it is more correct to compare with the height:
Unit：CM
80=9M-12M：length=35 bust=62 sleeve=27 shoulder=25.5; fits for: 70-80cm height / 9-12 Months

90=18M-24M：length=37 bust=64 sleeve=30 shoulder=27; fits for: 80-90cm height / 18-24 Months

100=3T：length=39 bust=66 sleeve=33 shoulder=28.5;fits for: 90-100cm height / 3 Years Old

110=4T：length=41 bust=68 sleeve=35 shoulder=30;fits for: 100-110cm height / 4 Years Old

120=5T：length=43 bust=70 sleeve=38 shoulder=31.5;fits for: 110-120cm height / 5-6 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.