We ordered the 6 month size, which says it's for babies 10-18 pounds. Our baby wears size 3-6 normally, and he weighs just under 14 pounds. He's in the 8th to 10th percentile all around proportionately (as in, not chunky legs or waist, he's actually rather petite for his age). But, this  swim diaper can barely be buttoned on him. It's too late for me to order a bigger size, as we need it for swim lessons this weekend, and the swim school requires a reusable swim diaper over a disposal one. I'm quite disappointed.