts in the waistband, which is what I had been doing before. There is no place to make a transition: leading to awkward puckering where the back of the pants meet the waistband. I think elastic can work better, provided the fabric is not a really heavy weight.

There is one brand where pants or jeans fit all over. I am very frugal and they were $$. They closed all their retail stores and only sell online now. The pants I bought during their clearance closedown of stores are/were my go-tos for everything. I have been lucky enough to find some at resale shops, too.

A Land's End store just opened near me. I can wear some of theirs without altering. But it depends on the cut and I just have to try them all on. Even some of those I bought recently (post holiday clearance) I have marked and put aside for adding elastic.

I held out from buying new pants for a couple of years. But I went up 2 sizes due to going off a medication, and got tired of having only a couple of pairs I could wear. So I bought some new but getting elastic put in them will take some time. I'm sure it will get faster as I do more. Just have a lot going on now, to devote several hours per pair in doing this.

I alter clothing in a store. The request I get more often is reducing the stretch in pant waistband, and I thread a piece of grosgrain ribbon in. You could do the same with elastic.

I open about an inch forward of the side seam on each side. Thread the elastic or ribbon from one side. Stitch one end in place, pull the other through to the other side with one of those elastic threaders. Stitch the other end in place, and close up the spots on the waistband you opened.

When I was learning to sew, (back in the Stone Age) women's, even GIRLS' clothes had curved waistbands. No, not facings, curved waistbands. Of course, straight ones use less fabric.