Enter your baby’s age, weight and length to get the US clothing size to buy — plus the size to buy up to, so you’re not paying for clothes they’ve already outgrown.
Babywear is sized by weight first, length second, and age barely at all. The “3 months” on the tag is a rough average; a chunky 2-month-old and a slim 5-month-old routinely wear the same size. So we match your baby’s weight to a size band, cross-check the length, and then tell you when the age on the label disagrees with the baby in front of you.
The recommendation bands are the midpoint of two published manufacturer charts: Gerber Childrenswear and Burt’s Bees Baby, both retrieved 2026-08-07. We publish both original charts in full below so you can see the gap. They differ by up to 1.5 lb at some sizes — an 8.5 lb baby is “0–3M” in one chart and still “Newborn” in the other. Carter’s, Old Navy, Target and H&M all publish their own, and none of them match exactly.
Confidence: medium. Use this to decide what to buy sight-unseen; check the specific brand’s own chart for anything fitted — zip sleepers, footed pajamas, sleep sacks and swimwear run the tightest and are where the disagreement bites. European brands generally run smaller and label by height in centimetres.
| US size | Our weight band | Our length band | Gerber Childrenswear | Burt’s Bees Baby | Age guide |
|---|---|---|---|---|---|
| Preemie | 0–5.5 lb | 0–17 in | Up to 5 lb · Up to 17 in | Up to 6 lb · Up to 17 in | Born early / under about 5.5 lb |
| Newborn (NB) | 5.5–8.5 lb | 17–21 in | 5-8 lb · 17-21 in | 6-9 lb · Up to 21.5 in | Birth to about 1 month |
| 0-3M | 8.5–12 lb | 21–24 in | 8-12 lb · 21-24 in | 9-12.5 lb · 21.5-24 in | About 1-3 months |
| 3-6M | 12–16.5 lb | 24–26 in | 12-16 lb · 24-26 in | 12.5-17 lb · 24-26.5 in | About 3-6 months |
| 6-9M | 16.5–20.5 lb | 26–28 in | 16-20 lb · 26-28 in | 17-21 lb · 26.5-28.5 in | About 6-9 months |
| 12M | 20.5–24.5 lb | 28–30 in | 20-24 lb · 28-30 in | 21-25 lb · 28.5-30.5 in | About 9-12 months |
| 18M | 24.5–28 lb | 30–32.5 in | 24-28 lb · 30-32 in | 25-28 lb · 30.5-32.5 in | About 12-18 months |
| 24M / 2T | 28–31.5 lb | 32.5–34 in | 28-32 lb · 32-34 in | 28-30 lb · 32.5-34 in | About 18-24 months |
Go by weight, not the age on the tag. Enter your baby’s weight above and we’ll map it to a size band using two published manufacturer charts. As a rough guide: Newborn is about 5.5–8.5 lb, 0–3M about 8.5–12 lb, 3–6M about 12–16.5 lb, 6–9M about 16.5–20.5 lb and 12M about 20.5–24.5 lb.
Weight, primarily, then length. The age on the label is an average that a lot of babies never match. Manufacturers publish weight ranges precisely because age is unreliable — Gerber’s 0–3M is 8–12 lb, Burt’s Bees Baby’s is 9–12.5 lb, and neither one is about being three months old.
Roughly 5.5–8.5 lb and up to about 21 inches, though the two charts we use disagree at the top end — Gerber ends Newborn at 8 lb, Burt’s Bees Baby at 9 lb. Newborn size typically lasts 2–4 weeks and some babies skip it entirely, so buy very few and keep the tags on until you meet your baby.
Fast at first, then slower. Newborn lasts a few weeks; 0–3M and 3–6M each last a couple of months; from 6–9M onward each size stretches further. Roughly speaking a baby doubles their birth weight by about 5 months and triples it by a year, which is why the early sizes turn over so quickly.
3–6M or bigger — never Newborn. Every new parent is drowning in Newborn and 0–3M from the shower and has nothing in the drawer beyond it. Also think about the season: a 6–9M outfit for an October baby will be worn in spring, not winter.
Because the age label is a population average and your baby isn’t average. A baby at the 90th percentile for weight will be a full size ahead of their age label; one at the 10th will be behind. Neither means anything is wrong — it just means the tag is a marketing convenience, not a measurement.