Baby Clothing Sizes by Age: A Simple Guide (and How Much to Buy)
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Short answer: baby clothes are sized by age (newborn, 0–3, 3–6 months and so on), but they really track weight and height — so treat the ages as rough guides, not rules. Big babies skip newborn size entirely; smaller ones live in it longer. The money-saving rule: buy only a week's worth per size, starting with newborn and 0–3 months, and size up as your baby grows rather than stockpiling ahead.
The most common baby-clothes mistake is buying a wardrobe of one size before the baby arrives. Here's how to size sensibly.
Rough size-by-age guide
- Newborn — roughly up to ~4.3kg / birth to a few weeks (many babies skip it).
- 0–3 months — roughly ~4.3–6kg.
- 3–6 months — roughly ~6–8kg.
- 6–9 / 9–12 months — from there it tracks weight and height more than age.
Brands vary, so always check the weight/height on the label over the age name — it's far more accurate.
How much of each size should I buy?
About a week's worth per size to start — because laundry happens often and babies outgrow sizes in weeks. Over-buying one size is the classic way to waste money on outfits that are never worn.
- Start with a small amount of newborn and more 0–3 months.
- Prioritise easy pieces: sleepsuits and vests over fiddly outfits.
- Restock the size that's working, rather than guessing ahead.
Buy for the season they'll actually be that size
A baby who'll be 6–9 months in winter needs warm clothes in that size, not summer ones — so think about the calendar when you size up, especially for gifts and hand-me-downs.
What about the soft basics?
Alongside clothes, the soft everyday layer gets constant use at every size — breathable muslins, and towels like a hooded towel for the newborn months. (Sorting your whole list? See the newborn essentials checklist.)
Frequently asked questions
How do baby clothing sizes by age work?
Sizes are labelled by age (newborn, 0–3, 3–6 months and so on) but actually track weight and height, so the ages are rough guides. Bigger babies skip newborn size; smaller ones stay in it longer. Always check the weight range on the label.
How many clothes should I buy in each baby size?
About a week's worth per size to start, beginning with a little newborn and more 0–3 months. Babies outgrow sizes in weeks and laundry is frequent, so buying a lot of one size ahead of time usually wastes money.
Should I buy newborn-size baby clothes?
Just a few. Many babies are born big enough to skip newborn size or move out of it within weeks, so buy a small amount and lean more on 0–3 months. Keep receipts in case your baby arrives larger than expected.
Written by the founder of Baby Zaza — a parent of one, and the person who tests everything in the shop. Browse soft baby basics in Little Wardrobe.