Measured: Children's sizes are named by month to 24M, then by T sizes, then by number. Every band is judged against a height and weight chart rather than the child's birthday.
The basics
What to know about kid's size range
Nothing about a child's age reliably tells you what size they wear. Two five year olds can stand four inches apart in height and differ by eight pounds while both grow normally. The age band on a label describes an average child at that stage, so treat it as a starting point rather than an answer.
Measure instead. Stand the child against a wall with shoes off, mark the top of the head, measure down to the floor, and weigh them the same morning. Add a chest measurement at the fullest part and a waist at the natural bend. Read those numbers against the chart of the brand you are buying, since one label's 5 can run a full size off another's.
The bands below run in order and overlap at every join on purpose. A 5T is cut fuller through the hip and shorter in the leg than a 5. That is because a toddler's shape is not a school child's. Where two bands cover your child's height, choose by body shape. One rule cuts across all of them. In the US, sleepwear above size 9 months and up to size 14 must either fit snugly or be flame resistant. Pajamas that look too small on the hanger are cut correctly.
