My daughter is really small for her age (24 lbs, 3 yrs old).  For swim wear she is wearing 18-24 months...but I took a chance and bought 2T in this swimsuit and fits PERFECTLY!!!  (for regular clothes she wears a 2T) I am really happy with this...so cute!  The bottom is the best part as it does not bunch up.  Usually the bottom bunches to leave room for those who are still wearing swim diapers. The "bikini" bottom is a perfect fit.  So if your child is average or normally wears a size larger than her age, I would order a size up.