Is a boy 18-20 size an adult small?

A boy's size 18-20 typically corresponds to a youth or large children's size, rather than an adult small. Adult sizes usually start from small (S) and often run larger than youth sizes. Therefore, a boy's size 18-20 is generally considered to be closer to an adult medium or large, depending on the brand and fit. It's always best to check specific sizing charts for accurate conversions.