By baby weight (typical, makers differ a little): NB 2 to 4.5 kg, S 4 to 8 kg, M 6 to 11 kg, L 9 to 14 kg, XL 12 to 17 kg, XXL 15 to 20 kg. Between two sizes, take the bigger one if you see leaks or red marks at the legs. This is a weight guide, not medical advice.
Diaper size guide
By baby weight (typical, makers differ a little): NB 2 to 4.5 kg, S 4 to 8 kg, M 6 to 11 kg, L 9 to 14 kg, XL 12 to 17 kg, XXL 15 to 20 kg.