What size — and what to actually buy?
By age, with weight + height ranges that brands actually use. Plus honest notes on which sizes are worth stocking and which to skip.
How baby clothing sizes actually work
Baby clothing labels (Newborn, 0-3 months, 3-6 months, and so on) are really just shorthand for a weight and height range — babies don't grow on a fixed schedule, so the month number on the tag is a rough guess, not a rule. Weight and height predict fit far better than age does. A chunky 4-month-old often needs 6-9 month clothes; a petite 9-month-old may still fit comfortably in 3-6 month sizes. When a tag's ranges don't match perfectly, the lower bound (the lightest/shortest a baby can be) usually matters more for fit than the upper bound.
Carter's, Old Navy, H&M, Gap, and Target's Cat & Jack all overlap on these size-by-age ranges within an inch or two, so the chart below is a reasonable brand-consensus reference no matter where you shop. The bigger source of variation is between cuts — regular, husky, and slim run differently even within the same label — and between countries, where a US "12 month" runs slightly smaller than an EU size labeled "74cm." Newborn (NB) sizing is also worth a specific warning: it's a common baby-shower gift size, but most babies outgrow it in just 2–4 weeks, so it's not worth overstocking. The 0-3 month size tends to last the longest for most babies, which makes it the size worth buying the most of.
| Age | Size label | Weight (lb) | Height (in) | Note |
|---|---|---|---|---|
| Preemie | Preemie | under 5 | under 17 | Some brands skip preemie; Carter's, H&M, and BabyGap carry it. |
| Newborn | NB | 5–8 | 17–21 | A common gift size that babies grow out of in 2–4 weeks. Don't overstock. |
| 0-3 mo | 0-3 mo | 8–12 | 21–24 | The longest-lasting size for most babies — buy more here. |
| 3-6 mo | 3-6 mo | 12–17 | 24–27 | H&M sometimes skips and pairs as 4-6m or 2-4m. |
| 6-9 mo | 6-9 mo | 17–20 | 27–28 | Crawling-stage. Stretchy waistbands matter more than cute prints. |
| 9-12 mo | 9-12 mo | 20–22 | 28–30 | Standing/cruising. Rompers get easier than separates. |
| 12-18 mo | 12-18 mo | 22–27 | 30–32 | Walking. Sized up vs equivalent "Toddler size 12 months". |
| 18-24 mo | 18-24 mo | 27–30 | 32–34 | Watch for "2T" overlap — same age, different cut. |
| 2T | 2T | 28–32 | 33–35 | "Toddler" cut: more room for diapers + standing. |
| 3T | 3T | 32–37 | 35–38 | |
| 4T | 4T | 37–42 | 38–41 |
Limitations: these are brand-consensus ranges, not a guarantee — an individual baby can fall outside them and still be perfectly healthy. Sizing source: brand size charts (Carter's, BabyGap, H&M); height ranges from WHO length-for-age 50th percentile.