Sizes below are of the shirt, not the child. Generally the size equates to age but obviously kids are different sizes. If you are still not sure, measure an existing item by laying it flat. Measure across the chest at the base of the armpit. Double that number and that number will match one of these chest sizes. When in doubt, go up one size. Kids grow and these shirts are cotton and can shrink if you hot wash or dry.

Size	Chest Circumference	Length
2	25"-26"	17.5"
4	27"-28"	19"
6	28"-30"	21"
8	30"-32"	22.5"
10	32"	23.5"
12	34"	24.5"
14	36"	25.5"
16	38"	26"
18	39"	27.5"