size	baby’s weight (kg)	baby’s height (cm)
nb	up to 4.1	up to 56
0 – 3m	5.7	62
3 – 6m	7.4	68
6 – 9m	8.8	74
9 – 12m	9.9	80
12 – 18m	11.3	86
18 – 24m	12.4	92
2 – 3y	14.5	98
3 – 4y	n/a	104
4 – 5y	n/a	110
5 – 6y	n/a	116

To measure a small baby lay them down & lay a measuring tape from the top of their head to the bottom of their heel.
For older littles ones, you can measure them standing.