it is more correct to compare with the height:
(Unit：CM)
80=9M-12M：top length=34 bust=59 sleeve=25 pants=43 hips=73, fits for: 70-80cm height / 9-12 Months

90=18M-24M：top length=36 bust=64 sleeve=27 pants=46 hips=76, fits for: 80-90cm height / 18-24 Months

100=3T：top length=38 bust=67 sleeve=29 pants=48 hips=79, fits for: 90-100cm height / 3 Years Old

110=4T：top length=40 bust=72 sleeve=31 pants=51 hips=82, fits for: 100-110cm height / 4 Years Old

120=5T：top length=42 bust=77 sleeve=33 pants=53 hips=85, fits for: 110-120cm height / 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.