The best way to find correct sizing is to choose a similar item (preferably swim) that fits your child well and measure that piece laid flat. Then compare them to our measurements.
Keep in mind swim fabric has stretch but also becomes looser when in the water. We designed our swim to fit snug.
You can size up when in doubt, but we do not recommend sizing up several sizes above your child's current size.
Bottoms on our two-pieces fit snug. We suggest buying your child's current size or sizing up if your child is in between sizes or are on the chunkier side. Also, size up if your child is at the end of the age range.
Model is about 27 lbs and 37" wearing size 2T in photos - fit is just right with no room to grow.