My son weighs 18 lbs and I ordered him the medium and they fit him perfect, but not for long. I love this concept of being able to wash his swim diaper. Its basically another layer of swimming trunks really. He could wear just this but we choose to put swim trunks on top of them. So much easier and cheaper than the disposable swim diapers. One caviat to this particular brand.. I found cuter swim diapers at Academy for half the price a day after these came in the mail. Ugh. .. when he grows out of these we'll buy them there.