it is more correct to compare with the height:
(Unit：CM)
80=6M-12M：length=40 bust=73, fits for: 70-80cm height / 6-12 Months

90=18M-24M：length=42 bust=78, fits for: 80-90cm height / 18-24 Months

100=3T：length=44 bust=83 , fits for: 90-100cm height / 3 Years Old

110=4T：length=47 bust=86, fits for: 100-110cm height / 4 Years Old

120=5T：length=50 bust=90, fits for: 110-120cm height / 5 Years Old

130=6T-7T：length=53 bust=95, fits for: 120-130cm height / 6-7 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.