Most U.S. clothing brands base their sizing on average baby weight at a given age. While exact measurements can vary by brand, here’s a general guide for baby clothing sizes by weight:

Size                 Weight                                Length / height

Preemie         Up to 5 lb / 2.3 kg           Up to 17 in / 43 cm

Newborn       5-8 lb / 2.3-3.6 kg           17-21 in / 43-53 cm

0-3M              8-12 lb / 3.6-5.4 kg         21-24 in / 53-61 cm

3-6M              12-17 lb / 5.4-7.7 kg         24-27 in / 61-69 cm

6-9M              17-21 lb / 7.7-9.5 kg         27-29 in / 69-74 cm

9-12M            21-25 lb / 9.5-11.3 kg       29-31 in / 74-79 cm

12-18M           25-28 lb / 11.3-12.7 kg     31-33 in / 79-84 cm

18-24M           28-30 lb / 12.7-13.6 kg   33-35 in / 84-89 cm

2 years (2T)   28-32 lb / 13-15 kg          33-35 in / 84-89 cm

3 years (3T)   32-35 lb / 15-16 kg          35-38 in / 89-97 cm

4 years (4T)   35-40 lb / 16-18 kg