If you've ever bought a "3-month" outfit that didn't fit at 3 months, you're not alone — it's the single most common reason baby clothes get returned. The fix is simple: size by your baby's weight and height, not just their age. This guide gives you an instant calculator, the full age-weight-height chart, how much to buy in each size, and exactly when to move up.
- Age is a starting point; weight is the most accurate signal for baby clothing size.
- Sizes are ranges with overlap — a big newborn may skip Newborn and start at 0–3M.
- Size up for comfort, longer wear, and bulky items like sleepwear and outerwear.
- Sizes vary by brand — always check the label's weight/height range, not just the age.
Baby Clothing Size Calculator
Baby clothing size chart by age, weight & height
Here is the full baby and toddler clothing size chart. Match your baby's weight first, then height, then age — if two disagree, choose the larger size for comfort and growing room.
| Size | Age | Weight (lb) | Weight (kg) | Height (in) |
|---|---|---|---|---|
| Preemie | preemie | up to 6 lb | up to 2.7 kg | up to 17" |
| Newborn | up to 1m | 6–9 lb | 2.7–4.1 kg | 17–21.5" |
| 0–3M | 1–3m | 9–12.5 lb | 4.1–5.7 kg | 21.5–24" |
| 3–6M | 3–6m | 12.5–17 lb | 5.7–7.7 kg | 24–26.5" |
| 6–9M | 6–9m | 17–21 lb | 7.7–9.5 kg | 26.5–28.5" |
| 9–12M | 9–12m | 21–25 lb | 9.5–11.3 kg | 28.5–30.5" |
| 12–18M | 12–18m | 25–28 lb | 11.3–12.7 kg | 30.5–32.5" |
| 18–24M | 18–24m | 28–30 lb | 12.7–13.6 kg | 32.5–34" |
| 2T | 24–36m | 30–33 lb | 13.6–15 kg | 34–36" |
| 3T | 36–48m | 33–36 lb | 15–16.3 kg | 36–38.5" |
| 4T | 48–60m | 36–40 lb | 16.3–18.1 kg | 38.5–41" |
How do baby clothing sizes work?
Baby clothes are sized in age bands — Newborn, 0–3M, 3–6M, and so on — but each band is really a weight and height range. Because babies grow at very different rates, two babies of the same age can wear sizes months apart. That's why the label always lists a weight range: it's the number to trust over the printed age.
After 24 months, sizing switches to toddler "T" sizes (2T, 3T, 4T). These are cut for a more mobile, potty-training toddler, so a 2T fits differently from a 24-month piece even though the ages overlap.
Should you size up in baby clothes?
In most cases, yes. Babies grow fastest in the first year, and roomier clothes are more comfortable and last longer. Size up when your baby is in the top third of a weight range, for sleepwear and outerwear, and for brands known to run small.
How many clothes do you need in each size?
Babies go through 2–3 outfits a day, so plan around laundry every 2–3 days. Buy the fewest in Newborn (many babies wear it only a week or two) and the most in 0–3M and 3–6M, the sizes they stay in longest.
| Size | Bodysuits | Sleepers | Outfits | Notes |
|---|---|---|---|---|
| Newborn | 4–6 | 4–6 | 2–3 | Buy light — many skip it |
| 0–3M | 7–10 | 6–8 | 4–6 | The workhorse size |
| 3–6M | 7–10 | 6–8 | 4–6 | Long-wear size |
| 6–12M | 6–8 | 5–6 | 4–5 | Add weather layers |
Do baby clothing sizes differ by brand?
Yes — sizing is not standardised, so a 3–6M from one brand can fit like a 6–9M from another. Always check the garment's stated weight and height range rather than the age on the front, and when a brand is known to run small, size up.
When should you move up a size?
Move up when outfits feel snug at the shoulders or thighs, when snaps strain at the crotch, when sleeves and legs ride up, or when your baby reaches the top of the current size's weight range. Re-check the fit every 4–6 weeks in the first year.
Frequently asked questions
What size does a newborn wear?
Is baby clothing sized by age or weight?
What size comes after 24 months?
Should I size up in baby sleepwear?
Now you know the size — shop it.
Browse Babbez baby clothing by exact size, from Newborn to 4T.
Shop baby clothes by size →Related reading: What to dress your baby in for each season · How many bodysuits does a baby need? · The baby romper & jumpsuit guide
This guide gives general sizing information to help you shop and reduce returns — it is not medical or growth advice. Every baby grows differently and clothing sizes vary between brands; when in doubt, size up. Sizing reflects standard US baby & toddler apparel ranges. For sleepwear safety and fit, follow the manufacturer's instructions.
