Kids grow in jumps. Measure over a thin shirt and compare to the chart before you pick a lettered or month size. When you are between sizes, size up for outer layers and true-to-size for onesies and pajamas.
0–3 months: up to 12 lb / 24 in · 3–6 months: 12–16 lb / 24–27 in · 6–12 months: 16–22 lb / 27–30 in · 12–18 months: 22–27 lb / 30–32 in
2T: 27–30 lb / 32–35 in · 3T: 30–33 lb / 35–38 in · 4T: 33–38 lb / 38–41 in · 5T: 38–43 lb / 41–44 in
4–6: 38–50 lb · 6–8: 50–60 lb · 8–10: 60–74 lb · 10–12: 74–87 lb. Utility pants and denim jackets have room in the seat for diapers on the smallest end and for layering on the largest.