it is more correct to compare with the height:
(Unit：CM)
6/80=9M/12M：length=40 bust=62 sleeve=37, fits for: 70-80cm height / 9-12 Months

8/90=18M/24M：length=43 bust=66 sleeve=39, fits for: 80-90cm height / 12-24 Months

10/100=3T：length=46 bust=72 sleeve=40, fits for: 90-100cm height / 2-3 Years Old

12/110=4T：length=49 bust=78 sleeve=42, fits for: 100-110cm height / 3-4 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.