So a few years ago my roommate needed a belt, so we broke out a retired (6mil) anchor, cut it to fit (we doubled it around his waste), and viola. I showed him how I would tie it, basically a 3 way sheet bend (since one end is just a bight). See the first photo of it tied in my 5 mil belt. He has since discovered he prefers to tie an overhand knot, as pictured in the second photo, again modeled on me. And yes, I realize if I were to use real models, I would probably get a lot more responses, or at least views.
Now before I get to the inevitable question, I want to endorse using rope as a belt. It works great, its stronger than I will ever need it to be, and if I am in need of some rope (and if you are on this forum, you know how frequently that can be) I have some. Better still, I am not distracted, using it to tie knots all day, because any time I use it for knot practice, my pants might fall down. Finally, it keeps a passion of mine, knot tying, close to my... ummm... heart.
But the question is, what knot is better for this application? They both take about the same amount of cord, which is required because the belts are cut almost exactly for that length. He thinks the overhand is easier to tie, I disagree, but he might be right for tying it around your waste, it probably is easier to keep tension while tying the overhand, and easier to learn/remember because its almost instinctual to tie an overhand. Also, when it comes to untying (occasionally the single most important issue) I again think they are similar, while he thinks the overhand is easier. They both seem to be plenty secure, he used one and then the other for at least a year with no wardrobe malfunctions.
My preference is clearly the sheet bend, maybe only because I prefer to be a little bit more clever than an overhand. But I gotta admit, he might be right about the overhand being a better choice, because its universally known and by every other measure they seem to be about equal.
And any other suggestions for knots? Like I said, belts are already cut, and neither of us are going to lose any weight, so the knot can't require more rope than what's is available.
And one last minute addition, growing up in scouts we tied "commando belts" by putting a large eye splice in one end of three strand sisal rope, and a small eye splice in the other around a toggle. The idea was to teach us splicing, not to use them as belts, because they weren't tight, and not to link them together. We were told commandos did it this way because if one or two guys had all the rope, and didn't make it to the cliff, then everyone else was screwed, so everyone carried some. Not that I think rope belts will catch on anytime soon, instead maybe my preference for rope belts originated then.