This is one of those persistent sartorial debates that doesn't have a clear-cut answer but does have plenty of opinion on either side. We'll break down both schools of thought first, then weigh in on what we think the right move is.
Why You Should
Belt loops without a belt can look odd, as if you forgot to finish getting dressed in the morning. That's all the more noticeable if you're wearing a tucked-in shirt.
It's generally considered the default. A simple black or brown leather belt that properly matches your shoes is a classic look that won't offend anyone's sense of style.
Practically speaking, they serve a real purpose. If your suit trousers are just a little too loose, a belt will keep them anchored in place.
Why You Shouldn't
It looks sleeker and more modern. There's less visual emphasis on your waistline and more on your perfect tailoring.
If your pants are more than a little loose, a cinched belt will bunch up the fabric of the waistband. That looks sloppy, and can damage the wool.
It's more formal. You'd never wear a belt with a tuxedo (in fact, most tux pants don't even have belt loops – but more on that in a minute) because it detracts from the perfectly polished, well-fitted image you're trying to project. You can apply the same logic to a suit in any kind of dressed-up situation.
There are merits for both sides, as you can see, but we say to lose the belt when you're suiting up fully.
Belts are inherently informal, and even slim, polished leather ones detract slightly from the perfectly put-together vibe of a well-fitted suit and tie. Leaving empty belt loops isn't ideal, but that's better than the alternative: adding too much of a visual break to your suit and drawing eyes to your waistline.
In a perfect world, your suit pants would have side tabs or no waist adjustment additions at all, making the belt debate a moot point. This is a suit, after all, and suits are meant to be tailored to your precise measurements – which should make too-loose pants a non-issue.
That said, if you're wearing a suit more casually – say, a cotton one with an open collar in the summer months – go ahead and add a belt, because it matches the relatively relaxed vibe.