it is more correct to compare with the height(cm):

80=9M-12M: length=50 bust=53 waist=51;fits for: 70-80cm height /9-12 Months

90=18M-24M: length=55 bust=57 waist=55;fits for: 80-90cm height /18-24 Months

100=3T: length=60 bust=61 waist=59;fits for: 90-100cm height /3 Years Old

110=4T: length=65 bust=65 waist=63;fits for: 100-110cm height /4 Years Old

120=5T: length=70 bust=69 waist=67;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.