it is more correct to compare with the height:
(Unit: CM)
80=9M/12M：length=36 bust=66 sleeve=34 shoulder=26, fits for: 65-75cm height / 9-12 Months

90=18M/24M：length=39 bust=70 sleeve=36 shoulder=28, fits for: 75-85cm height / 18-24 Months

100=3T：length=42 bust=74 sleeve=38 shoulder=30, fits for: 85-95cm height / 3 Years Old

110=4T：length=45 bust=78 sleeve=40 shoulder=32, fits for: 95-105cm height / 4 Years Old

120=5T：length=48 bust=82 sleeve=42 shoulder=33, fits for: 105-115cm height / 5 Years Old

130=6T：length=51 bust=86 sleeve=45 shoulder=35, fits for: 115-125cm height / 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.