it is more correct to compare with the height:
(Unit: CM)
6/80=6M-12M：length=33 bust=56 sleeve=29, fits for: 70-80cm height / 6-12 Months

8/90=12M-18M：length=36 bust=60 sleeve=30, fits for: 80-85cm height / 12-18 Months

10/100=18M-24M：length=39 bust=64 sleeve=31, fits for: 85-90cm height / 18-24 Months

12/110=3T：length=42 bust=68 sleeve=32, fits for: 90-100cm height / 3 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.