it is more correct to compare with the height:
(Unit：CM)
6/70=6M：length=34 bust=66 sleeve=29 shoulder=26, fits for: 66-73cm height / 6-9 Months

8/80=9M/12M：length=36 bust=68 sleeve=30 shoulder=27, fits for: 73-80cm height / 9-12 Months

10/90=18M：length=38 bust=70 sleeve=31 shoulder=28, fits for: 80-90cm height / 12-18 Months

12/100=24M：length=40 bust=72 sleeve=33 shoulder=29, fits for: 90-100cm height / 18-24 Months

14/110=3T：length=42 bust=74 sleeve=35 shoulder=30, fits for: 100-110cm height / 2-3 Years Old

16/120=4T：length=44 bust=76 sleeve=37 shoulder=31, fits for: 110-120cm height / 4-5 Years Old
Note:
1.There is 2-3% difference according to manual measurement.(1 inch = 2.54 cm)
2.All measurement in cm, please note 1cm=0.3937ch, 1 inch=2.540cm
3.Please check the figures carefully and order the right size, don't completely depend on the " age advice". It's just for reference.