If you've ever bought a “0–3 months” outfit only to find it either drowns your newborn or won't do up over their thighs, you've discovered the central frustration of baby clothes sizing: the labels are a rough guide at best, and they vary wildly between brands. A baby's age tells you surprisingly little about what size they actually wear. Their weight and height tell you far more. This guide explains how baby sizing really works, gives you a clear size chart, and shares the tricks that take the guesswork out of buying.
Why Baby Clothes Sizing Is So Confusing
The core problem: sizes are labelled by age, but babies of the same age vary enormously in size. A 3-month-old can be anywhere from the 5th to the 95th percentile in weight and still be perfectly healthy. A label that says “3 months” is really the manufacturer's guess at what an average baby of that age weighs and measures — and “average” describes very few real babies.
On top of that, sizing isn't standardised between brands or countries. One brand's “0–3 months” may be cut noticeably larger or smaller than another's, and US, UK, and EU sizing conventions differ. This is why the single most useful habit is to shop by weight and height rather than age wherever a brand provides those measurements.
Baby Clothes Size Chart (by Age, Weight & Height)
Use this as a general guide — always check the specific brand's own chart where available, since cuts vary:
| Size | Weight | Height |
|---|---|---|
| Newborn (NB) | Up to ~4.5 kg (10 lb) | Up to ~56 cm |
| 0–3 months | ~4.5–5.7 kg (10–12.5 lb) | ~56–61 cm |
| 3–6 months | ~5.7–7.5 kg (12.5–16.5 lb) | ~61–67 cm |
| 6–9 months | ~7.5–9 kg (16.5–20 lb) | ~67–72 cm |
| 9–12 months | ~9–10.5 kg (20–23 lb) | ~72–76 cm |
| 12–18 months | ~10.5–11.8 kg (23–26 lb) | ~76–83 cm |
| 18–24 months | ~11.8–13.2 kg (26–29 lb) | ~83–88 cm |
The Practical Rules That Save Money and Hassle
- Size up, almost always: When in doubt, choose the larger size. Babies grow astonishingly fast, a slightly big outfit is comfortable and gets more wear, and a too-small one is outgrown in days. The exception is newborn sleepwear, where a good fit matters for safety and comfort.
- Don't overbuy the smallest sizes: Newborn and 0–3 month sizes are outgrown the fastest — some babies skip newborn size almost entirely. Buy a small number to start and more of the 3–6 and 6–9 sizes that get worn longer.
- Buy ahead for the right season: A baby who is 3 months old in summer will need their 6–9 month clothes in autumn — so buy the next size up in the season it'll actually be worn. Sizing up a summer romper for a baby who'll be that size in winter leaves you with shorts in the cold.
- Check the weight range on the label: Where a garment lists a weight range, trust that over the age. Your baby's current weight is the most reliable guide to fit.
- Mind the cut for cloth nappies: Babies in cloth nappies often need to size up sooner for room around the bottom.
How to Tell If Something Fits
- Poppers do up without straining, and there's room to move at the shoulders and hips
- Sleeves and legs reach roughly to wrist and ankle with a little growing room — not bunching, not riding up
- The neckline isn't tight, and there's space to slide a finger comfortably
- Bodysuit crotch poppers close without pulling the shoulders down — the classic sign a bodysuit has been outgrown in length
- Nothing leaves red marks when removed
A Note on Sizing for Smaller and Premature Babies
For babies born early or who are very small, standard newborn sizes can still be far too big in the first weeks. Premature clothing is sized by weight rather than age, in dedicated tiny ranges — our guide to premature baby clothes and our detailed preemie size chart cover those smallest sizes in full.
Once you've got the size sorted, the next question is usually fabric. For the softest, most breathable options for everyday wear, see our guide to organic cotton baby clothes, and browse our current collections for pieces across every size.