“My son is 13 and he won’t touch broccoli. What size is he?” If only all 13-year-olds were the same size! In reality, boys come in all shapes and heights. We know a 13-year-old who’s 4’10” and 87 pounds—and his best friend, also 13, who’s 5’8” and 135 pounds. Same age, very different builds.

Luckily, just three measurements usually give us a good idea of the right size: height (without shoes), weight, and waist. 

The most accurate way to measure the waist? Grab a pair of pants he already wears and fits well. Button and zip them, lay them flat, measure straight across the waistband, and double it. That’s his waist (27” in the example photo).

A little extra room is okay—boys grow fast—but don’t go so big that the suit looks like it belongs to an older brother.

Fit Matters

The jacket fit is king. Pants can be adjusted more easily by a tailor. When you alter a suit, leave as much fabric as possible—it extends the life of the suit. Good fabrics usually handle sleeve or pant length adjustments without leaving a noticeable line, as long as the suit hasn’t been over-dry-cleaned or abused.

Slim, Skinny, or Trim—What’s the Difference?

Modern “trim fit” is all the rage—short jackets, low-rise skinny pants—but don’t confuse it with boys’ slim fit. True slim sizing for boys has always existed. It’s for tall, very thin boys: the waist is usually 2” smaller than a regular, with longer lengths. A modern trim-fit pant keeps the same waist as a regular but has a lower rise and narrower seat and leg.

Here’s the big question: how cool do we want our boy to be? Boys aren’t men—they’re still growing, often quickly. Do we go cutting-edge fashion that he might outgrow after a few wears, or a fit with just enough growing room that he can wear it for months? That’s up to you.