Buying baby clothes ought to be the simple part of getting ready for a baby. In practice it is one of the most quietly stressful bits — partly because baby clothing is sized by age, and babies have never once agreed to grow on schedule.
If you have ever unwrapped a 0–3 months sleepsuit that your two-week-old had already outgrown, or found a bag of 12–18 months clothes that never got worn at all, this guide is for you. Below you'll find a UK size chart by age, weight and height, how to measure your baby at home in about two minutes, and the situations where sizing up is sensible — and the one where it isn't.
Why baby clothes sizes are so confusing
Three things are working against you.
Age labels are averages
"6–9 months" describes the size of a typical baby at that age, not your baby at that age. Birth weight, feeding and genetics all shift the picture, and a perfectly healthy baby can sit two bands either side of their actual age.
Growth happens in bursts
Babies typically double their birth weight by around five months and grow fastest in the first six months, then slow considerably. That means the early size bands get outgrown quickly — sometimes in a matter of weeks — while 2–3 years might last most of a year.
Sizing conventions differ
UK and EU brands usually label by age band and often list a height in centimetres; US sizing runs differently again. Two garments both marked "12–18 months" can genuinely be different sizes.
The fix isn't to memorise a brand's chart. It's to know your baby's height and weight, and read the chart rather than the label on the front.
UK baby and toddler size chart by age, weight and height
Use this as your starting point. Height is usually the more reliable of the two measurements for clothing, because it drives sleeve and leg length — the things you actually notice when something no longer fits.
| Size | Typical age | Weight | Height / length |
|---|---|---|---|
| 0–3 months | Newborn – 3 months | Up to 6.5 kg (14 lb) | Up to 62 cm |
| 3–6 months | 3 – 6 months | 6.5 – 8 kg (14–17.5 lb) | 62 – 68 cm |
| 6–9 months | 6 – 9 months | 8 – 9 kg (17.5–20 lb) | 68 – 74 cm |
| 9–12 months | 9 – 12 months | 9 – 10 kg (20–22 lb) | 74 – 80 cm |
| 12–18 months | 1 – 1½ years | 10 – 11.5 kg (22–25 lb) | 80 – 86 cm |
| 18–24 months | 1½ – 2 years | 11.5 – 12.5 kg (25–27.5 lb) | 86 – 92 cm |
| 2–3 years | 2 – 3 years | 12.5 – 14 kg (27.5–31 lb) | 92 – 98 cm |
| 3–4 years | 3 – 4 years | 14 – 16 kg (31–35 lb) | 98 – 104 cm |
| 4–5 years | 4 – 5 years | 16 – 18 kg (35–40 lb) | 104 – 110 cm |
| 5–6 years | 5 – 6 years | 18 – 20 kg (40–44 lb) | 110 – 116 cm |
Swipe the chart sideways to see all four columns.
One rule to carry with you: if your baby is in the top third of a band on either height or weight, buy the next size up. You will get far more wear out of it.
How to measure your baby at home
You need two numbers, and you probably already have one of them.
Write the numbers in your phone with the date. In six weeks they will have changed, and having the old figures makes it obvious how fast your particular child is moving through the bands.
When to size up — and the one time not to
Sizing up is usually the right call:
- When they're near the top of a band. As above — the single most useful habit to get into.
- For anything you're buying ahead. Clothes bought in advance for a season three months away should be bought for the child they'll be then, not the child they are now.
- For outerwear and jumpers. These go over other layers, and a coat with a bit of room lasts two winters instead of one.
- For footed sleepsuits. Feet are almost always the first thing to run out of room. If the toes are pressing against the end, it's time to move up, even if the body still fits.
The exception is baby sleepwear in the first months. A sleepsuit or sleeping bag that's significantly too big has excess fabric that can ride up over the face, and a sleeping bag your baby can slip down inside is not safe. Sleepwear should fit properly now: snug around the neck and armholes, roomy over the legs and hips. Buy sleepwear to fit, and size up on everything else.
Sizing by garment type
Different pieces fail in different places, so it's worth knowing what to check.
- Babygrows and sleepsuits: check the feet first, then whether the poppers still fasten comfortably over a nappy. Length between shoulder and crotch is the real limit — if the fabric is pulling when they stretch out, size up.
- Bodysuits and vests: the popper gusset under the bottom is the tell. When it's straining, or the shoulders are pulling, they've outgrown it, regardless of what the neck opening suggests.
- Joggers and bottoms: elasticated waists give you months of grace, and cuffs can be rolled. These are the pieces where sizing up costs you nothing.
- Tops and tees: look for a wide or envelope neckline. Baby heads are proportionally large, and a narrow neck opening turns getting dressed into a wrestling match.
- Jackets and sweats: size up one band as standard so there's room for a jumper underneath.
What to do when they grow out of a size
This is the part of baby sizing nobody warns you about. Children move through the early bands so quickly that most pieces are outgrown long before they're worn out — especially good-quality organic cotton, which tends to still look new when it stops fitting. A drawer of 3–6 months babygrows in perfect condition is the normal outcome, not a sign you bought badly.
Rather than let those pieces sit in a loft, you can put them back into circulation through Super Looper, our pre-loved route run with our take-back partner Lloop. The idea is simple: send in the clothes your child has outgrown, they're individually checked and honestly described by real people, then relisted for another family to buy — and you get credit to spend rather than a bin bag by the door.
It's worth building into how you buy in the first place. If a piece is going to have a second life, sizing up slightly and buying fewer, better things stops being an extravagance and starts being the cheaper option per wear. It also works in the other direction: if your baby is racing through a band, buying that band pre-loved rather than new makes a lot of sense for something they'll wear for six weeks.
Super Looper — our Pre-Loved page · Shop and sell pre-loved at Lloop
How Lucy & Sam sizing works
Our range runs across ten size bands, from 0–3 months up to 5–6 years, so you can buy the size that fits now rather than settling for a band that's nearly right. Everything is made from 100% GOTS certified organic cotton, which is soft from the first wear rather than needing to be broken in.
Shop Clothing By Size
A note on washing, because it affects fit: organic cotton can relax or shrink slightly on its first wash. Washing at 30°C and skipping the tumble dryer keeps the fit and the fabric where they should be — and it's also what keeps a piece good enough to hand on or send to Super Looper later. Our full guidance is on our care page.
Read next: How To Care For Your Clothes
Common questions
What size does a newborn wear?
Most babies born at term start in 0–3 months, and many are in it from day one. Babies born small or early may need a separate newborn or tiny baby size for the first few weeks. If your baby is over about 3.5 kg at birth, expect 0–3 months to be a short-lived size.
Should I buy newborn size at all?
Buy a handful of essentials rather than a wardrobe — four or five sleepsuits and the same in vests. It's the size most likely to be outgrown before it's worn, and the one most likely to arrive as a gift.
Do organic cotton baby clothes shrink?
Only a little, and mostly on the first wash. Wash at 30°C, avoid the tumble dryer where you can, and reshape while damp. Hot washes and hot drying are what cause noticeable shrinkage in any cotton, organic or otherwise.
How many of each size do I actually need?
For the first year, roughly six to eight sleepsuits, six to eight vests and three or four day outfits per size is enough for most families — more if you're not doing laundry every couple of days. Buy fewer in the early sizes and more from 6–9 months onwards, when each band lasts longer.
What should I do with clothes my child has grown out of?
If they're still in good condition — which most well-made cotton pieces are — pass them on rather than storing them indefinitely. Siblings, cousins and friends are the easiest route. Beyond that, Super Looper, our pre-loved scheme with Lloop, takes outgrown clothes in, checks and photographs them, and relists them for another family, with credit back to you. It clears the drawer and keeps the clothes in use, which is the whole point of buying well in the first place.
Still unsure between two sizes? Go up. A slightly roomy babygrow gets worn for months; a slightly small one gets worn for a fortnight.
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